Plate Tectonics Glossary

Short definitions of the 52 terms used on our plate tectonics, Pangea and ocean current maps. Each one links to its source and to a map that shows it.

Last updated 11 October 2026

B

Bathymetry: The study of the floors of the ocean, lakes and rivers: how deep the water is and the shape of the bottom. Ocean floor map. Source: NOAA
Biome: A large group of ecoregions with similar vegetation and climate, such as tundra or tropical moist forest. RESOLVE uses 14 land biomes. Biomes map. Source: Bird (2003)

C

Caldera: A large basin left when a volcano collapses into its emptied magma chamber, such as Crater Lake in Oregon. Volcano map. Source: USGS
Cinder cone: The simplest volcano: a small, steep cone of loose lava fragments around a single vent, rarely more than about 1,000 feet high. Volcano map. Source: USGS
Cold current: A current that flows toward the equator, mostly along the west coasts of continents, and brings cool water from the polar regions. Ocean currents map. Source: NOAA
Convergent boundary: Plates move toward each other and crust is destroyed as one plate dives under another. Convergent boundaries. Source: USGS
Coriolis effect: Earth's rotation turns moving water and air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Ocean gyres. Source: NOAA
Country borders: Today's country outlines from Natural Earth, drawn with India's point of view on disputed borders. Tectonic plates map. Source: Source

D

Divergent boundary: Plates pull away from each other and new crust forms between them. Divergent boundaries. Source: USGS

E

Ecoregion: An area of land with a distinct set of natural communities and species. The RESOLVE 2017 map has 846 of them. Biomes map. Source: Bird (2003)
Epicenter: The point on the surface directly above the focus, the place underground where an earthquake rupture starts. Earthquake map. Source: USGS

G

Gondwana: Gondwanaland, the southern of the two landmasses Pangaea first broke into, as proposed by Alexander Du Toit. Pangea map. Source: USGS
Gyre: A large system of rotating ocean currents. There are five major gyres: the North and South Pacific, the North and South Atlantic, and the Indian Ocean. Ocean currents map. Source: NOAA

H

Hotspot: A small, long-lived, very hot region under a plate that feeds volcanoes far from plate edges, such as Hawaii. As the plate moves over it, a chain of volcanoes forms. Hotspot volcanoes. Source: USGS

K

Köppen-Geiger classification: A climate classification that sorts places into 5 main groups and 30 classes by monthly temperature and rainfall and how they change over the year. Climate zones map. Source: Bird (2003)

L

Latitude: Distance north or south of the equator, in degrees. Lines of latitude are always the same distance apart, about 111 km per degree. Latitude and longitude map. Source: NOAA
Laurasia: The northern of the two landmasses Pangaea first broke into, as proposed by Alexander Du Toit. Pangea map. Source: USGS
Lava dome: A rounded mound of thick, sticky lava that piles up over its vent instead of flowing away. Volcano map. Source: USGS
Longitude: Distance east or west of the prime meridian through Greenwich, England, in degrees from 0 to 180. Meridians run from pole to pole. Latitude and longitude map. Source: NOAA

M

Ma: Million years ago. Ma stands for megaannum, one million years. Pangea map. Source: USGS
Mantle convection: Slow circulation of hot mantle rock, heated by radioactive decay and heat left over from Earth formation. Why plates move. Source: USGS
Microplate: A small fragment of a plate, common in wide plate boundary zones such as the Mediterranean-Alpine region. How many tectonic plates. Source: USGS
Mid-ocean ridge: An underwater mountain range that forms along divergent plate boundaries, where new ocean floor is made. Mid-ocean ridge map. Source: NOAA
Mountain building: The folding and uplift of crust into mountain ranges, mostly where plates converge and one plate is pushed into or under another. Mountain ranges map. Source: USGS

N

Neighbors: Plates that share a boundary with this plate. Most plate movement happens along these narrow zones between plates. List of tectonic plates. Source: USGS

O

Ocean trench: A long, narrow trough in the sea floor, 8 to 10 km deep, where one plate bends down under another. Subduction zones map. Source: USGS

