1
Plate Tectonics
2
Continental Drift
In 1912, Alfred Lothar Wegener (1880-1930), a German meteorologist,
published the idea of continental drift. Wegener proposed that all of the
continents were once joined together as one supercontinent that he called
Pangea. He also compiled a great deal of evidence to support his
hypothesis, including the distribution of fossils and mountain ranges.
• Introduction
• Predecessor of plate tectonics: Continental drift hypothesis
• What is the theory of plate tectonics?
• What is a plate?
• How do we find their edges?
• How many plates are there?
• Why is plate tectonics important?
• Types of relative plate movements (and related stresses)
• Types of plate boundaries
• Plate boundaries and earthquake depths
• Measuring plate motions
• Why do plates move?
This Dynamic Earth - USGS
3
Fossil evidence for once
joined continents
Later studies have allowed the positions of the continents at different
times in Earth history to be determined, and have documented the
breakup of Pangea.
4
This Dynamic Earth - USGS
This Dynamic Earth - USGS
Plate Tectonics
Today, we know that it isn’t just the continents that have moved.
Rather, the Earth’s surface is broken into pieces of continental and/or
ocean crust called plates. Plate tectonics deals with the nature of these
plates, what happens at their boundaries, how and why they move, etc.
5
6
Plate tectonics also
explains the locations of
earthquake faults,
volcanoes, and major
mountain ranges.
Photos from: This Dynamic Earth
(USGS) and Understanding Earth
Fig 20.3 - Understanding Earth
1
Active continental margins (plate boundary)
vs.
Passive continental margins (no plate boundary)
7
Zones of progressively deeper earthquakes revealed the existence of 8
subduction. This led to an understanding of how ocean crust is recycled
at ocean trenches.
The western margin of North America is an active area with earthquakes
(yellow) and volcanic activity (red). In contrast, the eastern margin of North
America is quiet with no active volcanoes and few earthquakes. The eastern
margin is passive. We now know that the active zones coincide with the
boundaries between plates (blue).
Geology 2nd ed. Chernicoff
USGS
The discovery of extensive ocean ridges and young ages for the ocean
floor (in the 1940s and 1950s) provided another important clue.
9
10
What is a plate? – The brittle outer part of the earth (crust and uppermost
mantle) is called the lithosphere. The lithosphere is broken into plates that move on
the asthenosphere, a part of the mantle which is plastic (able to flow).
Fig 17.27 Understanding Earth
This Dynamic Earth - USGS
Types of relative plate movements and boundaries
and related stresses and fault types
11
12
Compression
Extension
Reverse and
thrust faults
Normal faults
Folding
Stretching and
thinning
Shear
Strike-slip faults
Shearing
This Dynamic Earth - USGS
2
The mid-ocean ridge (shown in red) winds its way between the
continents much like the seam on a baseball This Dynamic Earth - USGS
13
14
Fig 1.15 - Understanding Earth
This Dynamic Earth - USGS
Mid-Atlantic
Ridge at Iceland
15
16
Continental Rifting
Africa
East African Rift
SOUTH
Gulf of Aden
Red Sea
East African Rift
Continental rifting:
The Afar Triangle
Photo from Space Shuttle:
STS061-079-024 - NASA
This Dynamic Earth - USGS
Fig 20.3 - Understanding Earth
Next: 20-Plate Divergence.mov
(Red Sea rift)
17
Geology 2nd ed. - Chernicoff (originally from Bullard,
1969, The Origin of the Continents: Scientific American)
18
Transform Boundaries
This Dynamic Earth - USGS
3
Transform plate boundary
San Andreas fault, California
19
20
Ocean-ocean convergent boundary
This Dynamic Earth - USGS
Fig 1.17 - Understanding Earth
21
22
Ocean-continent convergent boundary
This Dynamic Earth - USGS
Japan, an example of an
oceanic island arc
Photo from Space Shuttle:
STS059-218-044 - NASA
23
Intrusive igneous rocks formed along an ancient volcanic arc: Part of the
Sierra-Nevada batholith in Yosemite National Park
24
Continent-continent convergent boundary
This Dynamic Earth - USGS
Next: 20-Ocean-Continent.mov (Andean subduction zone)
4
25
Formation of the highest
mountains on earth by
continent-continent
collision
26
Plate boundaries and earthquake depths
All types of plate boundaries can produce shallow earthquakes.
The deepest earthquakes (and some of the largest earthquakes) occur in
subduction zones.
This Dynamic Earth - USGS
Next: 20-Continent-Continent.mov
(India-Asia collision zone)
Measuring plate motions using hot spots
27
Fig 20.11 - Understanding Earth
28
Hotspots and plate movements
Geology 2nd ed.
- Chernicoff
Map of part of the Pacific ocean floor showing the 6,000-km-long Hawaiian
Ridge-Emperor Seamounts chain – This volcanic trail of the Hawaiian
“hotspot” could be related to a mantle plume
This Dynamic Earth - USGS
Ages of the ocean crust
Fig. 10-16 Chernicoff - Geology
Hot spots and the opening of the south Atlantic Ocean
http://earth.leeds.ac.uk/dynamicearth/plates_move/hotspots/index.htm
29
30
A GPS ground receiver -- here set up on the flank of Augustine Volcano
(Cook Inlet, Alaska) -- recording the signals sent by four or more of the
orbiting GPS satellites. Artist's conception of a Global Positioning
System satellite in orbit also shown. Source: USGS
5
31
32
Major plate boundaries and their rates of movement
Fig 20.12 - Understanding Earth
Projected future positions of the continents - 100 million years from now: If
current plate movements continue unchanged, part of California will become an
island off the coast of Washington, the Mediterranean Sea may close, and eastern
Africa may become an island. Geology 2nd ed. - Chernicoff
33
34
Plate Tectonics:
Why do plates move?
The general scientific consensus is
that plate movements are related
to convection within the Earth’s
mantle, although the direction and
speed of a moving plate doesn
doesn’tt
always equal the movement in the
mantle beneath it.
The convection results from the
escape of heat from the Earth’s
interior and involves the pull of
gravity on rocks of different
densities.
Possible mechanisms
to drive plate motions
Fig 20.25 - Understanding Earth
Fig 1.13 - Understanding Earth
Slab pull
- cold, relatively dense lithosphere is pulled downward at subduction
zones and pulls the rest of the plate along behind it
35
36
Ridge push
- sliding down slope away from the ridge which results in tension at the
rift valley and also pushes the rest of the plates apart
- the tension allows molten rock to rise toward the surface and form new
ocean crust
Lithosphere
Rift
Valley
Oceanic crust
partial
melting
upwelling
Asthenosphere
Image source: This Dynamic Earth – USGS
http://pubs.usgs.gov/publications/text/dynamic.html
6
37
This Dynamic Earth - U.S. Geological Survey
http://pubs.usgs.gov/publications/text/dynamic.html
Paleomap Project - Continent locations of the past, present and future.
http://www.scotese.com/
Plate Tectonics - U.S.G.S. Cascades Volcano Observatory
http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/framework.html
What is Plate Tectonics? – USGS and National Park Service
http://www2.nature.nps.gov/geology/usgsnps/pltec/pltec1.html
Plate Tectonics Animations – USGS and National Park Service
http://www2.nature.nps.gov/geology/usgsnps/animate/pltecan.html
7