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How Is Magma Different Than Lava?

Pockets of molten rock or magma may be trapped between the earth's cool crust and the much hotter mantle. Since it is less dense than the mantle or the crust, the molten rock forces its way up through the crust flowing along cracks and faults until it erupts on the surface as a volcano.

The difference between magma and lava is its location. Magma is molten rock held in a subterranean chamber. Once magma erupts through a volcanic opening, it becomes lava.

Magma
- The molten rock beneath the earth's crust collects in a magma chamber under a crack or weak area. As the hot, molten magma collects, the pressure increases until the magma bursts through the crust as a volcanic eruption. The temperature range of magma varies from 700-1300 degrees C. Once the magma flows out onto the surface of a volcano, it is called lava.
Lava
- The temperature range of lavas varies from 650-950 degrees C, but the consistency of lava does not depend on its temperature but on the minerals in its composition. Thicker lavas pile up around the volcanic vent and form a cone while the more fluid lavas flow for miles as thin sheets, which gradually cool. The most dangerous lavas are so thick that they cannot flow but plug up the vents until the pressure builds up and a violent explosion of hot gases, ash, and rock results.
Types of Magma
- Cooling magma crystallizes into two forms of igneous rocks: Magma erupting from volcanic openings cools on the earth's surface as lava; when magma cools deep below the earth's surface, it forms massive intrusive or plutonic rocks.
  
  The chemical composition of magma reflects the abundance of elements in the earth. They all contain 45-75 percent silicates and vary according to the iron, magnesium and calcium content. Three types of magma can be characterized by their composition: Basaltic or Gabbroic magma contains high amounts of iron and low amounts of potassium and sodium; Andesite or Dioritic magma is the intermediate in all the reference elements, and Rhyolitic or Granitic magma, which is low in iron and magnesium, but high in potassium and sodium.
Types of Lava
- The structure of a volcano depends on the type of lava (4) from which it forms. Conical, steep stratovolcanoes build up from a thicker lava; wide shield volcanoes form from the thin flows of fluid lavas, and the mounded lava domes are composed of chunks of thick lava and ash.
  
  Lava types are classified with Hawaiian names according to their flow patterns: A'a is pronounced "ah-ah." It is a slow lava flow that resembles jello with a rough surface and erupts at temperatures ranging from 1000-1100 degrees C.
  
  Pahoehoe ("pa-ho-ho") is a very fluid lava that can flow down the sides of a volcano like red-hot rivers of liquid rock. The surface cools to a smooth, thin crust after it erupts at temperatures of 1100-1200 degrees C.
  
  Pillow lava erupts from underwater volcanic vents where it cools rapidly and forms a hardened shell that may look puffy like a pillow. As more lava erupts, it cools under the previous lava shell, which cracks open to reveal the new pillow structure underneath.

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