Tectonic Plates

plate tectonics

the theory that Earth’s lithosphere is broken into tectonic plates that move relative to one another, concentrating earthquakes, volcanoes, and mountain building at plate boundaries and explaining the geological activity observed on Earth's surface.

lithosphere

Earth’s rigid outer shell (crust plus uppermost mantle) that is broken into moving tectonic plates

Asthenosphere

the liquid rock layer beneath the lithosphere that flows slowly in circular “convection currents” which moves the tectonic plates above it

Continental drift

Wegener’s early idea that continents were once joined and have moved over time

Pangaea

the supercontinent that included all present-day continents and began breaking apart about 200 million years ago

seafloor spreading

process where new oceanic crust forms at mid-ocean ridges and moves outward; seafloor rock gets older with increasing distance from the ridge

mid-ocean ridge

an underwater mountain chain (spreading center) where plates diverge and magma rises to create new oceanic crust

magnetic striping

alternating, symmetrical magnetic patterns (“stripes”) in seafloor rocks on both sides of mid-ocean ridges, supporting seafloor spreading

oceanic crust

thin, dense crust that forms the ocean floor and commonly subducts beneath less dense plates at convergent boundaries

continental crust

thicker, less dense crust that forms continents and typically resists subduction compared with oceanic crust

mantle convection

heat-driven circulation in the mantle (hot rises, cool sinks) that helps drive plate motion

divergent boundary

a plate boundary where plates move apart, producing new crust, shallow earthquakes, and volcanic activity (e.g., mid-ocean ridges)

convergent boundary

a plate boundary where plates move toward each other, causing subduction (often with volcanoes and trenches) or continental collision (mountain building)

subduction zone

a convergent boundary where one plate slides beneath another into the mantle, producing deep trenches, strong earthquakes, and often volcanic arcs

transform boundary

a plate boundary where plates slide past each other, producing earthquakes with little volcanism (e.g., San Andreas Fault)

hot spot

a volcanic source formed by a mantle plume melting through the lithosphere away from plate boundaries; can create a chain of volcanoes as a plate moves

earthquake

a sudden release of energy when rocks break and slip along a fault after stress builds up and overcomes friction

epicenter

the point on Earth’s surface directly above where an earthquake originates; shaking generally weakens with distance from it

fault

a fracture in Earth’s crust along which rocks can slip, producing earthquakes when stress exceeds friction

liquefaction

a process where saturated soils lose strength and behave like a liquid during strong shaking, increasing earthquake damage

volcano

a landform/vent where magma reaches the surface; eruptions can create new land and hazards such as ash and pyroclastic flows

pyroclastic flow

a fast-moving current of hot gas and volcanic matter that flows down the slopes of a volcano during an explosive eruption