Essential of Geology , 2 ed.

(2.1) Basic Plate Boundaries

Geologists define three types of plate boundary, based simply on the relative motions of the plates on either side of the boundary. These basic types-divergent, convergent, and transform plate boundaries-are shown in the following three-part animation.

Created by: Stephen Marshak

(2.2) What a Geologist Sees: Plate Boundaries

A photo from space shows the Sinai Peninsula, separated from Egypt to the west and the Arabian Peninsula to the east by rifts, narrow belts where the crust has stretched and broken apart. A geologist’s sketch highlights the plate boundaries.

(2.3) Formation of Ocean Crust

Magma (molten rock) forms in the asthenosphere beneath mid-ocean ridges. The magma rises into a magma chamber in the crust. As sea-floor spreading takes place, some of the magma cools slowly along the margins of the magma chamber to form gabbro (a coarse-grained igneous rock), while some intrudes upward to fill vertical cracks that form as new crust splits apart. Magma that cools in the cracks creates basalt dikes. Basalt is a fine-grained igneous rock, and a dike is a wall-shaped intrusion. Finally, some magma that makes it to the sea floor and extrudes as pillow basalt.

Created by: Stephen Marshak

(2.4) Transform Faulting

This animation shows the development of a transform fault along a divergent plate boundary. Plates slide past one another along a transform fault without the formation of new plate or the consumption of old plate. As this process occurs, new sea floor forms along the mid ocean ridge.

Created by: Stephen Marshak

(2.5) The Process of Subduction

At convergent plate boundaries or convergent margins, two plates, at least one of which is oceanic, move toward each other. But rather than butting each other like angry rams, one oceanic plate bends and begins to sink down into the asthenosphere beneath the other plate. This sinking process, termed subduction, is shown in the following animation.

Created by: Stephen Marshak

(2.6) Hot Spot Volcanoes

This animation shows how hot spot volcanoes arise. A mantle plume beneath an oceanic plate creates a hot spot at the base of the lithosphere, and a volcano forms. Because the hot spot remains fixed as the plate moves over it, this volcano eventually becomes extinct and a new one forms. In time, a chain of extinct volcanoes develops, with a live volcano over the hot spot as the last link in the chain.

Created by: Stephen Marshak

(2.7) The Process of Rifting

Rifting is the process by which a continent splits and separates to form a new divergent boundary. This animation shows the progressive formation and evolution of a continental rift, and the formation of a mid-ocean ridge.

Created by: Stephen Marshak