Practice7-Q6As geologists, one of our major areas of concern is rocks. The best way to define a rock is as matter composed of one or more naturally occurring substances. They can be anything from a tiny grain of one mineral to a massive compound made up o

Practice7-Q6

As geologists, one of our major areas of concern is rocks. The best way to define a rock is as matter composed of one or more naturally occurring substances. They can be anything from a tiny grain of one mineral to a massive compound made up of a large mix of minerals. There are two kinds of rock that I want to discuss today.

The first is igneous. These rocks start off as hot, liquid matter and cool to form what are commonly called fire rocks, or to us geologists, igneous rocks. Obsidian, that’s obsidian, represents this class well. It is a beautiful rock that forms when the lava from a volcano cools so rapidly that it does not allow any minerals within it to crystallize. Because no crystallization occurs, this rock takes on several unique properties: it often looks like beautiful black glass and can be chiseled or cut into sharp points, which is why it was often used as an implement for war in ancient societies.

The second is sedimentary. These rocks come from either the Earth’s surface or its oceans and seas. One great example is Halite. Seawater contains a high concentration of salt and halite is made when a body of seawater undergoes large amounts of evaporation. You see; as the water evaporates, the seawater can no longer hold as much salt as it once did. This salt then begins to fall out of solution as small salt rocks called Halite. If you cut these rocks in half, you won’t find a smooth surface; instead, you’ll find a mixture of separate pieces of rock that create a stratified or broken look, a common characteristic of sedimentary rocks.