Context: All volcanoes share the same basic features. First, mantle rock melts. The molten rock collects in magma chambers that can be 160 kilometers (100 miles) beneath the surface. As the rock heats, it expands. The hot rock is less dense than the surrounding rock. The magma rises toward the surface through cracks in the crust. A volcanic eruption occurs when the magma reaches the surface. Lava can reach the surface gently or explosively. Eruptions can be explosive or non-explosive. Only rarely do gentle and explosive eruptions happen in the same volcano. An explosive eruption produces huge clouds of volcanic ash. Chunks of the volcano fly high into the atmosphere. Explosive eruptions can be 10,000 times as powerful as an atomic bomb (Figure 8.6). Hot magma beneath the surface mixes with water. This forms gases. The gas pressure grows until it must be released. The volcano erupts in an enormous explosion. Ash and particles shoot many kilometers into the sky. The material may form a mushroom cloud, just like a nuclear explosion. Hot fragments of rock, called pyroclasts, fly up into the air at very high speeds. The pyroclasts cool in the atmosphere. Some ash may stay in the atmosphere for years. The ash may block out sunlight. This changes weather patterns and affects the temperature of the Earth. For a year or two after a large eruption, sunsets may be especially beautiful worldwide. Volcanic gases can form poisonous, invisible clouds. The poisonous gases may be toxic close to the eruption. The gases may cause environmental problems like acid rain and ozone destruction. Mt St. Helens was not a very large eruption for the Cascades. Mt. Mazama blew itself apart in an eruption about 42 times more powerful than Mount St. Helens in 1980. Today all that remains of that huge stratovolcano is Crater Lake (Figure 8.18). Some volcanic eruptions are non-explosive (Figure 8.7). This happens when there is little or no gas. The lava is thin, fluid and runny. It flows over the ground like a river. People generally have a lot of warning before a lava flow like this reaches them, so non-explosive eruptions are much less deadly. They may still be destructive to property, though. Even when we know that a lava flow is approaching, there are few ways of stopping it! Great volcanic explosions and glowing red rivers of lava are fascinating. All igneous rock comes from magma or lava. Remember that magma is molten rock that is below Earths surface. Lava is molten rock at Earths surface. Magma forms deep beneath the Earths surface. Rock melts below the surface under tremendous pressure and high temperatures. Molten rock flows like taffy or hot wax. Most magmas are formed at temperatures between 600o C and 1300o C (Figure 8.8). Magma collects in magma chambers beneath Earths surface. Magma chambers are located where the heat and pressure are great enough to melt rock. These locations are at divergent or convergent plate boundaries or at hotpots. The chemistry of a magma determines the type of igneous rock it forms. The chemistry also determines how the magma moves. Thicker magmas tend to stay below the surface or erupt explosively. When magma is fluid and runny, it often reaches the surface by flowing out in rivers of lava. The way lava flows depends on what it is made of. Thick lava doesnt flow easily. It may block the vent of a volcano. If the lava traps a lot of gas, the pressure builds up. After the pressure becomes greater and greater, the volcano finally explodes. Ash and pyroclasts shoot up into the air. Pumice, with small holes in solid rock, shows where gas bubbles were when the rock was still molten. Fluid lava flows down mountainsides. The rock that the flow becomes depends on which type of lava it is and where it cools. The three types of flows are aa, pahoehoe, and pillow lava. Aa Lava Aa lava is the thickest of the non-explosive lavas. Aa forms a thick and brittle crust, which is torn into rough, rubbly pieces. The solidified surface is angular, jagged and sharp. Aa can 
 Question: thickest type of non-explosive lava
Output: aa