Download the Lesson Plan
In this lesson, students will use the real life experiences of two young men featured in the documentary All the Difference to reflect on their plans post-high school and begin thinking about their futures, from college to careers. They will explore tools and strategies to guide this preliminary planning for life beyond high school.
Filmed over five and half years, All the Difference traces the paths of two promising young men, Krishaun Branch and Robert Henderson, as they navigate their lives in low-income, high-risk communities in Chicago. Statistics predict they will drop out of high school; both graduate and go on to college in spite of all the odds. The film explores the factors in their lives (mentors, teachers, parents and grandparents, role models and community support) that made all the difference. All the Difference is part of American Graduate: Let's Make It Happen, a national public media initiative made possible by the Corporation for Public Broadcasting to help local communities keep more students on the path to graduation.
This lesson touches on several key concepts presented in All the Difference and introduces students and teachers to the accompanying College Bound Resources. It is recommended that students and teachers delve into these materials for an extended college/career exploration. Note that the lesson plan does not focus on college as the sole option beyond high school, but allows students to consider other types of educational, training and career pathways.
Accompanying the film are College Bound Resources that support students and their families, and offer teachers instructional strategies that will help them guide their students on the college/career journey. This lesson plan draws on and references elements of these materials.
For Students: An online, interactive College Bound Students Handbook intended for first-generation students to use in their college prep and throughout their college careers. The handbook covers such topics as college selection, financial aid packages, time management, networking, academic majors and stumbling blocks.
For Educators: An online, interactive Facilitators Guide offers strategies and activities for using the film to start conversations with students and help them prepare for their college careers.
For Families and Caring Adults: Family Tips are tip sheets that offer advice and tips on how to support students and prepare to send them off to college, covering everything from how to throw a trunk party, to financial aid, to what to expect for a college freshman.
Embedded in the College Bound Students Handbook and Facilitators Guide are 31 video clips that reinforce the stories of the featured young men, with a focus on how they navigate college.
POV offers a lending library of DVDs that you can borrow anytime during the school year--FOR FREE! Borrow All the Difference to screen in your classroom. Get started by joining our Community Network.
By the end of this lesson, students will be able to:
- Describe what their futures might/could look like beyond high school
- Explore the benefits of a college education
- Identify and discuss how to address the potential challenges young people can experience as they prepare for life beyond high school, specifically if college is a goal
- Research college or alternative educational/training/career opportunities for post-high school planning purposes
GRADE LEVELS: 9-12
Note: Though aimed at high school students, this lesson can also be suitable for middle school students who have started to think about their futures.
College and Career Preparation
- All the Difference Facilitators Guide (for your reference)
- All the Difference College Bound Students Handbook (for students)
ESTIMATED TIME NEEDED
One 50-minute class period, with homework.
Film clips provided in this lesson are from the All the Difference College Bound Students Handbook (http://www.pbs.org/pov/allthedifference/college-bound/).
Clip 1: "All the Difference: Introduction" (5:11 min.)
In this clip, executive producer Wes Moore addresses students and introduces the All the Difference College Bound Students Handbook. His introduction is followed by the trailer for All the Difference.
Clip 2: "Aligning Student Strengths and Interests to the Right School Can Make All the Difference" (2:35 min.)
This clip features Krishaun Branch as he's deciding what he's looking for in a college in order to align his college choice with his career goals.
In preparation for this lesson, we recommend taking some time to review the All the Difference Facilitators Guide. Designed for educators, the Facilitators Guide offers tips, strategies, activities, discussion questions, homework assignments and more to help you lead students through the College Bound Students Handbook.
2. Thinking Forward
Invite students to reflect on what their futures might look like. Students can think about the future within the context of a timeline, beginning with their current grade. (If time permits, they might actually create illustrated timelines with visual benchmarks.) Have them think about what they might be doing as they move through high school, then beyond high school and what might come at the end of their timelines--jobs, degrees, books, crafts and so on. Make sure students know that it is fine not to have a particular end in mind, because reflecting and planning also allows a future vision to take shape over time. Invite students to share briefly their expectations/illustrated timelines.
3. College Thoughts
Point out that several students mentioned that college is likely to fit into their post-high school plans. Note that other students talked about the possibility of college or other types of educational/training options.
Briefly acknowledge the benefits of college (Reference: Facilitators Guide - Chapter 1, Expectations About College, Topic 2: Why College, Which College, and How to Get There). Ask students to add benefits not listed. Be sure to note that college may not be a desired goal for everyone and that there are other types of educational/training opportunities students can think about as they envision their life journeys beyond high school.
Ask students to think about challenges they might face (or are currently facing) when it comes to going to college or putting in place next steps after high school. Record these points.
Let students know that they're going to take steps towards making their futures a reality through a documentary called All the Difference and accompanying handbook based on the real-life experiences of two young men from Chicago, Robert Henderson and Krishaun Branch, who were both the first in their families to go to college. Show students Clip 1, which is the introduction in the College Bound Students Handbook - Introduction and Purpose. The clip includes a message from executive producer Wes Moore, followed by the film trailer. Have them briefly reflect on the key messages underscored in this segment.
Invite students to reference the clip and their personal experiences to reflect on the challenges they might face when it comes to planning for the future. Discussion prompts can include:
- How do these challenges play out in future plans?
- How is it possible to address and overcome those challenges?
- What guidance and support systems can help students through these challenges?
