Geography Reference
In-Depth Information
Within the mantle, there are massive, hot currents of liquid kept
molten by radioactivity within the Earth. These currents form cir-
culation cells within the mantle, a bit like the large atmospheric
convection cells described in Chapter 1. When the rising currents
reach the stiff lithosphere, they drag it along. In places the hot,
rising mantle material can force its way through the crust of the sea
floor. Here underwater volcanoes develop, forming a deep-sea
mountain range. The lava from these volcanoes forms new crust as
it cools. As the descending parts of the slow convection cell in the
mantle separate, they can slowly (1-10
cm per year) drag overlying
ocean crust with them (Figure 3.2). The great, deep trenches
around the edges of the oceans are zones where old crust is forced
down and melts into the Earth's mantle (Figure 3.2). The thickest
(and oldest) rocks on the ocean floor are also found nearest to the
continents. The oldest rocks on the ocean floor have been dated as
208 million years old. Therefore the ocean floor all over the world
is young compared to the age of the continents where the rock is
often between two to four billion years old.
There are several stiff plates moving across the Earth's surface
which grind or rub against each other (Figure 3.3). Earthquakes
occur most at the boundaries of these plates. The continents are
Mid-ocean ridge
Figure 3.2
Mid-ocean ridge, sea floor spreading and ocean trench subduction
zones.
