Plate Tectonics

Class Notes: Geo 1330: Dr Dupre¥

Plate Tectonics

(see also first weekís class notes and USGS Dynamic Earth)

Q. Who is Alfred Wegener ?

Wegener's evidence for continental drift included:

The Breakup of Pangaea: Dance of the Continents (fig. 20.24):

Q. What kept Continental Drift from being widely accepted in the first half of the century?

Q. Who is Harry Hess?

Q. What are Echo Sounders used for?

Hess discovered an extremely complex bathymetry in the Pacific Ocean, including trenches, seamounts, guyots, and rises.

German oceanographic expedition in the 1920ís was the first to describe the mid-Atlantic ridge.

Q. Where are the worldís longest mountain chains located?

Q. How does the Theory of Sea-Floor Spreading differ for the Theory of Continental Drift?

Convection in the mantle producing sea-floor spreading -: a possible mechanism for continental drift?

An Interlude - Paleomagnetism: the study of the earth's past magnetic field as preserved by magnetic minerals of rocks.

The Earth's Magnetic Field is similar to that of a Bar magnet (fig. 19.11)

The location of the earthís magnetic north pole can be determined by two measurements:

1) magnetic : the angle made between the earthís magnetic field and horizontal.

2) magnetic : horizontal angle between magnetic north and true north.

Q. What is the magnetic inclination at the magnetic equator, magnetic north pole, and magnetic south pole?

Q. What has magnetite got to do with all of this?

Fig. 19.13

Q. What is an "apparent polar wandering curve" and what is it's significance?

Q. What is a "magnetic reversal" (fig. 19.13)?

Q. What is meant by ìnormalî and ìreversedî polarity?

Q. What is a magnetic epoch?

Q. What is the evidence for a possible magnetic reversal about 30,000 years ago? (fig. 19.12)

Q. Where might you go to record evidence of magnetic reversals in the rock record?

Q. What is the Geomagnetic Time Scale; how was it put together, and how does it work?

Magnetic Stripes off the Pacific Northwest (USGS)

Magnetic Stripes along the Mid-Atlantic Ridge (USGS & fig. 20.9)

How are Magnetic Stripes formed? (fig. 20.10)

Q. The Glomar Challenger Drilling Ship was used to test what hypothesis?

Q. How does the age of the world's oceanic crust vary with distance from mid-ocean ridges (and how do we know)? (fig. 20.11)

Global Earthquake Epicenters

Global Plate Boundaries (fig. 20.3)

Fundamentals of plate tectonics include:

1) The outermost shell of the earth consists of a rigid lithosphere composed of the crust and upper mantle

2) The lithosphere rides over a weak, partly melted portion of the mantle - the asthenosphere.

3) The lithosphere is broken into a series of large plates that move relative to each other, driven by convection in the mantle.

4) Most of the deformation and volcanic activity on earth occurs along plate boundaries

NB. Not all plate boundaries are the same!

Types of Plate Boundaries include:

ï Divergent

ï Convergent

--Ocean-Ocean

--Ocean-Convergent

--Continent-Continent

ï Transform

Divergent Plate Boundary: characterized by:

Convergent Plate Boundary: characterized by:

Transform Plate Boundaries characterized by:

Divergent Plate Boundary (fig. 20.4b)-example?

Another Divergent Plate Boundary (fig. 20.4a)- example?

Ocean-Ocean Convergent Plate Boundary (fig. 20.6b)-example?

Ocean-Continent Convergent Plate Boundary (fig. 20.6a)-example?

Continent-Continent Convergent Plate Boundary (fig. 20.6c)-example?

Tranform Plate Boundaries mark the offset of spreading centers (fig. 20.7)

J. Tuzo Wilson: Father of Transform faults, Hotspots, and the Wilson Cycle.

The Wilson Cycle

Embryonic Stage ñ e.g.

Immature Stage - e.g.

Mature Stage - e.g.

Declining Stage - e.g.

Remnant Stage - e.g.

Suture Stage - e.g.