Introduce the College Bound Students Handbook to students. Point out that planning for life beyond high school (college, career) is a process and that the handbook provides a helpful step-by-step framework to guide them through that process over the next several years, using real-life experiences and advice from Robert and Krishaun, who have been through many of the challenges they brought up earlier in the class. Briefly walk through the contents so they become familiar with the important elements of this planning journey on which they will eventually embark. Direct students to the explanation of the optional Self Scoring tasks in the handbook. Encourage them to complete the tasks and add up their final scores in order to determine their college/career readiness and to begin building plans for their future lives. You can also assign this as a long-term homework assignment.
4. Figuring it Out
Probe with students where the starting point is for thinking about the future. What do they need to do, for example, if college is a consideration? Have students share several ideas.
Show students Clip 2, from the College Bound Students Handbook - Chapter 1, Expectations About College, Topic 2: Why College, Which College, and How to Get There. After viewing the clip, ask students to share some of the college-search strategies presented in the segment.
Using the homework assignment at the end of Topic 2 as a framework, instruct students to create, as a class, a list of the top 10 things to consider when thinking about college as a post-high school option. Items might include thinking about what to study and finding schools that address that interest, or meeting college representatives. If college is not an option, students can create a list of tasks one must take if pursuing an alternate educational/career prep route. Have each student share one element from the list that is their first priority and offer one step they will take to begin addressing that element.
(Note that these tasks will require a few days or more to complete; you might want to review via a "check-in" within a set timeframe to monitor how students are progressing):
Option A: Students can conduct preliminary research into and identify up to 10 colleges that jibe with their interests and expectations. Use the Topic 2 Homework activity in the Facilitators Guide, which provides a set of questions students can use to guide this research.
Option B: If students are not thinking about college at this time, but do have ideas for other types of educational/training pathways, they can conduct similar research. Consider modifying the homework activity so that questions reflect these alternatives.
Option C: Students can select one section of the handbook they view as integral to the journey and do some or all of the tasks to help direct their planning processes.
1. How They Did It
For this activity, reference Chapter 1: Expectations about College, Topic 1: Thinking About Expectations in the Facilitators Guide (for teachers) and the College Bound Students Handbook (for students).
Students seeking to map out paths for their futures can learn from others with whom they might share similar experiences (struggles, obstacles, goals, desires, direction). Students identify and interview a family or community member they view as someone who can inform next steps in the educational and career journey. Note that this individual does not need to have graduated from college or followed a traditional educational/career path.
Small student groups convene to write a series of relevant interview questions, which can include those the College Bound Students Handbook addresses: college expectations and choices; support systems; those who inspired and/or mentored them; obstacles/challenges they experienced, tackled and overcame; mistakes they made; how they prepared for their future choices. Students should present a description of whom they interviewed, what they learned and what lessons seemed best to inform steps they will take to frame out their futures. For additional inspiration, students can look at stories from:
The Black List: Volume One
Makers: Women Who Make America
2. Making and Saving Money
For this activity, reference Chapter 1: Expectations About College, Topic 4: Financing College in the Facilitators Guide (for teachers) and the College Bound Students Handbook (for students).
While the emphasis in the film and the accompanying materials is on financing college, learning how to make and save money in life in general is critical. Students might think first about something they need or want in the immediate future and how they plan to acquire it. How much money will they need; how will they get that money (budgeting, saving, earning, spending, investing)? This can be more of a brainstorm task that gets students thinking about what is involved in saving funds and beginning the grander challenge of financing college. Some of the following resources can support this task:
Your Financial Plan: Where it All Begins
What Can I Afford?
Get Schooled: Money Talks
3. Practicing the Personal Essay
For this activity, reference Chapter 1: Expectations About College, Topic 3: Writing the Essay for Your College Application in the Facilitators Guide (for teachers) and the College Bound Students Handbook (for students).
Students can try their hand at personal essays on topics of their choice. High school students can focus on writing college application essays, if preferred. As part of this process, they can read a variety of high-quality personal essays and think about their components in order to recognize what makes them compelling.
4. Making and Moving Past Mistakes
For this activity, reference Chapter 4: Slips, Stumbles and Getting Up Again, Topic 2: Managing Crises in the Facilitators Guide (for teachers) and the College Bound Students Handbook (for students).
All the Difference emphasizes that making mistakes is part of life's journey. Everybody makes them, but the goal is to ensure that those mistakes are not permanently debilitating. Learning how to accept and get past mistakes is critical to moving forward. Students can either write reflections or work in small groups to share brief stories about mistakes they have made and how they tackled them in order to rectify problems and redirect themselves. They can create a set of tips that guides peers through mistake making, with emphasis on the positive outcomes that can emerge from mistakes made.
POV: All the Difference
The film's official POV site includes a discussion guide with additional activity ideas and resources.
All the Difference College Bound Resources:
College Bound Students Handbook
Introduced by Wes Moore and intended for first-generation, college-bound high school students, the handbook covers such topics as college selection, financial aid packages, time management, networking, academic majors and stumbling blocks. The guide was written by Marcia Cantarella, author of I CAN Finish College: The Overcome Any Obstacle and Get Your Degree Guide.
For educators, guidance counselors and college prep programs, the guide offers strategies and activities geared to using the film to start conversations with students and help them prepare for college. It was written by Marcia Cantarella, author of I CAN Finish College: The Overcome Any Obstacle and Get Your Degree Guide.
For parents, guardians and/or other adult family members, these tip sheets offer insight and advice on everything from how to throw a trunk party, to financial aid, to what to expect for a college freshman. Written by Joy Thomas Moore, JWS Media Consulting and executive producer of All the Difference.
POV: Media Literacy Questions for Analyzing POV Films
This list of questions provides a useful starting point for leading rich discussions that challenge students to think critically about documentaries.
