CONTINENTAL ACCRETION

Continents can grow when fragments of crust, such as island arcs or small continental blocks, collide with their edges. These fragments can become attached as tectonic plates converge, a process called continental accretion.

HOT-SPOT VOLCANISM

Some volcanoes form above unusually hot mantle regions called hot spots. Rising mantle material can partially melt, supplying magma that reaches the surface. As a tectonic plate moves over a long-lived hot spot, successive volcanoes can form a chain, as in Hawaii. 

DIVERGENT BOUNDARIES

At divergent boundaries, tectonic plates move apart. On land, stretching can create a rift valley, while in oceans, spreading builds oceanic crust along mid-ocean ridges. Over time, rifting can split a continent and open a new ocean basin.

TRANSFORM BOUNDARIES

At transform boundaries, tectonic plates slide horizontally past one another. This side-to-side movement occurs along strike-slip faults, where rocks on opposite sides move mainly parallel to the fault and built-up stress can be released as earthquakes.

CONVERGENT BOUNDARIES

At convergent boundaries, tectonic plates move toward one another. Dense oceanic crust can descend beneath another plate in a process called subduction, creating earthquakes and volcanoes. When buoyant continental crust collides, it can shorten and thicken into mountain ranges over millions of years.

IMPACT EVENTS

Fast-moving asteroids or comets can excavate huge craters when they strike a planet. A raised rim and minerals altered by extreme shock pressure can help geologists identify an ancient impact site.

ICE AGES

During an ice age, large ice sheets persist on land, expanding during colder glacial periods and retreating during warmer interglacial periods. As glaciers move, they erode the landscape, carving valleys and reshaping mountains. Growing ice sheets store water on land and lower global sea level; when that land ice melts, sea level rises again.

SUPERCONTINENTS

A supercontinent forms when plate motions bring most of Earth’s continental land together. Matching rocks, fossils, and mountain belts on separate continents preserve evidence of those former connections. Over geologic time, continents have repeatedly assembled and broken apart.