The Structure of the Earth The planet Earth is composed of a number of concentric layers. At the centre is the core (a solid inner core and a liquid outer.

The Structure of the Earth

The planet Earth is composed of a number of concentric layers. At the centre is the core (a solid inner core and a liquid outer core). Surrounding the core is the mantle (similar to mafic minerals like olivine and the ultramafic peridotite). The outer most layer is the crust (composed of the continental crust and oceanic crust).

The Lithosphere

Geologists use the term lithosphere to mean an outer Earth zone, or shell, of rigid, brittle rock. It includes not only the crust, but also the cooler, upper part of the mantle that is composed of brittle rock.

The rigid, brittle lithosphere rests on top of a soft, plastic underlayer named the asthenosphere.

The lithospheric shell consists of large pieces called lithospheric plates.

The Lithosphere

(a) Idealized cross section of the Earth's crust and upper mantle. (b) Details of the crust and mantle at the edge of a continent, including the types of rocks found there. Also shown are the lithosphere and asthenosphere.

Continents and Ocean Basins

The major relief features of the Earth are the continents and ocean basins. The continental masses consist of two basic subdivisions: (1) active belts of mountain-making, and (2) inactive regions of old, stable rock.

Continents and Ocean Basins

Principal mountain arcs, island arcs, and trenches of the world and the midoceanic ridge.

The Ocean Basin

A typical ocean basin is characterized by a central ridge structure that divides the basin in about half. The midoceanic ridge consists of submarine hills that rise gradually to a rugged central zone.

The continental margins are narrow zones that separate ocean crust from continental crust. From the ocean to the land it is composed of the continental rise, then the continental slope, then the continental shelf.

The Ocean Basin

This block diagram shows an inner wedge of sediments beneath the continental shelf and an outer wedge of deep-sea sediments beneath the continental rise and abyssal plain.

This schematic block diagram shows the main features of ocean basins. It applies particularly well to the North and South Atlantic oceans.

Plate Tectonics

The study of the motion of lithospheric plates and their interactions at their boundaries is plate tectonics. Tectonics refers to the breaking and bending of the entire lithosphere, including the crust.

*See the Animation on Plate Tectonics found on WileyPLUS or your text’s website.

Plate Tectonics

Diagram (a) is greatly exaggerated in vertical scale, and emphasizes surface and crustal features. Diagram (b) is drawn to true scale. Here the actual relationships between lithospheric plates can be examined, but surface features are too small to be shown.

The Lithospheric Plates

The rigid outer lithosphere is divided into a number of lithospheric plates of various shapes and sizes. The boundaries also differ according to whether plates are coming together (converging), moving apart (spreading), or moving alongside each other (transforming).

To explore the lithospheric plates further, see the Animated 3D Globe found on WileyPLUS or your text’s website.

Tectonic Plate Boundary Relationships

There are three major kinds of active plate boundaries:

1. Spreading boundaries: New lithosphere is being formed by accretion.

2. Converging boundaries: Subduction is in progress, and lithosphere is being consumed.

3. Transform boundaries: Plates are gliding past one another on a transform fault.

*See the Animation on Tectonic Plate Boundary Relationships found on WileyPLUS or your text’s website.

Continents of the Past

The distribution of continents was very different in the past than it is today. A single supercontinent, named Pangea, existed about 250 million years ago. It split up over time, and as the continents drifted we arrived at today’s pattern of land and sea.

*See the Animation on Continents of the Past found on WileyPLUS or your text’s website.