Career Exploration Lessons for Sixth and Seventh Grades
Career Readiness Partner Council
CareerTech: "Middle School Career Development Lessons"
Getting Started: Career/College Planning Guide for Ninth Grade Students
Kids.gov: "Jobs and Careers"
U.S. Department of Labor: "Career Planning for High Schoolers"
ASCD: "What Makes a Student College Ready?"
The College Board: "Big Future"
Connections Academy: Getting Ready for College: A Four-Year Checklist for High-School Teens"
Disabilities, Opportunities, Internetworking and Technology: "Preparing for College: An Online Tutorial"
Federal Student Aid: "Getting Ready for College or Career School Can Be Easier than You Think"
I'm First: "Find Colleges"
ideas42: "Nudging for Success: Using Behavioral Science to Improve the Postsecondary Student Journey."
Indiana Afterschool Network: "College and Career Readiness"
Mapping Your Future: "Success in College Guide"
The New York Times: "Tip Sheet: An Admissions Dean Offers Advice on Writing a College Essay"
Peterson's: "College Planning Timelines"
Quintessenti: "Next Step After High School? Some Alternatives to College"
Forbes: "5 Proud Alternatives to Going to College"
The Huffington Post: "How to Build a Successful Life Without a Four-Year Degree"
PBS NewsHour: "Why I'm Telling Some of My Students Not to Go to College"
Bloomberg: "Fading College Dream Saps U.S. Economy of Productivity Miracle."
The College Fix: "MYTH: More Black Men in Prison Than in College"
FiveThirtyEight: "Race Gap Narrows in College Enrollment, But Not in Graduation"
One Day Magazine: "Preparing for the College Shock"
University of Pennsylvania Graduate School of Education: "Black Male Student Success in Higher Education"
American Graduate: Let's Make It Happen
Getting Smart: "Smart List: 30 Orgs Boosting College Access & Success"
Common Core State Standards for English Language Arts & Literacy in History/Social Studies, Science and Technical Subjects
SL.6.1 Engage effectively in a range of collaborative discussions (one-on-one, in groups and teacher-led) with diverse partners on grade 6 topics, texts and issues, building on others' ideas and expressing their own clearly.
SL.6.1.C Pose and respond to specific questions with elaboration and detail by making comments that contribute to the topic, text or issue under discussion.
SL.6.1.D Review the key ideas expressed and demonstrate understanding of multiple perspectives through reflection and paraphrasing.
SL.6.2 Interpret information presented in diverse media and formats (e.g., visually, quantitatively, orally) and explain how it contributes to a topic, text or issue under study.
SL.7.1 Engage effectively in a range of collaborative discussions (one-on-one, in groups and teacher-led) with diverse partners on grade 7 topics, texts and issues, building on others' ideas and expressing their own clearly.
SL.7.1.C Pose questions that elicit elaboration and respond to others' questions and comments with relevant observations and ideas that bring the discussion back on topic as needed.
SL.7.1.D Acknowledge new information expressed by others and, when warranted, modify their own views.
SL.7.2 Analyze the main ideas and supporting details presented in diverse media and formats (e.g., visually, quantitatively, orally) and explain how the ideas clarify a topic, text or issue under study.
SL.8.1 Engage effectively in a range of collaborative discussions (one-on-one, in groups and teacher-led) with diverse partners on grade 8 topics, texts and issues, building on others' ideas and expressing their own clearly.
SL.8.1.C Pose questions that connect the ideas of several speakers and respond to others' questions and comments with relevant evidence, observations and ideas.
SL.8.1.D Acknowledge new information expressed by others, and, when warranted, qualify or justify their own views in light of the evidence presented.
SL.8.2 Analyze the purpose of information presented in diverse media and formats (e.g., visually, quantitatively, orally) and evaluate the motives (e.g., social, commercial, political) behind its presentation.
SL.9-10.1 Initiate and participate effectively in a range of collaborative discussions (one-on-one, in groups and teacher-led) with diverse partners on grades 9-10 topics, texts and issues, building on others' ideas and expressing their own clearly and persuasively.
SL.9-10.1.C Propel conversations by posing and responding to questions that relate the current discussion to broader themes or larger ideas; actively incorporate others into the discussion; and clarify, verify or challenge ideas and conclusions.
SL.9-10.1.D Respond thoughtfully to diverse perspectives, summarize points of agreement and disagreement and, when warranted, qualify or justify their own views and understanding and make new connections in light of the evidence and reasoning presented.
SL.9-10.2 Integrate multiple sources of information presented in diverse media or formats (e.g., visually, quantitatively, orally) evaluating the credibility and accuracy of each source.
SL.11-12.1 Initiate and participate effectively in a range of collaborative discussions (one-on-one, in groups and teacher-led) with diverse partners on grades 11-12 topics, texts and issues, building on others' ideas and expressing their own clearly and persuasively.
SL.11-12.1.C Propel conversations by posing and responding to questions that probe reasoning and evidence; ensure a hearing for a full range of positions on a topic or issue; clarify, verify or challenge ideas and conclusions; and promote divergent and creative perspectives.
SL.11-12.1.D Respond thoughtfully to diverse perspectives; synthesize comments, claims and evidence made on all sides of an issue; resolve contradictions when possible; and determine what additional information or research is required to deepen the investigation or complete the task.
SL.11-12.2 Integrate multiple sources of information presented in diverse formats and media (e.g., visually, quantitatively, orally) in order to make informed decisions and solve problems, evaluating the credibility and accuracy of each source and noting any discrepancies among the data.
Content Knowledge (http://www2.mcrel.org/compendium/browse.asp) a compilation of content standards and benchmarks for K-12 curriculum by McREL (Mid-continent Research for Education and Learning).
