Plate Tectonics

Dr. Pamela Gore
Georgia Perimeter College

1. Explain the basics of the theory of plate tectonics.
2. Explain what is meant by the terms "lithosphere" and "asthenosphere", and the difference between them in terms of plate tectonics.
3. Discuss the major evidence in support of the theory of plate tectonics.
4. Explain what is meant by "seafloor spreading".
5. List the three types of plate boundaries and describe the type of motion involved at each.
6. Explain what Pangea was.
7. Describe what convection currents are, how they form, and how they move.
8. Explain how convection currents are related to plate tectonics.

The Theory of Plate Tectonics

Map of the Earth's tectonic plates from the US Geological Survey.
Credit: U. S. Geological Survey [http://pubs.usgs.gov/publications/graphics/Fig1.gif]

The lithosphere consists of the Earth's crust and part of the uppermost mantle. The lithosphere is divided into plates.

Asthenosphere is a partially molten part of the mantle. below the lithosphere.

The rates and directions of plate movements vary.

All plates are moving. As plate movement occurs, the plates sometimes stick together and then slip. This sudden slippage causes vibrations known as earthquakes.

Types of plate boundaries:

1. Divergent - where the plates are moving apart.

   Examples: mid-ocean ridges such as the Mid-Atlantic Ridge (the site of sea-floor spreading), and continental rifts such as the east African Rift system.

   
   Animation of divergent plate motion.

2. Convergent - where the plates are moving toward one another.

   Examples: subduction zones which occur at deep sea trenches such as the Marianas Trench, and sites of continental collision forming mountain belts, such as the Himalaya Mountains, the Ural Mountains, the Appalachian Mountains, and the Alps.

   
   Animation of convergent plate motion.

3. Transform - where the plates are sliding past one another, such as one sliding to the north and the adjacent plate sliding to the south.

   Examples: transform faults (easily seen where they cut at right angles to the mid-ocean ridges); includes the San Andreas fault.

   
   Animation of transform plate motion.

Evidence in support of the Theory of Plate Tectonics:

1. Shape of the coastlines - Africa vs. South America, for example

2. Faunal similarities

3. Fossil evidence
   1. Glossopteris flora - a type of Late Paleozoic seed ferns (plant fossils) that were found in Gondwanaland (India, Africa, Australia, S. America, Antarctica)
      
   2. Mesosaurus, a freshwater aquatic reptile whose fossils were found in South America and Africa

   
   As noted by Snider-Pellegrini and Wegener, the locations of certain fossil plants and animals on present-day, widely separated continents would form definite patterns (shown by the bands of colors), if the continents are rejoined. Diagram from the US Geological Survey, [http://pubs.usgs.gov/publications/graphics/Fig4.gif].

4. Rift Valleys of Africa - (evidence for a continent breaking up)

5. Geologic similarities between S. America and Africa
   
   1. Same stratigraphic sequence (i.e. same sequence of types of layered sedimentary rocks)
      
   2. Mountain belts and folded rocks would line up if you could push the continents back together

6. Paleoclimatic evidence (Paleo = "ancient", climatic = "climate")
   If you could push the continents back together, the ancient climatic zones, as indicated by the rock types, would match up.
   Layers of glacial deposits are found at same place in sequence of rocks
   Note directions of glacial ice movement as indicated by striations or grooves in the rock.

   Glaciers start to form on continents from the buildup of snow. As they grow through snow accumulation, they begin to move outwards. They do not form in the sea and move onto the land.

   Directions of glacial striations on the Gondwanaland continents only make sense if you could push the continents back together to form a single landmass in the past.
   The directions of glacial movement would not make sense in the absence of plate tectonics.

