Discussion 3

John Aubert  American River College Plate Tectonics Physical Geography Lecture Notes Page 1 of 7 aubertj@arc.losrios.edu

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Plate Tectonics (from the Greek word tektonikos, meaning “to build”)

I. The Theory (in a nutshell): Continents and ocean floors are fractured into separate lithospheric

plates (i.e. crust + uppermost mantle) that move independenly and in different directions… This results in earthquakes, volcanic activity, and the building of mountains.

II. Origins of the Theory

A. Abraham Ortelius was first to speculate on continental fit (1596)

B. Alfred Wegener (called Father of Continental Drift / Plate Tectonics): This German

geophysicist and meteorologist publicly presented his theory in 1912, then published it in 1915. The theory was considered revolutionary. He studied rocks, plants, fossils, and the “fit” of the continents. This was NOT sufficient proof for scientists.

III. The Evidence to Support the Theory (list is roughly in chronological order)

A. Matching Coastlines (Continental Fit): The shapes of the coastlines – especially

around the Atlantic Ocean – appear to match. Modern underwater surveys have documented a much closer fit beneath the surface than is observed at the surface.

B. Matching Geology: Scientists have documented numerous examples of RARE AND

UNIQUE rock samples separated by thousands of miles of ocean.

C. Matching Biology (both living and fossilized): Scientists have documented

hundreds of examples of related species (and/or groups of species) of plants and animals which are separated by oceans; in many of these cases the species ARE NOT able to float, swim or fly. Also, many RARE AND UNIQUE fossilized species are found separated as well.

D. Ocean Floor Topography (Bathymetry): Long mid-ocean ridges and long

oceanic trenches found in linear patterns around the world.

E. Global Earthquake Patterns: Linear patterns of earthquakes match pattern of mid-

ocean ridges and oceanic trenches around the world. The patterns actually form shapes!!!

F. Linear Chains of Volcanoes: Linear volcanic mountain ranges and volcanic island

chains match pattern of oceanic trenches around the world.

G. Ocean Floor Rock ALL VOLCANIC!!!! All ocean floor rock comprised of basalt

– a volcanic rock!!!!

H. Heat Emanating from Mid-Ocean Ridges: Likely from slow cooling of lavas, which

can take millions of years.

I. Age of Ocean Floor Rock: Rock at mid-ocean ridges is consistently the youngest,

becoming older as you travel away in either direction. In general, there is no “old” rock on the ocean floor (oldest is ONLY 208 millions years).

J. Earthquake Depth Pattern near Trenches : Earthquake origins shallowest at

trenches, and increasingly deeper toward associated volcanoes…evidence of “diving” plate.

K. Extrusion of Magma at the Sea Floor: It was only recently (last fifteen years) that

magma was actually observed erupting at spreading centers.

John Aubert  American River College Plate Tectonics Physical Geography Lecture Notes Page 2 of 7 aubertj@arc.losrios.edu

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IV. The Basics of Plate Tectonic Theory: Theory became widely accepted in the late 1960's;

renamed Plate Tectonics (previously called continental drift).

A. Structures Critical to Understanding Plate Tectonics (review)

1. Lithosphere (Crust + Uppermost mantle): review earlier description

2. Asthenosphere: review earlier description

B. Processes Critical to Understanding Plate Tectonics

1. Sea Floor Spreading (accretion): Magma rises along zones beneath the sea

floor. It then splits the plate, fills the new gap, and finally cools, creating new ocean floor made of mostly basalt. Accretion refers to the addition of new crust material. The Atlantic Ocean is growing due to this process (e.g. at the Mid Atlantic Ridge).

2. Subduction (consumption): Where one plate dives beneath another and is

consumed (melted). Consumption is the subtraction (or destruction) of plate material. The Pacific Ocean is shrinking due to this process. (e.g. at the Peru-Chile, Japan, Marianas, and Aleutian Trenches).

C. Past Continents, beginning with Pangaea (approx. 225mya): Wegener

called his original supercontinent Pangaea (meaning “all Earth”). It split into:

1. Laurasia: Laurasia eventually became N. America, Europe, and Asia.

2. Gondwana: Gondwana eventually became S. America, Africa, Australia, and

Antarctica.

D. Movement Continues: Lithospheric plates are in constant motion, usually moving in

sudden jumps, felt as earthquakes. The lithospheric plates move at an approx rate of 5- 10cm/year (50-100km per million years).

E. Plate Boundaries (simplified summary)

Divergent Boundaries (forces of extension):

 Sea Floor Spreading

 Continental Rupture

Convergent Boundaries (forces of compression):

 Oceanic vs. Continental (subduction)

 Oceanic vs. Oceanic (subduction)

 Continental vs. Continental (continental collision or “suture”)

Transform Boundaries (shearing forces)

These are the mountain

building processes!!!

