Plate Tectonics: Boundaries, Stresses, and Earth's Layers

Theory of Plate Tectonics

The theory which states that pieces of Earth's lithosphere are in constant motion, driven by convection currents in the mantle.

Plates

The Earth's crust and upper mantle (Lithosphere) are broken into sections called plates.

Convection Currents

Currents in the mantle that move the plates as the core heats the slowly-flowing asthenosphere.

Plate Boundaries

The edges of Earth's plates meet at plate boundaries, which extend deep into the lithosphere.

Fault

Breaks in Earth's crust where rocks have slipped past each other.

Divergent Boundaries

A plate boundary where two plates move away from each other.

Convergent Boundaries

A type of plate boundary where two plates move towards each other.

Transform Boundaries

A type of plate boundary where two plates slide past each other.

Seafloor Spreading

The process that occurs at divergent boundaries where new oceanic crust is formed as plates move apart.

Normal Fault

A fault that occurs when rock drops down as it breaks due to tension stress.

Rift Valleys

Geologic features that may form on continents at divergent boundaries.

Mid-ocean ridges

Underwater mountain ranges formed by seafloor spreading at divergent boundaries.

Fissure volcanoes

Volcanoes that may form at divergent boundaries due to the movement of plates.

Stress of Tension

The stress that occurs when rock is pulled apart at divergent boundaries.

Asthenosphere

The elastic/plastic-like part of the mantle over which the lithosphere moves.

Lithosphere

The rigid outer layer of the Earth, consisting of the crust and upper mantle.

Rock thinning

The process that occurs in the middle of divergent boundaries as rock is pulled apart.

Rifting

The process of the lithosphere being pulled apart at divergent boundaries.

Divergent

A term describing the movement of plates away from each other.

Geologic features

Structures that form as a result of tectonic activity, such as rift valleys and mid-ocean ridges.

Compression

The stress that occurs at convergent boundaries when two plates collide.

Subduction Zone

The process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary.

Volcanoes

Formed at subduction zones where one plate sinks under another.

Trench

A subduction zone created when an ocean plate collides with a more dense ocean plate.

Collision Zones

Areas formed when a continental plate collides with another continental plate, leading to folded and thrust faulted mountains.

Folded Mountains

Mountains that may form at collision zones, such as the Himalayas or the Rockies.

Reverse Fault

A fault that occurs when rock is forced upward due to compression.

Shearing

The stress that occurs at transform boundaries when rock is pushed in two opposite directions.

Earthquakes

May occur at transform boundaries when rock snaps from pressure.

San Andreas Fault

A famous fault located at a transform boundary in California.

Strike-Slip Fault

A fault that occurs when rocks on each side slip past each other due to shearing.

Ocean Plate vs. Continental Plate

A type of convergent boundary where an ocean plate collides with a less dense continental plate.

Ocean Plate vs. Ocean Plate

A type of convergent boundary where an ocean plate collides with another ocean plate.

Continental Plate vs. Continental Plate

A type of convergent boundary where two continental plates collide.

Mountain Ranges

Formed from the collision of continental plates at convergent boundaries.

Shearing

means cutting

Crust

Outermost shell of a terrestrial planet, divided into older, thicker continental crust and younger, thinner oceanic crust.

Continental crust

About 50 - 100-km thick, composed of granitic rocks with a density of about 2.7 g/cm³, referred to as 'sial'.

Oceanic crust

About 5-10 km thick beneath the ocean floor, composed of basaltic rocks with a density of about 3.0 g/cm³, referred to as 'sima'.

Mantle

About 2,885 km thick, subdivided into the upper and lower mantle.

Core

Inner part of the Earth, consisting mostly of iron (Fe) and nickel (Ni), subdivided into the liquid outer core and the solid inner core.

Outer core

About 2,210-km thick, the only layer that is liquid, made up of molten nickel and iron.

Inner core

About 1,216-km thick, composed mostly of solid iron.

Crust temperature

Continental crust - 20 ºC to 600 ºC; Oceanic crust - 20 ºC to 1,200 ºC.

Mantle temperature

Upper mantle - 900 ºC; Lower mantle - 1,000 ºC to 3,700 ºC.

Outer core - 3,700 ºC to 4,300 ºC;
Inner core - about 6,000 ºC.

Core temperature

Heat source of Earth's interior

Heat accumulated from the radioactive decay of uranium, thorium, and other radioactive isotopes.

Mohorovicic discontinuity

Boundary between the crust and mantle.

Gutenberg discontinuity

Boundary between the mantle and outer core.

Lehmann discontinuity

Boundary between the liquid outer core and solid inner core.

100-km deep crust

1% of the Earth's mass.

Crust Mass

made of solid rocks and minerals.

Crust composition

2.7 g/cm³.

Density of continental crust

3.0 g/cm³.

Density of oceanic crust

900 ºC.

Temperature of the upper mantle is approximately

1,000 ºC to 3,700 ºC.

Temperature of the lower mantle ranges from