7. Youth of ocean basins and sea floor.
   Before the seafloor rocks were sampled and dated, the hypothesis is that they would be very old and would be covered by thick accumulations of sediment. The deep sea drilling efforts have revealed that the basaltic rocks of the seafloor are actually quite young. Seafloor basalt dates to less than 200 million years (most is younger than 150 million years). In addition, there is only a thin layer of sediment on the basalt.
   The seafloor is much younger than we expected. This is because new seafloor is continually forming at the mid-ocean ridges and spreading outward away from the ridge in conveyor belt fashion. The Earth is not expanding. Older seafloor rocks have been subducted, or carried down into the Earth's mantle in deep sea trenches and melted.

8. Evidence for subsidence in oceans -
   
   1. guyots - flat-topped sea mounts (erosion when at or above sea level)
   2. Chains of volcanic islands that are older away from site of current volcanic activity -
      Hawaiian Islands and Emperor Sea Mounts
      (also subsiding as they go away from site of current volcanic activity)

9. Mid-ocean ridges located near ocean centers
   1. High heat flow
   2. Seismic wave velocity decreases due to high temperatures
   3. Valley along center of ridge (graben)
   4. Volcanoes along ridge
   5. Earthquakes along ridge

10. Benioff Zones - inclined zone of earthquake foci (plural of focus) near deep sea trenches

11. Magnetic stripes on the sea floor

Contributions to plate tectonic theory from paleomagnetism

1. Recently magnetized rocks show alignment of magnetic field consistent with Earth's current magnetic field

2. Magnetization in older rocks has different orientations (as determined by magnetometer towed by ship)
   Can determine direction to north magnetic pole and distance to north magnetic pole from inclination and declination of magnetic field in the rock

3. Polar wandering curves
   Different polar wandering paths seen in rocks of different continents.
   Put continents "back together" and the polar wandering curves are superimposed (match up)

4. A test of the hypothesis of sea floor spreading (Vine and Matthews, 1963)
   Magnetic reversal "stripes" are SYMMETRICAL about the ridge.

5. Magnetic reversal time scale -
   Pattern of reversals in sea floor basalts matches known reversal time scale as determined from rocks exposed on land.
   Width of magnetic stripes on sea floor is related to time.
   (Wide stripes = long time; narrow stripes = short time)

In 1962, Harry Hess proposed the hypothesis that midocean ridges represent narrow zones where ocean crust forms.

Spreading centers

Mantle material moves upward, carrying heat.
Heat causes expansion and rising of sea floor.
Volcanism occurs (also earthquakes)

Convection in the mantle drives the system
Large scale thermal convection in the mantle.
Convection cells. Roughly circular.
Mantle heat probably due to radioactive decay

If the rising part of a convection cell is beneath a continent, it will cause it to RIFT apart.
Also causes seafloor to rift apart at mid-ocean ridge.

Continents move alont with ocean crust, and do not plow through it.

Many seemingly unrelated geologic facts are unified by the plate tectonic theory. At subduction zones, cold lithosphere descends into the asthenosphere in deep sea trenches
Associated with volcanoes and deep-focus earthquakes (over 190 miles deep).
"Ring of Fire" - around Pacific Rim.

Island Arcs = chains of volcanoes adjacent to subduction zones.

Hot spots - thermal plumes (heat rising in mantle).
Plates move over hot spots creating a chain of volcanoes.
Hawaiian Islands, Emperor Sea Mounts

Triple Junction - where three plate boundaries meet.
Associated with divergent plate boundaries.
Rift zones
Two plate boundaries tend to be most active, and one less active (failed rift)

Regions of continental rifting have:

1. Normal faults
2. Mafic dikes and sills (basalt - rich in Mg and Fe)
3. Thick continental sedimentary sequences in fault-block basins (normal faults)

Pangea

Diagram of five maps of the Earth showing Pangea and the positions of the continents as they split apart over time, from the US Geological Survey. According to the continental drift theory, the supercontinent Pangaea began to break up about 225-200 million years ago, eventually fragmenting into the continents as we know them today. Continental drift was the forerunner of the theory of plate tectonics.

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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Adapted July 22-23, 2000 from original web page created April 10, 1996. Modified February 10 and 24, 2001