Basalts are dark colored, fine-grained extrusive rock. The mineral grains are so fine that they are impossible to distinguish with the naked eye or even a magnifying glass. They are the most widespread of all the igneous rocks. Most basalts are volcanic in origin and were formed by the rapid cooling and hardening of the lava flows. Some basalts are intrusive having cooled inside the Earth's interior.
Dacite is an extrusive igneous rock. The principle minerals that make up dacite are plagioclase, quartz, pyroxene, or hornblende.
Gabbro is a dark-colored, coarse-grained intrusive igneous rock. Gabbro is very similar to basalt in its mineral make up. It is composed mostly of the mineral plagioclase feldspar with smaller amounts of pyroxene and olivine.
Granite is an igneous rock that is composed of four minerals. These minerals are quartz, feldspar, mica, and usually hornblende. Granite forms as magma cools far under the earth's surface. Because it hardens deep underground it cools very slowly. This allows crystals of the four minerals to grow large enough to be easily seen by the naked eye.
Obsidian is a very shiny natural volcanic glass. When obsidian breaks it fractures with a distinct conchoidal fracture. Obsidian is produced when lava cools very quickly. The lava coo ls so quickly that no crystals can form. When people make glass they melt silica rocks like sand and quartz then cool it rapidly by placing it in water. Obsidian is produced in nature in a similar way.
Pumice is a very light colored, frothy volcanic rock. Pumice is formed from lava that is full of gas. The lava is ejected and shot through the air during an eruption. As the lava hurtles through the air it cools and the gases escape leaving the rock full of holes.
Rhyolite is very closely related to granite. The difference is rhyolite has much finer crystals. These crystals are so small that they can not be seen by the naked eye. Rhyolite is an extrusive igneous rock having cooled much more rapidly than granite, giving it a glassy appearance. The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende.
CLASSIFICATION OF ROCKS