Four Basic Concepts of the Theory of Plate Tectonics

The entire theory can be reduced to Four basic concepts really only three, since the fourth is the third turned inside out and sdrawkcab! backwards!

Concept 1: Plates are formed from the lithosphere Layers Defined by Physical Properties Lithosphere rigid (breaks when stress is applied) forms the plates. Asthenosphere plastic (flows when stress applied) allows the plates to glide horizontally.

Layers Defined by Composition Crust continental: thicker, less dense. oceanic: thinner, denser. Mantle

Model showing Earth’s inferred internal structure.

Concept 2: Plates move in one of three ways relative to each other. Divergent motion Convergent motion Transform motion It’s as simple as putting your hands side by side, palms down. Pull them apart. Move them toward one another. Slide one hand past the other. Descriptions of each type follow.

Divergent Boundary

Convergent boundary

Transform Boundary

Plates and continents are not the same!

Concept 3: Most internally-driven geologic activity occurs at or near plate boundaries. Divergent boundary Description Associated geologic phenomena Examples

Most divergent boundaries are located along the crests of oceanic ridges

Rising magma forces the crust apart As the crust is pulled apart, large slabs of rock sink producing a rift zone.

Eventually, an expansive ocean basin and ridge system are created. Further spreading generates a narrow sea. Eventually, an expansive ocean basin and ridge system are created.

East African rift valleys and associated features.

Convergent boundary Description Associated geologic phenomena: details depend on type (below)

Convergent boundary (continued) Variations: there are only three ways that two entities can relate to each other….if the entities are ocean lithosphere and continental lithosphere, what are these three combinations? Answer: ocean to continent ocean to ocean continent to continent

Convergent boundary (continued) The details of geologic phenomena depend on the specific type of convergent boundary. Ocean to continent: Explosive volcanoes Earthquakes from shallow to deepest. Folded/faulted mountain building

earthquakes from shallow to deepest. no folded/faulted mountains. Convergent boundary (continued) Ocean to ocean: explosive volcanoes earthquakes from shallow to deepest. no folded/faulted mountains.

continent to continent: no volcanism Convergent boundary (continued) continent to continent: no volcanism earthquakes of variable depth but scattered. folded/faulted mountain building

Ocean - continent convergent boundary

Cascade volcanoes form near a ocean - continent convergent boundary.

Ocean - ocean convergent boundary

Continent - continent convergent boundary

Head-on collision of India and Asia produced the Himalayas Mountains.

Test yourself: Which types of plate boundaries are suggested by oceanic ridges? By trenches?

Which types of plate boundaries are suggested by only shallow-focus earthquakes? By deep-focus quakes?

Transform boundary Description Associated geologic phenomena Examples

Transform fault boundary, in this case offsetting a divergent boundary.

Some West Coast transform faults.

Concept 4: Interiors of plates are relatively quiet geologically Exceptions exist, including… Intra-plate earthquakes Hot spots Volcanic activity Evidence for plate motion

Hot Spot Tracks... Magma rises from mantle through the lithosphere and erupts to form a volcano. Volcanic activity only occurs at the site above the hot spot.

Hawaiian Islands: older to northwest, younger to southeast

Hot Spot Tracks... As the plate moves over the hot spot the volcano moves off the hot spot and goes extinct.

Of course, externally-driven geological activity occurs away from plate boundaries! Weathering Erosion by streams by glaciers by wind directly by gravity Deposition of sediment (delta, floodplain, etc. Flooding Storms

How this applies to your class: Understanding where earthquakes occur most commonly is closely related to knowledge of plate tectonic theory. Also, knowing the particular type of plate boundary will help you understand if an earthquake is likely to be shallow and if it might be very large.

How this applies to your class: Understanding where volcanic eruptions occur and knowing about the specific type of eruption and hence specific volcanic hazards is closely related to knowledge of plate boundaries.

How this applies to your class: Understanding where some mineral deposits are formed is related to plate tectonics.

How this applies to your class: Recognize that geologic hazards related to surface processes differential solar heating and wind gravity can occur any place conditions are favorable for them, including in the middle of a lithospheric plate!

The End