Volcanoes have always scared and interested people. Today, volcanoes still fascinate people, especially volcanologists . Volcanoligists are geologists who study volcanoes. They study volcanoes to learn things about the inside of the earth because the things that come out of volcanoes come from the center of the earth. A volcano is an higher opening in the earth's surface where rocks, gasses, and ash fly out when there is too much pressure. Sometimes the rock is so hot it is melted (molten) into lava. Once in a while, lava will come out quietly. Other times, the expanding gas will make explosions. The explosions throw huge pieces of rock into the air. It comes down in a giant shower around the opening. Both kinds are called volcanic eruptions. The opening is called a volcanic vent.
Magma is the geological name for molten rock. It contains gas. The gas may be dissolved in the magma like the gas in a pop bottle or it might be like bubbles like in a pop bottle after it has been opened.
Sometimes, magma contains crystals of the minerals that are forming inside of it. The crystals dissolve when the magma is really hot, like sugar dissolving in boiling water. When the magma cools, sometimes the crystals separate from it. They measure up to an inch (three centimeters) across!
Magma freezes and hardens into solid rock when it hits the surface of the earth. When magma is above the earth's surface, it is called lava. The rock formed by the cooling lava is called lava rock.
If magma cools fast, the crystals are small, and if it cools really fast, there might not be any crystals at all. Instead, there would be a natural glass called obsidian. Lava rock is usually made of a mixture of small crystals and obsidian. Inside the rock are bigger crystals that grew slowly inside the earth. They were carried up with the rising magma.
If a bottle of cold pop is gently opened, the gas inside of it will separate out as bubbles and escape it. It's the same way with magma. When it reaches the top gently, but if it is warm and has been shaken up, it is different. When you take off the cap, the gas will rush out violently, carrying a spray of liquid with surface, the gas might bubble out gently or it might explode out violently. When magma flows out gently, it creates lava flows, or gently formed streams of molten rock. When gas rushes out of the vent, it carries fragments of rock with it. Some are solid and some are not. The pieces fall back down to the ground and form pyroclastic rocks (fire broken rocks).
There are several different types of lava flows. Two common types of lava flows have Hawaiian names because they often occur on Kilauea and Mauna Loa in the Hawaiian Islands. Their names are aa and pahoehoe.
Pahoehoe lava has a smooth, gently rolling surface. Sometimes, it is wrinkled into things that look like rope in coils. As the liquid lava pours down the mountain, it moves beneath the frozen crust through pipelike tubes called lava tubes. The supply of liquid rock is cut off at the end of the vent at the end of the eruption. An open tube is made when the lava drains out. Most lava tubes' diameters are 1/2 meter (1.5 feet), but some can be as big as 9 meters (30 feet)! The very big ones look like small tunnels and are common in volcanic areas like the lava beds of central Washington, Oregon, Idaho, and northeast California, also Hawaii. Lava tubes in California were furnished into hiding places where Captain Jackand his Indian followers hid to escape from the United States Army in the Modoc War.
Aa lava has a very rough surface. It has tons of bristling fragments. When the lava flows, the surface layer is carried on top of the flowing liquid. the lava freezes into a layer of solid rock with spiny fragments on top.
A third kind of lava is block lava. It has a surface of fragments like aa lava, but they are smooth. Block lava is the most common type of lava flow in the continents. There are a lot of them in the Cascade Range in the United States.
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