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 coerce vast tectonic plates to converge, collide, split, and separate.

The solid Earth has a layered structure: a dense, metallic core (a solid inner core
immersed in a molten outer core) surrounded by a mantle of deformable ultramafic
rock which is covered by a brittle crust composed of ocean floor mafic rocks and
continental felsic rocks.

Geologists use the term lithosphere to refer to the Earth’s outer layer of rigid, brittle
rocks which extend from the upper mantle to the surface crust. Beneath the
lithosphere lies the asthenosphere, a layer of soft, plastic mantle rock.

The history of the Earth can be subdivided into various time intervals using the
geologic time scale. Precambrian time includes crustal rocks that range in age
between 4.6 billion years to 570 million years. The Paleozoic, Mesozoic and
Cenozoic Eras include crustal rocks that range in age from 570 to 245 million years,
245 to 66 million years and 66 million years to present, respectively.

Most of the landscape features of the Earth’s surface developed during the Cenozoic
Era.

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
and 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 nine 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 Wilson Cycle, named in honour of the Canadian geophysist J. Tuzo Wison,
accounts for the observed logical time sequence of plate tectonic events.
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.