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Igneous Rock Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Magma

What is magma, anyway? Magma is the sea of melted rock found in the mantle. This super-heated liquid is hotter and cooler depending on its location and activity within the mantle’s circulation currents.

Geologists use pyrometers to measure the temperature of lava from a distance. A pyrometer is an optical measuring device that allows temperature measurements to be taken safely. Freshly blasted magma has been measured at temperatures between 1000 and 1200°C. Once magma arrives, it cools. The cooler lava gets, the greater its viscosity and the slower it moves. But don’t get too close, lavas that are barely moving have been measured at temperatures of 800°C.

Viscosity is the resistance that a fluid has to flow because of its chemical and structural composition.

Temperature plays a big part in magma’s viscosity. Think of pancake syrup or molasses; the hotter it gets, the runnier it gets. Heat excites the atoms and adds energy.

With magma, the silica content is also a big factor. Silicate minerals have a basic tetrahedral (pyramid-shaped) structure. They are linked together by shared oxygen molecules. However, silicate molecules in hot magma form crazy chains, sheets, and big matrices. As these linked silicate molecules get larger, the magma becomes more and more viscous and doesn’t want to flow. The number of tetrahedral bonds that can be formed into linked molecule groups depends on the amount of silica present in the magma. Put simply, silica in magma gets hard when it cools.

↑ the number of tetrahedral, ↑ the linked group silicates, ↑ the viscosity

Some temperatures recorded at different sample locations are hotter in some areas of the crust than others. This tells geologists that the thickness of the crust changes and produces more volcanic activity in some areas than others. In active volcanic areas like Hawaii, the temperatures at 40 km have been recorded as high as 1000°C, while in more stable areas, the temperature at the same depth is only 500°C. After magma flows from the depths of the mantle out onto the crust, it is called lava .

Magma chambers are pockets of molten rock formed in the lithosphere. These chambers may be formed as surrounding rock is pushed down during plate interaction and melted. The outline of magma chambers have been seen while recording earthquake waves from active volcanoes. The depth, size, and overall shape of magma chambers can be figured out based on these readings.

Magma is the origin of all volcanic rock. It has been around since the formation of the Earth.

When scientists studied the texture of quickly cooled magma, they found it took on two distinct forms: fine crystalline rock or glassy rock with no visible crystals. This is the magma blown violently from volcanoes during eruptions.

Some magma is very fluid and rains down fine molten fire, which cools quickly into ash. Some magma mixes with groundwater and creates super-heated steam and land-leveling mudflows.

Lava that slowly blobs out in bubbles and globs like slow moving molasses has a different texture. Since slow flowing lava streams and lakes below ground have a longer trip to the surface, they have time to form crystals. The longer lava cools, the larger and more complex the crystals can grow without interruption.

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