Self-Regulation, Standard 2: Performs self-appraisal.
Grade K-12 Benchmarks:
5: Determines appropriate behaviors that are used and should be adopted to obtain wants and/or needs.
6: Knows personal strengths and weaknesses and techniques for overcoming weaknesses.
8: Understands how hobbies, personal interests and aptitudes can lead to a career.
Thinking and Reasoning, Standard 6: Applies decision-making techniques.
Grade 6-8 Benchmark 1: Identifies situations in the community and in one's personal life in which a decision is required.
Grade 9-12 Benchmark 5: Evaluates major factors (e.g., personal priorities, environmental conditions, peer groups) that influence personal decisions.
American School Counselor Association National Standards for Students (http://static.pdesas.org/content/documents/asca_national_standards_for_students.pdf)
Standard A: Students will acquire the attitudes, knowledge and skills that contribute to effective learning in school and across the life span.
Standard B: Students will complete school with the academic preparation essential to choose from a wide range of substantial post-secondary options, including college.
Standard C: Students will understand the relationship of academics to the world of work and to life at home and in the community.
National Standards for Family and Consumer Sciences Education (http://www.nasafacs.org/national-standards-and-competencies.html)
Career, Community and Family Connections, Comprehensive Standard
Integrate multiple life roles and responsibilities in family, work and community settings.
Career, Community, and Family Connections, Standard 1.1
Analyze strategies to manage multiple roles and responsibilities (individual, family, career, community and global).
This lesson plan includes content adapted from the All the Difference College Bound Students Handbook and Facilitators Guide, written by Marcia Cantarella, Ph.D., author of I CAN Finish College: The Overcome Any Obstacle and Get Your Degree Guide • Edited by Anne Llewellyn, Outreach Extensions • Community Engagement Resources • Produced by Judy Ravitz, Outreach Extensions, and executive produced by Joy Thomas Moore, JWS Media Consulting
ABOUT THE AUTHOR
Michele Israel owns Educational Writing & Consulting (www.micheleisrael.com), where she works with large and small educational, nonprofit and media organizations to bolster products and programs. Her rich career spans more than 25 years of successful experience developing educational materials and resources, designing and facilitating training, generating communication materials and grant proposals and assisting in organizational and program development. Her long list of clients includes Tiffany & Co., Frost Valley YMCA, Teaching Tolerance, the Public Broadcasting Service, the Association for Supervision and Curriculum Development, the New York City Department of Health and Mental Hygiene, WETA Public Television, Religion & Ethics NewsWeekly and the Harm Reduction Coalition.
Chapter 13:Evolution of Continents and Oceans
The theory of plate tectonics is nowadays more or less universally accepted by geologists, and I have mentioned the basic idea briefly at the beginning of this class. The basic thought is, that instead of being permanent fixtures of the earth's surface, the continents and ocean basins undergo continuous change. Both are parts of lithospheric plates that move against each other, and in the process new crust is created at midoceanic ridges (spreading centers), and old crust is consumed at convergent plate boundaries (subduction zones). Even before the theory of plate tectonics, there were a variety of geologic observations that suggested that the continents were on the move, but because nobody had a good idea what the underlying driving mechanisms might be, the idea languished in obscurity for the first half of the 20th century. For now we will take plate tectonics as a theory with a broad observational data base in its support, and will assume that it essentially works as outlined in Chapter 3.
Alfred Wegener, the pioneer of continental drift, thought that the continents as plates move through the oceanic crust, implying thus that the shorelines of the continents are the margins of the continental plates. However, even though that may be initially a reasonable assumption (the shorelines being major geographic features), continental margins need not necessarily be plate margins. Today scientists have a fairly good understanding of how the plates move and how such movements relate to earthquake and volcanic activity. Most movement occurs along narrow zones between plates where the results of plate-tectonic forces are most evident. There are basically three different types of plate boundaries (divergent, convergent, transform), and a fourth type (boundary zones) is sometimes designated when it is difficult to define a clear boundary:
- Divergent boundaries -- where new crust is generated as the plates pull away from each other.
- Convergent boundaries -- where crust is destroyed as one plate dives under another.
- Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other.
- Plate boundary zones -- broad belts in which boundaries are not well defined and the effects of plate interaction are unclear.
The three principal types of plate margins and various associated features are illustrated in the picture above.
|Divergent plate boundaries occur along spreading centers where plates are moving apart (white arrows) due to mantle convection and new crust is created by magma pushing up from the mantle.|
Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This submerged mountain range, which extends from the Arctic Ocean to beyond the southern tip of Africa, is but one segment of the global mid-ocean ridge system that encircles the Earth. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years. This rate may seem slow by human standards, but because this process has been going on for millions of years, it has resulted in plate movement of thousands of kilometers. Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today.
The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, offers scientists a natural laboratory for studying on land the processes also occurring along he submerged parts of a spreading ridge. Iceland is splitting along the spreading center between the North American and Eurasian Plates, as North America moves westward relative to Eurasia. The consequences of this type of plate movement are easy to see around Krafla Volcano, in the northeastern part of Iceland, and the Thingvellir Fissure Zone.
