Page 32 - Curriculum Visions Dynamic Book

Just as convection of air powers the water
cycle in the atmosphere, so convection currents power the very slow plastic flow of rocks below the surface. In this case the energy comes from the heat flowing up from the center of the Earth and also from radioactive decay of minerals within the rocks themselves.
Heat rises in some places. As it does so, it melts some rocks, which then flow upward. In some places, such as at the edges of plates, the liquid material, known as magma, reaches the surface. We called it ash or lava depending on whether
it solidifies as it is blown into the atmosphere or whether it flows from volcanoes.
To balance the rising material, rock also sinks back into the Earth in other places. That happens along long lines at the edges of plates. They are called sUbdUction zones, and they are marked by more volcanoes and also by extensive earthqUakes. The earthquakes are caused as the crust moves in periodic jerks while it reenters the Earth.
ash Fragments of lava that have cooled and solidified between when they leave a volcano and when they fall to the surface.
convection The circulating flow in a fluid (liquid or gas) that occurs when it is heated from below.
earthqUake The shock waves produced by the sudden movement of two pieces of brittle crust.
lava Hot, melted rock from a volcano. magma Hot, melted rock inside the Earth that,
when cooled, forms igneous rock. mineral A solid crystalline substance.
plastic The ability of certain solid substances to be molded or deformed to a new shape under pressure without cracking.
radioactive decay The change that takes place inside radioactive materials and causes them to give out progressively less radiation over time.
sUbdUction zones Long, relatively thin, but very deep regions of the crust where one plate moves down and under, or subducts, another. They are the source of mountain ranges.
 The main features of the Earth’s tectonic plates and the possible mechanism— convection—that drives them across the Earth’s surface.
Mountain belt
Direction of movement of colliding plate
Subduction zone
Continental plate
Direction of flow of convection current
Ocean plate
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