Plate tectonics theory

Plate tectonics theory
• Earth’s lithosphere* broken into huge slabs or “plates”
* lithosphere = crust and upper most mantle
• Plates are moving
• boundaries classified as divergent, convergent, transform
• Plates move as coherent units
•strong and rigid internally, major stress/deformation on edges
• deformation includes EQ’s, volcanoes, mountains
• Not all evidence supports plates tectonics theory, ex., “hot spots”

How did the earth’ s plates
move over geologic time?
See these links for the answer:
http://www.ucmp.berkeley.edu/geology/tectonics.html
http://www.odsn.de/odsn/services/paleomap/animation.html

Explaining plate movement
• continental drift “theory” (really a hypothesis)
• presented evidence that plates had moved, but could
not explain how.
•Theory lacked a “mechanism” for plate movement.
• modern plate tectonic theory
• provided mechanism (how) and driving force (why)

Continental Drift Theory / Hypothesis

Other evidence
discussed in text
Continental drift was only one
hypothesis proposed to
explain fossil evidence

Chemically
1. Crust
2. Mantle
3. Core
Physically
1. Lithosphere
2. Asthenosphere
3. Mesosphere

A simple lab model of the earth’s lithosphere

ρ Styrofoam

1.25”
Styrofoam
ρ

Mountains
60-90
miles
~150
miles
Continental
Lithosphere
Ocean Floor
Oceanic
Lithosphere
3 - 60 miles
Asthenosphere

Crust is thin upper “skin” of lithosphere
Mountains
Continental Crust
Asthenosphere
Oceanic
crust

Continental crust is thicker than oceanic crust
Mountains
Continental Crust 40 miles
25 miles
Ocean
Floor
Asthenosphere
5 miles
thick
Oceanic
crust

Continental crust is lighter (less dense)
than oceanic crust
Continental Crust
ρ = 2.7 g/cc
Asthenosphere
ρ = 3.3 g/cc
Oceanic crust
ρ = 3.0 g/cc

Summary: Two types of crust:
Continental
• thicker (25-70 miles), less dense
• rides over top of oceanic crust at convergent boundaries
• different composition than oceanic crust
• Oceanic
• thin (5 miles), more dense
• sinks below continental crust at convergent boundaries
• different composition than cont. crust

Continental margins:
transitions between continental and oceanic crust
Continental Crust
Continental
Margin
Oceanic crust
Asthenosphere

Passive continental margins:
Oceanic and continental crust on same plate,
firmly joined, moving together.
Continental Crust
Continental
Margin
Oceanic crust
Asthenosphere

Passive continental margins:
Oceanic and continental crust on same plate,
firmly joined, moving together
Atlantic Ocean bordered
by two passive margins

Active continental margins:
Oceanic crust and continental on separate plates,
Not joined, and moving relative to each other.
Continental Crust
Continental
Margin
Oceanic crust
Asthenosphere

Active continental margins:
If movement is convergent …
Continental Crust
Continental
Margin
Oceanic crust
Asthenosphere

Subduction ! (C-O)
Causes earthquakes, volcanoes, mountain building

Active continental margins:
Oceanic and continental crust on separate plates,
not joined, and moving relative to each other.

Subduction ! (O-O)
Causes earthquakes, volcanoes, mountain building

Now know of three types of motion at plate boundaries
and two types of crust:
• Convergent
• Divergent
• O-O, C-O, C-C
• O-O, C-C
C = Continental crust
O = Oceanic crust
• Transform
• C-C, O-O, C-O

An oceanic-continental convergent
plate boundary (C-O convergent)
Melting of subducting plate
“destructive boundary”

An oceanic-oceanic convergent plate
boundary (O-O convergent)

A continental-continental convergent
plate boundary (C-C convergent)

Divergent boundaries are located mainly
along oceanic ridges (O-O divergent)
“seafloor spreading”
Creation of new plate
“constructive boundary”

The East African rift – a
divergent boundary on land
(C-C divergent)

A continental-oceanic transform plate
boundary (C-O transform)

Heat flow: driving mechanism
behind plate tectonics
Sources:
• Residual heat
• Radioactive heat
on ocean floor

Testing Plate tectonics model
• Paleomagnetic field patterns
on ocean floor
• Hot spots (e.g, Hawaii)