|Lava fountains (10 m high) spouting from eruptive fissures during the October 1980 eruption of Krafla Volcano in Iceland. At Krafla, existing ground cracks have widened and new ones appear every few months. From 1975 to 1984, numerous episodes of rifting (surface cracking) took place along the Krafla fissure zone. Some of these rifting events were accompanied by volcanic activity; the ground would gradually rise 1-2 m before abruptly dropping, signaling an impending eruption. Between 1975 and 1984, the displacements caused by rifting totaled about 7 m.|
|Aerial view of the area around Thingvellir, Iceland, showing a fissure zone (in shadow) that is the on-land exposure of the Mid-Atlantic Ridge. Right of the fissure, the North American Plate is pulling westward away from the Eurasian Plate (left of the fissure). Large building (near top) marks the site of L�gberg, Iceland's first parliament, founded in the year A.D. 930.|
|The evolution of a divergent plate boundary has three recognizable stages. The birth of a divergent boundary requires that an existing plate begins to divide. This is happening today in east Africa, in an area known as the East African Rift zone. The African continent is slowly splitting in two. As the continental crust divides, magma from the asthenosphere fills in the gap. Several volcanoes are present in the rift zone. Eventually the gap will form a narrow ocean (youth) much like the Red Sea to the north of the East African Rift Zone. The Red Sea separates Saudi Arabia from Africa.|
East Africa may be the site of the Earth's next major ocean. Plate interactions in the region provide scientists an opportunity to study first hand how the Atlantic may have begun to form about 200 million years ago. Geologists believe that, if spreading continues, the three plates that meet at the edge of the present-day African continent will separate completely, allowing the Indian Ocean to flood the area and making the easternmost corner of Africa (the Horn of Africa) a large island.
|A similar narrow sea, the Gulf of California (see image at right), lies between much of Mexico and Baja California. The view to the south along the Gulf of California, between Baja peninsula (right) and the mainland of Mexico (left). The Gulf is spreading, pushing Baja further away from the Mexican mainland.|
It takes millions of years to form a mature ocean, as rates of plate motions are slow (10-100 mm/yr). At such rates it would take millions years to form even a narrow ocean.
The size of the Earth has not changed significantly during the past 600 million years, and very likely not since shortly after its formation 4.6 billion years ago. The Earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being created. Such destruction (recycling) of crust takes place along convergent boundaries where plates are moving toward each other, and one plate sinks (is subducted) under another. The location where sinking of a plate occurs is called a subduction zone. The type of convergence (some call it a very slow "collision") that takes place between plates depends on the kind of lithosphere involved. Convergence can occur between an oceanic and a largely continental plate, or between two largely oceanic plates, or between two largely continental plates.
Convergence between continental and oceanic crust
|Off the coast of South America, along the Peru-Chile trench, the oceanic Nazca Plate is pushing into and is being subducted under the continental part of the South American Plate. In turn, the overriding South American Plate is being lifted up, creating the towering Andes mountains, the backbone of the continent. Partial melting of the subducted oceanic crust gives rise to andesitic volcanism parallel to the subduction zone. Because continental crust is less dense than oceanic crust, oceanic crust will always be subducted under continental crust. Strong, destructive earthquakes and the rapid uplift of mountain ranges are common in these region. Earthquakes are often accompanied by uplift of the land by as much as a few meters.|
|The convergence of the Nazca and South American Plates has deformed and pushed up limestone strata to form the towering peaks of the Andes, as seen|
here in the Pachapaqui mining area in Peru.
Convergence between oceanic and oceanic crust
|As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other (the older one is subducted because of its larger density), and in the process a trench is formed. The Marianas Trench (paralleling the Mariana Islands), for example, marks where the fast-moving Pacific Plate converges against the slower moving Philippine Plate. The Challenger Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth's interior (nearly 11,000 m) than Mount Everest, the world's tallest mountain, rises above sea level (about 8,854 m).|
Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. The trenches are the key to understanding how island arcs such as the Marianas and the Aleutian Islands have formed and why they experience numerous strong earthquakes. Magmas that form island arcs are produced by the partial melting of the descending plate and/or the overlying oceanic lithosphere. The descending plate also provides a source of stress as the two plates interact, leading to frequent moderate to strong earthquakes.
|The Himalayan mountain range dramatically demonstrates one of the most visible and spectacular consequences of plate tectonics. When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways.|
|The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate. After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights. Most of this growth occurred during the past 10 million years. The Himalayas, towering as high as 8,854 m above sea level, form the highest continental mountains in the world. Moreover, the neighboring Tibetan Plateau, at an average elevation of about 4,600 m, is higher than all the peaks in the Alps except for Mont Blanc and Monte Rosa, and is well above the summits of most mountains in the United States.|
|The collision between the Indian and Eurasian plates has pushed up the Himalayas and the Tibetan Plateau. The cross sections show the evolution of the Himalayas and the displacement of slivers of continental crust during this collision. The reference points (small squares) show the amount of uplift of an imaginary point in the Earth's crust during this mountain-building process.|
|The zone between two plates sliding horizontally past one another is called a transform-fault boundary, or simply a transform boundary. The concept of transform faults originated with Canadian geophysicist J. Tuzo Wilson, who proposed that these large faults or fracture zones connect two spreading centers (divergent plate boundaries) or, less commonly, trenches (convergent plate boundaries). Most transform faults are found on the ocean floor. They commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes.|
|However, a few occur on land, for example the San Andreas fault zone in California. This transform fault connects the East Pacific Rise, a divergent boundary to the south, with the South Gorda -- Juan de Fuca -- Explorer Ridge, another divergent boundary to the north.|
The Blanco, Mendocino, Murray, and Molokai fracture zones are some of the many fracture zones (transform faults) that scar the ocean floor and offset ridges.
