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Nazca "^vS Plate

The main tectonic plates are outlined by areas of geological activity. Earthquakes and volcanos are marked by red dots.Arrows indicate the direction of motion of the plates. [NASA]

consistency of plastic. This means that if you press on it, the material doesn't deform instantly, as a liquid would. However, under a steady pressure, it will flow slowly. The solid layer above the plastic region is called the lithosphere. The Earth's lithosphere is broken into plates. The name is meant to suggest that they are much larger in extent along the surface than they are thick. The plates float on top of the plastic layer.

As they float, the plates move slowly. Since they carry the continents with them as they move, we refer to this motion as continental drift. The general term for any process involved in the movement or deformation of planetary surfaces is tectonics, so continental drift is also called plate tectonics. Fig. 23.6 shows the Earth's main tectonic plates. Their motion is being driven by material being forced up from below into some narrow gaps between the plates. One such region, shown in Fig. 23.7, is called the mid-Atlantic ridge. Throughout the ridge, fresh material is appearing on the sea floor, as the plates move away from the ridge.

The regions where the plates meet are characterized by a high level of geological activity.

When one plate is forced under another, the resulting upward pressure can build great mountain ranges, such as the Himalayas, as shown in Fig. 23.8. These plate boundaries also show a high frequency of volcanoes and earthquakes. The volcanoes result from material being pushed upward. The earthquakes result from the fact

A computer generated image of what the mid-Atlantic ridge would look like if there were no water in the ocean. [USGS]

A computer generated image of what the mid-Atlantic ridge would look like if there were no water in the ocean. [USGS]

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