This is rock formed by the cooling and solidification of hot, molten material.
Earth was originally molten. The very first rocks of the continents and ocean basins must have been igneous. Igneous activity has continued actively throughout the history of Earth. Therefore Igneous rocks are the most abundant type constituting 80% of the Earth's crust. Often Magma does not find its way to the Earth's surface. Exposed Igneous rock is called Extrusive Rock while rock cooled within the Earth's interior is called Intrusive Rock.
El Capitan in Yosemite National Park shown below is an example of an Igneous Extrusive rock.
Earth's temperature becomes 1 degree Fahrenheit for every 46 meters (150 feet) of depth. This works out to 35 degrees for every mile. If the increase would continue at the same rate, the center of the earth would be 140,000 degrees. But as there are several layers, the interior is no where near as hot. But why is Earth hot? Pressure produces heat. But there is not enough pressure to account for all the heat. Another reason could be the interior heat caused by the molten origin is insulated. But the main reason is the radioactivity. The decay of radioactive elements releases energy. Earth as a whole is solid, but there are local packets of molten materials. The temperature of the lava (800 to 1200 degrees Celsius) indicate that these pockets must be deep. As rock melts, it expands reducing its density. So it is squeezed to the surface by the surrounding rock.
Igneous rocks are classified according to texture and mineral composition.
Texture, or grain size is determined by the rate at which lava cools. Slow cooling yields large grains, while rapid cooling yields small or no grains. Three factors control the rate of cooling.
1. Location. Lava exposed to cool atmosphere loses heat quickly and develops small grains.
2. Size. A small volume of magma
cools more quickly than a large one.
3. Shape. Thin magma bodies lose
heat more rapidly than those with the same volume but a more compact shape.
Mineral composition. Rocks rich in Silica (SiO2)
contain minerals with abundant silicon, sodium, and potassium. These
rocks are light in color. Rocks low in Silica are rich in minerals
containing iron, magnesium, and calcium are are darker in color.
The table below gives examples of various igneous rocks based on their
classification.
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Quartz dominant; Some Hornblende, Biotite, and Muscovite |
Plagioclase Dominant; Abundant Hornblende |
Plagioclase and Pyroxene Dominant |
Olivine Dominant |
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Volcanic breccia |
Volcanic breccia |
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Volcanic activity and the formation of Igneous Rocks:
Igneous rocks formed below the surface of the Earth by solidification of magma are known as plutons. It is discordant if it cuts across the grain of the surrounding rock. Batholith is a huge discordant rock. To be classified as a batholith, it must be at least 103 km2. The Coast Range Batholith in Western Canada is more than 1600 km long and as much as 160 km wide. Dikes are discordant rocks formed from magma that has filled fractures that are vertical or nearly so, and their shape is tubular, thin in one dimension and extensive in the other two.
An igneous rock parallel to the grain of the surrounding rock is called Concordant. A sill has the same shape as a dike but is concordant. A laccolith is a concordant pluton formed from a blisterlike intrusion that has pushed up the overlying rock layers.
Volcanic eruptions: There are three products in a volcanic eruption.
1. Gas. Gas is expelled during all stages of an eruption, (early infancy, height of activity, final dying moments) with steam accounting for 90% of the gasses.
2. Lava. Could be fluid which flows easily and quietly with little explosive violence, or if much gas escapes simultaneously, explosions may hurl incandescent lava fountains into the air, or there could be small volumes of lava so stiff and viscous that it can barely flow.
3. Tephra. They are solid particles ranging in size from finest dust to huge boulders.. They can also include gas-laden material that the volcano ejects in molten form but hits the ground as a solid. Such debris is blasted into the air by violently explosive volcanos.
The most violent explosion in recent times was that of Tambora, on the island of Soembawa just east of Java on April 10 to 20, 1815. The whole mountain was blown off, with 145 km3 of solid debris thrown into the air darkening the sky for several days for hundreds of miles around. The Sun's rays were blocked and 1816 was called the year without a summer. For more details of Tambora, click below:
Low-silica magmas, which cool to form Basalt, are much more fluid and can flow more easily than high-silica magmas, which for felsite. The rate of flow depends on its viscosity. Chemical composition and temperature are the most important of many factors that determine the viscosity of a magma or a lava. Viscosity is a property of a fluid that resists the force causing it to flow. A thick, slow moving liquid such as molasses as high viscosity. More fluid magma are low in Silica and form igneous rocks such as basalt, while low flowing high-silica magmas form felsites. For details of other volcanos, click below.
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