Chapter 12
The Lithosphere and Plate Tectonics
This chapter introduces the grand cycle of plate tectonics. This cycle explains how the
continents and ocean basins of the Earth’s surface slowly change as forces deep within
the Earth cause vast tectonic plates to converge, collide, split, and separate.

The solid Earth has a layered structure: a dense, metallic core surrounded by a
mantle of ultramafic rock which is covered by a crust with mafic rocks exposed on
the ocean floor and felsic rocks exposed on the continents.

Geologists use the term lithosphere to refer to the Earth’s outer layer of rigid, brittle
rocks which extend through the crust to the upper mantle. Beneath the lithosphere lies
the asthenosphere: the layer of soft, plastic rock in the mantle.

The history of the Earth can be subdivided into various time intervals using the
geologic time scale. Most of the landscape features of the Earth’s surface developed
during the Cenozoic Era which began sixty-five million years ago.

The major relief features of the Earth’s surface are the continents and ocean basins.

The continents contain young, dynamic alpine belts and old, stable continental
shields.

The ocean basins consist of extensive, smooth abyssal plains marked by long, narrow
midoceanic ridges.

Shallow continental shelves are found beneath the ocean next to continental shields,
while deep oceanic trenches are found adjacent to alpine belts.

The two basic tectonic processes are compression and extension.

The lithosphere is broken into six great plates and several lesser plates that move
relative to one another.

Spreading boundaries exist where plates move apart, converging boundaries where
they collide, and transform boundaries where they move past one another.

Spreading boundaries are marked by midoceanic ridges on the ocean floor and rift
valleys on continents. New ocean crust is formed along spreading boundaries.

Subduction occurs where continents meet the ocean floor along a converging
boundary and the denser rock of the ocean floor plunges beneath the continent. An
oceanic trench marks the subduction zone.

Subduction along continental margins leads to the formation of island arcs and
alpine belts as subducted ocean crust melts and rises to the surface in volcanoes, and
sediment from the ocean floor is folded and faulted as it accumulates on the
continental margin.

Plate movement is thought to be driven by convection currents in the plastic rock of
the asthenosphere.
The plate tectonics cycle ties together major relief features, volcanic and Earthquake
activity, and patterns of rock age and type at the Earth’s surface.