The Theory of Plate Tectonics: The Earth is composed of several lithospheric plates that float atop the asthenosphere and are in constant motion.
See "What is a Plate" web site
See "Understanding Plate Motions" web site
Refer to the diagram for the interior of the Earth.
Lithosphere: made up of the crust and upper mantle it is a rigid solid. The lithosphere material is divided into two types:
Oceanic: more dense, higher concentration of iron minerals
Continental: less dense, higher concentration silicate minerals
Asthenosphere: made up of the lower mantle, it is a "plastic solid" much like Play-Doh.
Mechanism for plate movement: mantle plumes and convection in the asthenosphere are believed to be the mechanism for plate movement. Convection is caused by heat transfer from the hot interior of the Earth outward.
The boundaries between adjacent plates have distinct features and geologic activity.
A) DIVERGENT BOUNDARY: where two plates move away from one another.
1. Ocean/Ocean: where tectonic boundary is in oceanic material. Characterized by a mid-ocean ridge with a rift down the center.
Features: undersea mountain range or ridge system with rift in center
Geologic effects: shallow focus earthquakes, volcanism
Example: The Mid-Atlantic Ridge
2. Continent/Continent: where tectonic boundary is in continental material. Characterized by a rift zone.
Features: Volcanic mountain range split by a rift valley, basalt flows
Geologic effects: shallow focus earthquakes, volcanic activity
Example: East African Rift
B) CONVERGENT BOUNDARY: where two plates move toward one another.
1. Ocean/Ocean: where tectonic boundary is in oceanic material. One plate will SUBDUCT or sink back into the asthenosphere.
Features: deep ocean trench, volcanic island arc
Geologic effects: deep focus earthquakes, subduction of one plate and explosive volcanism
Example: Philippines
2. Ocean/Continent: where tectonic boundary is between oceanic and continental material. Characterized by subduction of the OCEAN plate.
Features: deep ocean trench adjacent to shoreline, volcanic mountain range inland from the coast
Geologic effects: deep focus earthquakes, subduction of the ocean plate, explosive volcanism
Example: Andes Mountains, Cascade Range
3. Continent/Continent: where tectonic boundary is in continental material.
Features: large mountains
Geologic effects: extensive earthquake activity, NO subduction, NO volcanism, highly deformed, folded and metamorphosed rock units
Example: Himalayas
C) TRANSFORM BOUNDARY: where tectonic plates move past one another. Not differentiated by plate material.
Transform Boundary: two tectonic plates move past one another.
Features: usually none, occasionally an earthquake scar
Geologic effects: extensive earthquake activity
Example: San Andreas Fault
Evidence that Supports the Theory of Plate Tectonics
1: Fit of the Continents
2: Paleomagnetism across a spreading center
3: Age of the rock units across a spreading center
4: Fossil evidence
5: Continuity of rock units across ocean basins
6: Continuity of geographic features across ocean basins
7: Paleoclimate
8: Turtle Migration
The theory of plate tectonics can be used as a model for the Earth providing an explanation for as well as prediction about geologic phenomenon. Plate tectonics can also be used to try and predict where important economic resources may be located.