The offset that is marked by the San Andreas Fault also implies that there is mantle upwelling beneath Southwestern North America. The resulting extension is seen at the surface in form of a series of NE-SW trending mountain ranges and valleys, the so called Basin and Range Province, a result of Horts and Graben tectonics.
|The San Andreas fault zone, which is about 1,300 km long and in places tens of kilometers wide, slices through two thirds of the length of California. Along it, the Pacific Plate has been grinding horizontally past the North American Plate for 10 million years, at an average rate of about 5 cm/yr. Land on the west side of the fault zone (on the Pacific Plate) is moving in a northwesterly direction relative to the land on the east side of the fault zone (on the North American Plate). |
The picture at left shows and aerial view of the San Andreas fault slicing through the Carrizo Plain in the Temblor Range east of the city of San Luis Obispo.
Other Pictures from the San Andreas Fault:
Not all plate boundaries are as simple as the main types discussed above. In some regions, the boundaries are not well defined because the plate-movement and deformation occurs over a broad belt (called a plate-boundary zone). One of these zones marks the Mediterranean-Alpine region between the Eurasian and African Plates, within which several smaller fragments of plates (microplates) have been recognized. Because plate-boundary zones involve at least two large plates and one or more microplates caught up between them, they tend to have complicated geological structures and earthquake patterns.
|Regardless of these complications, however, it is now a well established fact that the Earth's crust is broken into a dozen or so rigid slabs (called tectonic plates by geologists) that are moving relative to one another.|
The cause of plate movement is not accessible to direct observation. The various features of plate movement, and the increased heatflow along midoceanic ridges are consistent with the idea that plate movement is caused by convection in the mantle. The driving force behind the convection is heat generated by radioactive decay in the earth. The heat released by this decay (radiogenic heat) is transferred by convection (slow movement of hot, plastic rock) to the surface of the earth. Friction between the convecting mantle and the lithosphere (includes the rigid crust and that part of the mantle that lies above the plastic/soft behaving astheneosphere) causes the crustal plates (form the top of the lithosphere) to move according to the movement of the convection currents. Heat production in the earth will cease as radioactive decay diminishes, and then convection will cease and the final cooling phase of the Earth will begin. No more mountain ranges will be built, and the continents will become very flat. Eventually the oceans may cover the continents again (shallow seas, buildup of carbonate platforms, change of seawater composition because terrestrial input cut off, possibly a new stage in evolution). Tectonic movements will still occur, but this time they will mainly be a response to differential cooling of the earth (surface already cold, but interior shrinks now as well, volume reduction, pressure ridges will form due to shrinking, may resemble folded mountain belts).
THE OCEAN BASINS, THEIR EVOLUTION AND IMPORTANT FEATURES
Most of the earth's surface is covered by oceans, but for a long time the oceans have been an essentially white spot on the map of the world. Early expeditions like that of the Beagle (Charles Darwin) brought some preliminary knowledge, compilations of data by ship captains brought some initial knowledge about ocean currents and migration of fish swarms (mention Melville, Captain Ahab), but by and far we did not know much about the topography of the ocean floor, much less about its geological features. Starting at around 1930, however, a vast amount of knowledge has been gathered about the oceans, about their water chemistry, the cycling of elements, biological aspects, bathymetry, bottom sediments and their stratigraphy.
Though much less spectacular and not as well publicized, the progress in knowledge about the oceans is far more important for the future of mankind than to send a few men to the moon. Ocean research has implications for food resources, the supply of raw materials for a growing population, and possibilities of ocean population by man (giant raft cities in shallow seas, platforms moving with food-rich ocean currents, etc.). Even populating the deep sea is probably cheaper and more feasible than to have people live in colonies on the moon.
Work on the bathymetry of the ocean basins (mainly with echo-sounding devices) has revealed many morphologic features that were previously unknown, such as oceanic ridges, abyssal plains (and hills), seamounts, trenches, and continental margins, all of these features are now easily explained by plate tectonics.
|Map of the Atlantic and Eastern Pacific Basin. Mid-Oceanic Ridges (marked with white arrows) are extensive. These are the youngest portions of the ocean basins where new ocean crust is generated through mantle upwelling and plate divergence. Taken together the oceanic ridge system of the earth is about 65000 km long and extend all around the globe.|
|Map of the Pacific Basin and parts of the central Atlantic. Continental Shelf = flooded edges of the continents; Continental Margin = the edge/border region of the continent; Deep Sea Trenches = deepest parts of ocean basins (due to subduction of oceanic crust); Abyssal Plains = older parts of oceanic crust, smoothed due to sediment deposition; Seamounts = submarine volcanic cones; the can also form linear arrangements, so called Seamount Chains.|
Continental margins are in a geological sense not part of the oceanic crust. They consist of continental crust and material that was eroded from the continents and is now piled up along the margins of the continents. The margins are subdivided into CONTINENTAL SLOPE and SHELF with the latter simply being a submerged part of shield or platform.
