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Welcome
to GLG101C Introduction to Geology
Fall 2004
Professor James Tyburczy |
Department
of Geological Sciences |
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Chapter
2 Plate Tectonics |
Chapter
2 PLATE TECTONICS: The Unifying Theory
Plate
Tectonics - The Earth's surface is broken up into a small number of major plates
that move about on the surface. Major geological activity (volcanism, earthquakes,
mountain building) occurs at the boundaries between these plates. This concept
combines the ideas of Continental Drift and Sea Floor Spreading
Evidence
for Continental Drift - some of these ideas and observations had been around
since late 1800's
- Geographic
fit of continents
- Fossil
evidence-Distribution of various fossil types (for example, the distribution
of Glossopteris (a fern) and Mesosaurus (a fresh water reptile) fossils make
sense if continents were connected
- Rock
type and structural similarity - Appalachian rocks are similar to North African
Mts
- Paleoclimatic
evidence - distribution of Late Paleozoic, early Mesozoic glaciers (polar
regions) , coal (equatorial swamps), and evaporite deposits make sense if
continents were connected.
- Problem:
The mechanism was not understood initially - how could whole continents move!
Observations
of the sea floor leading to idea of Sea Floor Spreading
- Stem
from exploration of the sea floor, in part due to World War II
- Topography
of the ocean floor
- Deep
sea trenches
- Mid-ocean
ridges
- New
material (new lithosphere) is created at the mid-ocean ridges and lithosphere
is pushed back inside the earth and consumed at subduction zones (deep sea
trenches)
- Paleo
magnetism of continental rocks and apparent polar wander
- Marine
geomagnetic anomalies (magnetic stripes), magnetic reversals
- Age
of the ocean floor - 200 million years maximum (determined by anayzing magnetic
stripes and from analysis of deep sea drilling samples)
- One
of originators of the idea was Robert Dietz, who spent 15 years here at ASU
(the other originator was Harry Hess)
Combine
continental drift idea with seafloor spreading idea (mechanism)
to give modern Plate Tectonics - Earth's surface is covered by a small number
of plates, most geologic activity occurs at plate boundaries. New material (new
lithosphere) is created at the mid-ocean ridges and
Lithospheric
plates
- Lithospheric
plate or lithosphere - the major unit of plate tectonics, contains crust and
uppermost mantle, rigid, about 100 km thick
- Asthenosphere
- upper mantle below the lithosphere, plastic, flows
- Three
types of plate boundaries - divergent,
transform, and convergent plate boundaries
- Rate
of sea-floor spreading = rate of movement of plates (a few cm per year)
Divergent
plate boundaries
- Mid-ocean
ridge (spreading center) system - Gulf of California, Mid-Atlantic Ridge,
Iceland
- Continental
rift zones - East African Rift system
- Creation
of new lithosphere (new plate material)
Transform
plate boundaries - Transform faults
- Plates
slide past each other
- San
Andreas fault in California
Convergent
plate boundaries
- Ocean-ocean
convergence- island arc volcanoes, subduction zone, oceanic trench (examples,
Aleutian Arc in Alaska, southeast Pacific, Japan, west coast of South America)
- Ocean-continent
convergence - Continental volcanic arc, subduction of oceanic plate(West
coast of South American, Oregon & Washington Coast of North America)
- Continent-continent
convergence - no subduction, mountains pushed up by force and deformation,
example Himalayas
Determining
Past Plate Locations and Motions
Transform
faults that offset mid-ocean ridge indicate the directions of plate motion
Age of ocean
floor (isochrons) gives speed of plate motion in the geologic past
We can measure
present day motions in 'near real time' using the Global Positioning Satellite
(GPS) system
Plate Configurations
During
the late Paleozoic (280 million years ago) plates were joined together to form
a super continent - Pangaea.
Through
geologic and geomagnetic reconstructions, we can reconstruct
plate motions and continental configurations back almost 800 million years (early
supercontinent called 'Rodinia')
Tests,
Successes of Plate Tectonic Model
- Distribution
and depth of global seismic and volcanic activity - along plate margins
- Origins
and distribution of young mountain ranges
- Features
of the sea floor - mid-ocean ridges, oceanic trenches
- Earthquakes
on transform faults of mid-ocean ridge system - only occur on the part of
the fault between ridge segments
- Thickness
(age) of sediments on ocean floor, age of ocean floor
- Intraplate
('hotspot') volcanics - fixed beneath plates - not predicted by plate tectonics,
but used to determine plate velocities and directions in geologic past
Mechanism
of Plate Tectonics
- Temperatures
of earth's interior and surface heat flow indicate that the mantle must be
flowing or circulating (like a pot simmering soup on a stove) to release the
heat that is inside the earth. This circulation is called Mantle Convection
(remember - the mantle is solid and crystalline, but over geologic time it
can flow like a very thick fluid).
- Simplest
model of mantle convection - up at mid-ocean ridges, down at subduction zones
is too simple.
- Arguments
- does mantle convection occur in two distinct layers (top layer down to 700
km, and deepr layer from 700 to 2900 km depth) - called stratified convection,
or does the whole mantle (down to 2900 km deep) all circulate (whole mantle
convection)? Results from from seismology (Seismic tomography - detailed 3D
seismic images of Earth's interior) - suggest that subducted plates do penetrate
down in to the deepest mantle (but this result is preliminary).
Forces
acting on the plates
- Gravity
- slab pull (trench pull) and ridge push forces - probably dominate plate
motions - this means that the plates play an active role in determining plate
tectonic motions.
- Conveyor
belt forces (passively dragging plate along with mantle flow ) are probably
less important.
Mantle
plumes (hot spots, intraplate volcanoes, such as Hawaii) may come from very
deep in the mantle (maybe as deep as the core-mantle boundary). The tracks of
hot spots tell us the relative motions of the plates.
©2004, James A. Tyburczy, Department of Geological Sciences, Arizona
State University
If you have any questions or concerns regarding this page, please address
them to jim.tyburczy@asu.edu.
Be specific in your description of the problem!
Last update 8/27/2004
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