Plate Tectonics

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. 

1. Origins of the Theory
   1. Abraham Ortelius was first to speculate on continental fit (1596)
   1. 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 NOTsufficient proof for scientists.

1. The Evidence to Support the Theory (list is roughly in chronological order)
   1. 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.

• Matching Geology:   Scientists have documented numerous examples of RARE AND UNIQUE rock samples separated by thousands of miles of ocean.

• 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.

• Ocean Floor Topography (Bathymetry):    Long mid-ocean ridges and long oceanic trenches found in linear patterns around the world.

• Global Earthquake Patterns:    Linear patterns of earthquakes match pattern of mid-ocean ridges and oceanic trenches around the world.  The patterns actually form shapes!!!

• Linear Chains of Volcanoes:      Linear volcanic mountain ranges and volcanic island chains match pattern of oceanic trenches around the world.

• Ocean Floor Rock ALL VOLCANIC!!!!       All ocean floor rock comprised of basalt – a volcanic rock!!!!

• Heat Emanating from Mid-Ocean Ridges: Likely from slow cooling of lavas, which can take millions of years.

1. 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).

• Earthquake Depth Pattern near Trenches :   Earthquake origins shallowest at trenches, and increasingly deeper toward associated volcanoes…evidence of “diving” plate.
  
  
  • Extrusion of Magma at the Sea Floor:  It was only recently (last fifteen years) that magma was actually observed erupting at spreading centers.

1. 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

• Asthenosphere: review earlier description

• 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).

• 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).

• 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.

• Gondwana:  Gondwana eventually became S. America, Africa, Australia, and Antarctica.

• 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).

• Plate Boundaries (simplified summary)

Divergent Boundaries (forces of extension): Sea Floor SpreadingContinental 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)

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 RidgeGulf of California	normal faultingshield volcanoeseffusive 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 faultingcomposite volcanoesexplosive eruptions
Oceanic vs. Oceanic (subduction)	Marianas Trench and IslandsPhilippines Trench and IslandsJapan 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.

Profile Diagram of Sea Floor Spreading and Oceanic vs Continent Subduction
<<<This is critically important!!!!!>>>

The Tectonic Setting of Western North America

                                        (adapted from Fig. 10-11a, pg 322)

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.

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!!!

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