|Closeup of central Pacific Basin. Shows how the Hawaiian Islands (Hawaii marked with white arrow) are the youngest portion of a long chain of seamounts. The linear arrangement of many seamounts indicates that they formed because the plate moved over a stationary site of magma upwelling, a so called mantle "Hot Spot". Seamounts are submarine volcanoes that may finally build above the water level (e.g. Hawaii), in which case they are called islands. If seamounts rise above sea level (rises for two reasons, buildup of material in a cone, upwelling mantle pushes up plate), they are subject to wave erosion and colonization by reefs, with both processes tending to create a flat top on the original volcanic cone. Later, when the oceanic plate cools down and the island finally drowns we get flat-topped seamounts, so called GUYOTS.|
|Closeup of the eastern Pacific Basin. Shows triple junction of spreading ridges in center. Also shown are the subduction zone/trench along the western edge of central America, and the associated slope and shelf regions. The trenches are the deepest parts of the oceans and are the topographic expression of subduction zones. They are marked by intense volcanism (island arcs, volcanic mountain ranges, e.g. Andes, Cascades), and high frequency of earthquakes. hey are usually asymmetrical with a gentle slope towards the subducted plate, and a steeper slope towards the subducting plate. Some trenches are as deep as 11 km, and may extend for thousands of kilometers across the seafloor.|
|A map of the ocean basins where the locations of some major deep sea fans are marked. Deep Sea Fans are large sediment accumulations that are deposited on the slope and the adjacent seafloor. The sediments are supplied to the slope regions through submarine canyons, deep incisions in the continental shelf that probably originated during prior episodes of low sea level (ice ages). Along the continental margins sediment that is conveyed to the deep sea via submarine canyons (sliding, mass movement, turbidity currents) forms large cone-shaped or fan-shaped sediment accumulations at the toe of the continental slope, so called SUBMARINE FANS or DEEP-SEA FANS (not unlike alluvial fans). Turbidity currents move down these fans, spread out on the abyssal plain, decelerate, and deposit graded sand and silt layers (so called turbidite sequences). Sediment spreading by turbidity currents helps to smoothen the relief in abyssal plain regions.|
The floor of the ocean basins (abyssal plains) is essentially basaltic crust that is covered by sediment (settling from suspension, of organic material such as foram tests, radiolarian tests, etc., and also clay swept in from the rivers, volcanic ash [large ashclouds may circle the globe several times], and material transported by winds from the continents [Atlantic west of Sahara desert]). We call that material PELAGIC SEDIMENT.
COMPOSITION AND STRUCTURE OF OCEANIC CRUST
The oceanic crust is not simply a pile of basalt, but can be subdivided into several distinct layers, that form in response to the processes operating at a midoceanic ridge.
|The top layer (1.) consists of pelagic sediments that were deposited above the basalts of the oceanic crust. The second layer (2.) consists of lavas that were extruded onto the ocean floor at the spreading center. These lavas are called pillow basalts, because of the way they appear in cross-section. The molten basalt is extruded onto the ocean floor through fractures (extension), and as soon as the molten material comes in contact with seawater it will cool down and solidify. The next batch of lava will come out to the side of the first one, and also will solidify, etc. We will slowly pile up small batches of magma, that in their geometric arrangement are not unlike a pile of sausages, or squirts out of a toothpaste tube. In cross section we will have mainly elliptical cross-sections (pillow shape), thus the name pillow basalt. The surface topography of this layer is irregular and rough. The third layer (3.) consists essentially of complexly cross-cutting, near vertical basaltic dikes, which are the feeder channels for the pillow basalts. They form as fractures at the spreading center (highest extensional stress), and finally fill up with basalt and become part of the sheeted dike complex as they move away from the spreading center. The fourth layer (4.) consists of the magma chambers that feed the dikes of layer three, and these leftover magma chambers are filled by the plutonic equivalent of basalt, gabbro. The magma itself originated by partial melting in the mantle below the spreading center (higher heatflow, rising of accumulating melt). Below that layer is the mantle (asthenosphere), consisting of peridotite.|
That the oceanic crust is layered has been known from seismic refraction data, but nobody has ever drilled through the oceanic crust (too hot). Fortunately, once in a while bits and pieces of oceanic crust are incorporated into the uplifted material of flooded mountain belts, and is thus available for direct and detailed study. In Iceland, where the Mid-Atlantic Ridge rises above the sea surface, is another opportunity to examine the structure of the oceanic crust.
As new oceanic crust forms at mid-oceanic ridges, cold sea water invades the hot new crust through the abundant fractures (crustal extension). As the sea water heats up its density decreases and it rises upwards. When it leaves through fractures at the seafloor we have submarine hot springs, better known as black smokers. These hot springs have created quite a bit of excitement in the scientific community because they open up all sorts of unexpected angles on the chemistry of the oceans, the transfer of chemical elements between the oceans and the oceanic crust (elemental cycles), and the origin of life. The latter was prompted by the discovery of unusual communities of microbes, worms, clams, and crustaceans that live at hot spring sites and instead of sunlight depend on energy supplied by the hot springs in the form of sulfides.
EVOLUTION OF CONTINENTS
The characteristic features of continents are shield areas, stable platforms, and folded mountain belts (introduced earlier in this lecture). With the theory of plate tectonics we can now relate these features to each other and describe them as different phases in the evolution of continents.
|When we examine the continental crust in some detail, we see that in many areas (e.g. Texas) it consists of a thin surface cover of horizontally stratified sediments that is underlain by complexly deformed metamorphic rocks that have been intruded by granites. In places where vast areas of this lower complex of rocks are exposed, we speak of a "shield". In places where the shield material is covered by sediments we speak of a "stable platform". This kind of situation is typical for large portions of continents, except along some of the margins where we have subduction and compression. In the latter case mountain ranges develop end we have "folded mountain belts".|
Pertinent features of the continental crust are:
- It consists overall of material with granitic composition (granites and gneisses of granitic composition, other compositional rock types, such as basalts are present, but volumetrically not important)
- From the travelling velocity of seismic waves in the continental crust we know that the lower portions of the continental crust are denser than the upper portions, probably because of a downwards increase of rocks of more basaltic composition
- Continental crust is thickest beneath mountain ranges (root zones, 50-60 km), elsewhere the thickness is about 30 km.
- The structure of the continental crust is considerably more complex than the simple layer structure of the oceanic crust. It consists of intensely deformed metamorphic rocks (derived from sediments and volcanic rocks) that are intruded by granites, and may have been partially remolten to granites.
- The oldest continental crust has been determined to be about 3.8 b.y. old, and it appears that the continents grew throughout geologic history. They cover nowadays about one third of the earth's surface, but initially the proportion of the oceans may have been much larger.