John Aubert  American River College Plate Tectonics Physical Geography Lecture Notes Page 3 of 7 aubertj@arc.losrios.edu

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Detailed Plate Boundary Review

Major Plate

Boundary Types Sub-Types Selected Important

Global Examples Selected Important

Regional Examples

Associated

Faulting and

Volcanism**

Divergent

(extensional forces)

Sea Floor Spreading*  Mid-Atlantic Rift and

Ridge  Gorda Rift and Ridge

 Gulf of California  normal faulting

 shield volcanoes

 effusive eruptions Continental Rupture  East African Rift

 Great Basin (from Gulf of CA northward into Nevada)

Convergent

(compressional forces)

Oceanic vs. Continent*

(subduction)  Peru-Chile Trench and

Andes Range

 Cascade Trench and Cascade Range (Lassen, Shasta, Hood, Baker, Rainier, St. Helens…)

 reverse and thrust faulting

 composite volcanoes

 explosive eruptions

Oceanic vs. Oceanic

(subduction)

 Marianas Trench and Islands

 Philippines Trench and Islands

 Japan Trench and Islands

 Aleutian Trench and Islands (Alaska)

Continent vs. Continent

(continental collision)

 Himalayas (India vs. Asia)

 Alps

 Rockies

Transform

(shearing forces)

(some on land, some on ocean floor, all are similar)

 North Anatolian (in Turkey)

 San Andreas Fault

 strike-slip faulting (right and left lateral)

 volcanic activity uncommon

*You must be able to draw a profile diagram of these processes – see next page. **These topics will be covered in depth in next module.

John Aubert  American River College Plate Tectonics Physical Geography Lecture Notes Page 4 of 7 aubertj@arc.losrios.edu

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Profile Diagram of Sea Floor Spreading and Oceanic vs Continent Subduction

<<<THIS IS CRITICALLY IMPORTANT!!!!!>>>

asthenosphere

asthenosphere asthenosphere

oceanic crust

asthenosphere

uppermost mantle

oceanic crust

continental crust

uppermost mantle

uppermost mantle

moho

moho moho

Magma:

rises

splits

fills

cools

mid-ocean rift and ridge (ex. GORDA RIDGE)

oceanic trench (ex. CASCADE TRENCH)

volcanic mountain range (ex. CASCADE RANGE)

ex. PACIFIC PLATE ex. GORDA PLATE ex. NORTH AMERICAN

PLATE

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The Tectonic Setting of Western North America (adapted from Fig. 10-11a, pg 322)

Cascade Volcanic Range

Gorda

Plate

John Aubert  ARC Plate Tectonics Physical Geography Page 6 of 7 Lecture Notes

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Practice Map for Locating Plate Boundaries

See next page for sample questions >>>>>>

John Aubert  ARC Plate Tectonics Physical Geography Page 7 of 7 Lecture Notes

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Sample Questions: Questions similar to these will be on your exam. As you study you should

anticipate how I might use these questions to create new questions on the same concepts.

1

Which of the following was NOT used as evidence in support of Plate Tectonic Theory?

A. Global map showing the location of earthquakes

B. Global map showing the location of landslides and mudflows

C. Global map of showing the location mid-ocean ridges and oceanic trenches

D. Global map showing the age of ocean floor rock

E. Hundreds of examples of related plants and animals – unable to float, swim, or fly – which are separated by 1000s of miles of

ocean.

2 Where on the ocean floor will I find the oldest rock? A. near sea floor spreading centers B. far from sea floor spreading centers

3 What type of forces are evident at convergent plate boundaries? A. shearing B. extensional C. compressional D. recessional

4 What was the name for Wegener’s original supercontinent? A. Laurasia B. Gondwana C. Rodinia D. Pangaea E. Fossillia

5 At what type of plate boundary might I find reverse faulting?

6

A volcanic island chain such as the Marianas Islands is formed at what specific type of plate boundary?

A. divergent B. convergent (oceanic vs. oceanic) C. convergent (oceanic vs. continental) D. convergent (continental vs. continental)

7

Which land mass was once connected to Laurasia?

A. India B. North America C. South America D. Antarctica E. Australia

8

Which land mass was once connected to Gondwanaland?

A. Greenland B. Asia C. North America D. Antarctica E. Europe

9

Approximately how fast do lithospheric plates move across the Earth? A. 0.5 mm/yr B. 5cm/yr C. 5 meters/yr D. 5km/yr E.

500km/yr

10

Which of these processes is probably most responsible for driving (or causing) tectonic plate movement?

A. upwelling of magma at divergent boundaries B. tidal fluctuation C. upwelling of magma at “hot spots” D. subduction

11 Where is subduction occurring? (You will be shown a map on the exam.)

12 At which location did a continental collision (also called continental suture) occur in the past? (You will be shown a map on the exam.)

13 At which location is a continental rupture (split) occurring? (You will be shown a map on the exam.)

14 POSSIBLE ESSAY QUESTION: In the future the Atlantic Ocean will likely become larger than the Pacific. Explain this statement

using plate tectonic terminology and give details and examples.

15 CRITICAL DIAGRAM QUESTION: Provide a thorough and detailed profile (sideview) diagram of sea floor spreading and subduction.

GOOD NEWS!!! The practice questions above - plus many, many more – can be found in the online practice

quizzes discussed in your syllabus and in class. You can take each quiz multiple times, and each time you will get some new questions. Once submitted, the quizzes are graded automatically, with the correct answers provided immediately. This is a great way to prepare for the exams!!!

SERIOUSLY, TAKE THE ONLINE PRACTICE QUIZZES!!!