Page 117 - Introduction to Petroleum Engineering

GEOLOGIC HISTORY OF THE EARTH 103

Eurasian
plate
North
American
plate

Pacific African
plate plate
Nazca South
Indo-Australian plate American
plate plate

Antarctic
plate

FIGURE 6.3 Tectonic plates.

The history of the lithosphere depends on the dynamic relationship that exists bet-
ween the lithosphere and the asthenosphere. The lithosphere consists of a set of rigid
plates floating on the semimolten asthenosphere. Crustal plates move in relation to
one another at the rate of a few inches per year. Molten material in the asthenosphere
can enter the lithosphere through cracks between plates. This transfer of material can
be by violent volcanic eruptions, or it can be gradual through the extrusion of basaltic
lavas at the boundaries between plates.
Satellite measurements of the gravitational field of the Earth show boundaries
between continents and tremendous mountain ranges rising from ocean floors. The
shapes of the boundaries are suggestive of vast plates, as depicted in Figure 6.3. Only
the largest of the known plates are depicted in the figure. Many of these plates are
associated with continental land masses.
Oceanic mountain ranges are sources of basaltic extrusion and seafloor
spreading. As seafloors spread, the continental plates are forced to move.
Movement of continental plates is known as continental drift and was first
proposed by the German meteorologist Alfred Wegener (1880–1930 ce) in 1915.
A collision of two plates can form great mountain ranges, such as the Himalayas.
The boundary at the interface between two colliding plates is a convergent plate
boundary. Alternatively, a collision can deflect one plate beneath another. The
boundary where one plate is moving under another is a subduction zone. Material
in the subduction zone can be forced down through the Moho and into the semi-
molten asthenosphere. The separation of two plates by the extrusion of material at
the mid‐Atlantic Ridge is an example of seafloor spreading. Together, seafloor
spreading and subduction zones are the primary mechanisms for transferring
material between the crust and upper mantle.