Evolution of a Mature Ocean

Embryonic Stage: volcanic and non-marine sediment deposited in rift valleys (fig. 20.17a)

Immature Stage: continued rifting creates new oceanic crust and a long, linear marine seaway. Cooling and subsidence of the rifted margin allow sediment to be deposited.(fig. 20.17b)

Carbonate platforms may develop on the rifted platforms as the land-derived sediment supply decreases (fig. 20.17c)

Mature Stage: A broad ocean develops and the continental margin continues to grow as sediment derived from erosion of the land is deposited (fig. 20.17d)

The Atlantic is a ìtypicalî mature ocean.

It is fringed by ìpassiveî continental margins such as those on the Atlantic and Gulf coast of the US.

Compare the continental margin of Texas with that of Oregon. Why are they different?

Q. What is happening along the margins of the Pacific Ocean and how does it differ from along the margins of the Atlantic Ocean?

A Remnant Ocean (fig. 20.20a)

Q. What is a Suture Zone? (fig. 20.20b)

Selected examples of present-day plate boundaries.

East African Rift System (fig. 20.4b)

The view from space (fig. 20.5a)

East African Triple Junction

East African Rift System

Active Volcano in the East African Rift System

The Great Rift Valley of East Africa (an "embryonic ocean")

Q. What type of faulting would you expect to occur in the Rift Valleys?

The Red Sea started out like the rift valleys, but now is an "immature ocean"

The Atlantic Ocean started out like the Red Sea, but now is a ìmature oceanî.

Most Divergent Plate Boundaries are located along the crests of Mid-Ocean Ridges

Mid-Atlantic Earthquake Epicenters

Mid-Atlantic Ridge System through Iceland (USGS)

Icelandic Rift Valley and Volcanism (USGS)

Bathymetry the Mid-Ocean Ridge (USGS)

Deep Sea Submersibles (USGS)

"Black Smokers" and Life at a Mid-Ocean Vent (USGS)

Q. What is ìChemosynthesisî and how does it differ from ìPhotosynthesisî. Why is it important?

The Gulf of California is another ìimmatureî ocean formed by sea-floor spreading (fig. 20.5b)

Q. The Pacific ocean is getting smaller every day. How is that possible?

Composite volcanoes along the Pacific "Ring of Fire" (USGS)

Andesitic volcanism is typically associated with oceanic trenches (e.g. the Japan Trench)

Some details of convergent margins (fig. 20.18)

The Andes: an example of an Ocean-Continent Convergent Margin. ( and home of the Andesite)

Evolution of an Andean-type Margin from a ìpassiveî margin to an ìactiveî margin.

How are the following related and where would you find them?

Trench

Accretionary Wedge

Forearc Basin

Magmatic Arc

Fig. 20.19

What does the Juan de Fuca Ridge have to do with the Cascade Range?

Origin of the Cascade Mountains

Mt. Ranier in the Cascades

Continued subduction and the formation of a Remnant Ocean (fig. 20.20a)

Eventually the ocean closed and continents collide! (fig. 20.20b)

Like India and Asia Ö..

To form Mount Everest, the Himalayas, and the Tibetan Plateau

Sometimes the collision is between a large continent and a smaller arc or fragment of a continent (micro-continent) ñ fig. 20.21a-b.

This forms an ìaccreted microplate terraneî (fig. 20.21c)

The accretion of such terranes has resulted in significant widening of the western margin of North America over the past 200 years (fig. 20.22)

Plate Tectonics in the Western United States

San Andreas Transform Fault System

Dead Sea Fault Zone

Transform faults with a "bend" can form:

Origin of the Dead Sea

Global Hot Spots (USGS)

Active basaltic volcanism in Hawaii (USGS)

Hawaiian Islands from Space (NASA)

Hawaiian Island Chain (USGS)

Hawaiian Hotspots (USGS)

Loihi: Hawaiiís Newest Volcano (USGS)

Pacific Hot Spot Tracks

Q. Where is the United Stateís other Hot Spot?