- The continental crust is the end product of planetary differentiation (accumulation of light materials), and within the crust of each continent we can distinguish three basic components: shields, stable platforms, folded mountain belts.
SHIELDS contain the bulk of the rock record of continental evolution and growth, and are thus the key to the understanding of the origin of continents. As noted earlier, they are essentially flat and consist of a complex arrangement of igneous and metamorphic rocks. The mere fact that these rocks are exposed at the surface now, implies that many kilometers of rock were eroded from the continent before these rocks finally came to the surface. If the shield rocks of a continent are studied with respect to their metamorphic age, it often turns out that those on the center are the oldest ones, and that there are several belts of metamorphic rocks that get progressively younger outward. The oldest portions of the shields consist of a mixture of volcanic rocks (basalts, andesites) and volcanic derived sediments (erosion of volcanoes), and the rocks show similarity to the material accumulating in modern day island arcs. Only when these basically mafic rocks were later on intruded by granites, did the overall composition become granitic (75% granite). Later metamorphic belts were accreted onto these old continental cores (will discuss a little later) and have overall a considerably more granitic composition (because the sediment was derived from a crust that was already 75% granite).
STABLE PLATFORMS As time goes by, the shields are eroded down to within a few tens of meters of sea level, and any rise of sea level will lead to flooding of vast areas of the shield (plate tectonics, increased spreading, rise of ridges, flooding). At present only 18% of the continental crust is flooded, but there were times in the past where vast portions of the continents were covered by a shallow sea (interior of North America).
FOLDED MOUNTAIN BELTS are usually found along the margins of continents, and the folding and thrusting indicates that as much as 30% of crustal shortening has taken place during their formation. We know now that his shortening is a direct reflection of the compressive stress regime and subduction of oceanic crust along convergent plate margins, but before plate tectonics the missing crust was very troublesome thing to explain. The location of these fold belts along continental margins implies that by convergence of plates material is piled up along the continents, and finally becomes part of the continental crust. Fold belts that are terminated abruptly at the continental margin, such as the Appalachians and the Caledonides, suggest that he fold belts were once much longer, and have been separated when continents broke up by continental rifting.
From Mountain Belt to Continent
|When a mountain belt is formed along a continental margin by subduction, sedimentary and volcanic rocks are buried deeply and undergo high-pressure and high-temperature metamorphism in the root zone of the mountain belt. Also, parts of the buried material as well as of the subducted oceanic plate melt, and granitic and andesitic magmas rise. A considerable portion of the granites never rises to the upper portions of the mountain range, and crystallizes within the realm of the metamorphic rocks in the lower portions.|
|The newly formed mountain range (A) is of course in isostatic equilibrium with the mantle (that's why we have a root zone), but as erosion wears down the top portions of the fold belt, the root zone has to rise in order that equilibrium is maintained (B&C). In that way the volcanic and sedimentary unmetamorphosed portions of the range are eroded away, and the metamorphosed and granite intruded lower portions move upwards (B&C). This process continues until the fold belt is eroded down to sealevel, then erosion stops and isostatic uplift ceases (D). By that time the outcropping rocks will be the high grade metamorphics and granites of the root zone. We started with a folded mountain belt, and through continued erosion we have produced a new piece of shield material.|
Formation of a fold belt and a metamorphosed root zone on convergent plate boundaries is also known as orogeny (or creation of mountain ranges). Within the context of different types of plate convergence (mentioned earlier) we can distinguish three different main types of orogeny (ocean/ocean = island arc; ocean/continent = fold belt/volcanic arc; continent/continent = fold belt/high plateaus).
EVOLUTION OF A CONTINENT
We can use these different types of orogenies and the underlying plate tectonic processes to explain the evolution of continents and the continental crust.
|Initially (A) we might for example have only oceanic crust, convergence of oceanic plates and formation of island arc complexes (andesitic material, too light to be again subducted). Sediment is shed from the arc (B), is compressed and pushed against the arc, the mountains rise, and the root zone grows, until finally high-P/T metamorphism and granite plutonism occur (C). We start accreting material (folded mountain belts) to the initial arc, an embryonic continent is formed (C). The continent is eroded and quartz, feldspar, and clay-rich sediments accumulate around its margins. Renewed subduction pushes up new folded mountain belts, accompanied by metamorphism and granite plutonism. Finally the new fold belt is worn down and another segment has been added to the growing continent (D). Continued accretion etc. etc., the cycle repeats and the continent grows (D).|
Crustal recycling and the differentiation of the continental crust is intimately related to the composition of the oceans, the supply of nutrients for the global biomass, and thus is also linked to those global feedback mechanisms that we consider essential for climate regulation (carbon cycle etc.). In part, the biosphere has adapted opportunistically to whatever chemical components were provided in the process, but it also has an active role through the weathering of continents, the deposition of carbonate banks, the carbon cycle feedbacks with climate, etc.
I hope that in the course of this lecture you have gained insights into three topical complexes:
- the Earth system really is highly complex, and consists of many nested and interlinked element cycles and feedback loops
- we are a long way from understanding how the Earth system works in detail, but we are making progress
- the biosphere is an important component of the Earth system. Simply through evolutionary selection pressures it may have evolved to participate in climate regulation for most of Earth history.
Eventually, all things merge into one, and a river runs through it.
The river was cut by the world's great flood and
runs over rocks from the basement of time.
On some of the rocks are timeless raindrops.
Under the rocks are the words, and some of the words are theirs.
I am haunted by waters.
Norman Maclean, from "A River Runs Trough It"