P

Paleomagnetism: The record of Earth's ancient magnetic field kept in magnetic minerals, such as magnetite, in rocks. Continental drift evidence. Source: USGS
Panthalassa: The one ocean that surrounded Pangaea about 250 million years ago. Pangea map. Source: NOAA
Plate boundary zone: A broad belt where the edges of plates are not clearly defined and the boundary is spread across many small blocks. Plate boundaries map. Source: USGS
Plate colors: The Earth's outer layer is broken into moving plates that carry the continents and oceans. Each plate gets one color, and no two neighbors share a color. Tectonic plates map. Source: USGS
Plate model: A set of plate outlines and rotations that moves features on each plate back to where they were at a past age. Continental drift map. Source: Müller et al. (2019)

Q

Quaternary fault: A fault with evidence of surface movement during the Quaternary, the past 1.6 million years, as mapped in the USGS Quaternary Fault and Fold Database. Fault lines map. Source: Bird (2003)

R

Relative plate velocity: How fast one plate moves compared with its neighbor across a boundary, in millimeters per year. Plate motion map. Source: Bird (2003)
Ridge push: The push on a plate from new magma rising and spreading at a mid-ocean ridge. Why plates move. Source: USGS
Rift: A valley where the crust is being pulled apart, such as the East African Rift, which may in time become a new ocean. Mid-ocean ridge map. Source: USGS
Ring of Fire: The belt of volcanic arcs and ocean trenches that partly rings the Pacific Ocean, where earthquakes and eruptions are frequent. Ring of Fire map. Source: USGS
Rodinia: A supercontinent that assembled about 1,000 to 870 million years ago and broke apart about 870 to 650 million years ago. Rodinia map. Source: Müller et al. (2022)

S

Seafloor spreading: New ocean crust forms at the crest of a mid-ocean ridge and moves away from it, so the sea floor gets older away from the ridge. Continental drift evidence. Source: USGS
Shield volcano: A broad, gently sloping volcano built almost entirely of runny lava flows. Mauna Loa in Hawaii is the largest. Volcano map. Source: USGS
Slab pull: The pull of a cold, heavy plate edge sinking into the mantle at a subduction zone, now seen as the main driving force. Why plates move. Source: USGS
Stratovolcano: A tall, steep composite volcano built of layers of lava, ash and cinders, such as Mount Fuji or Mount St. Helens. Volcano map. Source: USGS
Subduction zone: Where one plate sinks under another and back into the mantle, usually marked by a deep ocean trench. Subduction zones map. Source: USGS
Supercontinent: A large landmass made of most or all of the continents joined together, such as Pangea. Supercontinents. Source: USGS

T

Tethys: The Tethys Sea, a vast ocean that separated India from Asia about 225 million years ago. It closed as India moved north. Pangea map. Source: USGS
Thermohaline circulation: Deep ocean flow driven by differences in water density, which depend on temperature (thermo) and salt (haline). Also called the global conveyor belt. What are ocean currents. Source: NOAA
Transform boundary: Plates slide horizontally past each other. Crust is neither made nor destroyed. Transform boundaries. Source: USGS
Tsunami: A series of sea waves set off by an earthquake, an undersea landslide or, less often, an island volcano. Not related to tides. Cascadia subduction zone map. Source: USGS

U

Upwelling: Wind pushes surface water away and cold, nutrient-rich water rises from below to replace it. What are ocean currents. Source: NOAA
UTC offset: How many hours and minutes local standard time is ahead of or behind Coordinated Universal Time, the time at the prime meridian. Earth turns 15 degrees of longitude per hour. World time zone map. Source: NOAA

V

Volcanic arc: A curved chain of volcanoes that runs parallel to an ocean trench, always on the landward side, fed by the plate sinking below it. Where is the Ring of Fire. Source: USGS

W

Warm current: A current that flows from the equator toward the poles, mostly along the east coasts of continents, and carries warm tropical water. Ocean currents map. Source: NOAA
Western boundary current: The strong, narrow, warm current on the western side of a gyre, such as the Gulf Stream. The eastern side has a weak, broad current. Gulf Stream map. Source: NOAA