what are the characteristics of igneous rocks.../?

Question

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Answer 1

Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle or crust. Typically, the melting is caused by one or more of the following processes -- an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust. The word "igneous" is derived from the Latin igneus, meaning "of fire".

Characteristics of Igneous Rocks


Introduction

As described in some of the previous topics, igneous rocks are produced by the crystallization and solidification of molten magma. Magma forms when rock is heated to high temperatures (between 625 and 1200° Celsius) beneath the Earth's surface. The exact temperature needed to melt rock is controlled by several factors. Chemistry of the rock material, pressure, presence of gases (like water vapor) all influence when melting occurs. Most of the heat required to melt rock into magma comes from the Earth's central internal region known as the core. Scientists estimate that the temperature of the Earth's core is about 5000° Celsius. Heat moves from the Earth's core towards the solid outer crust by convection and conduction. Convection moves hot plumes of magma vertically from the lower mantle to the upper mantle. Some of these plumes melt through the Earth's solid lithosphere and can produce intrusive igneous features and extrusive igneous features on the surface. Heat can also be generated in the lower lithosphere through friction. The tectonic movement of subducted crustal plates can generate enough heat (and pressure) to melt rock. This fact explains the presence of volcanoes along the margin of some continental plates.



Types of Igneous Rocks

The type of igneous rocks that form from magma is a function of three factors: the chemical composition of the magma; temperature of solidification; and the rate of cooling which influences the crystallization process. Magma can vary chemically in its composition. For example, the amount of silica (SiO 2 ) found in magma can vary from 75 % to less than 45 %. The temperature of cooling determines which types of minerals are found dominating the rock's composition. Rocks that begin their cooling at low temperatures tend to be rich in minerals composed of silicon, potassium, and aluminum. High temperature igneous rocks are dominated by minerals with higher quantities of calcium, sodium, iron, and magnesium. The rate of cooling is important in crystal development. Igneous rocks that form through a gradual cooling process tend to have large crystals. Relatively fast cooling of magma produces small crystals. Volcanic magma that cools very quickly on the Earth's surface can produce obsidian (see image link) glass which contains no crystalline structures.

Geologists have classified the chemistry of igneous rocks into four basic types: felsic, intermediate, mafic, and ultramafic. Igneous rocks derived from felsic magma contain relatively high quantities of sodium, aluminum, and potassium and are composed of more than 65 % silica. Rocks formed from felsic magma include granite (see image link), granodiorite (see image link), dacite, and rhyolite (see image link). All of these rock types are light in color because of the dominance of quartz, potassium and sodium feldspars, and plagioclase feldspar minerals (Figure 10e-1). Dacite and granodiorite contain slightly more biotite and amphibole minerals than granite and rhyolite. Rhyolite and dacite are produced from continental lava flows that solidify quickly. The quick solidification causes the mineral crystals in these rocks to be fine grained. Granite and granodiorite are common intrusive igneous rocks that are restricted to the Earth's continents. Large expanses these rocks were formed during episodes of mountain building on the Earth. Because granite and granodiorite form beneath the Earth's surface their solidification is a relatively slow process. This slow solidification produces a rock with a coarse mineral grain.

Mafic magma produces igneous rocks rich in calcium, iron, and magnesium and are relatively poor in silica (silica amounts from 45 to 52 %). Some common mafic igneous rocks include fine grained basalt (see image link) and coarse grained gabbro (see image link). Mafic igneous rocks tend to be dark in color because they contain a large proportion of minerals rich in iron and magnesium (pyroxene, amphiboles, and olivine). Basalt is much more common than gabbro. It is found in the upper portion of the oceanic crust and also in vast continental lava flows that cover parts of Washington, Oregon, Idaho, and California. Gabbro is normally found in the lower parts of oceanic crust and sometimes in relatively small intrusive features in continental crust.

Andesite (see image link) and diorite are intermediate igneous rocks that have a chemistry between mafic and felsic (silica amounts between 53 to 65 %). These rocks are composed predominantly of the minerals plagioclase feldspar, amphibole, and pyroxene. Andesite is a common fine grained extrusive igneous rock that forms from lavas erupted by volcanoes located along continental margins. Coarse grained diorite is found in intrusive igneous bodies associated with continental crust.

Ultramafic igneous rocks contain relative low amounts of silica (< 45 %) and are dominated by the minerals olivine, calcium-rich plagioclase feldspars, and pyroxene. Peridotite is the most common ultramafic rock found in the Earth's crust. These rocks are extremely rare at the Earth's surface.

Answer 2

Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle or crust. Typically, the melting is caused by one or more of the following processes -- an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust. The word "igneous" is derived from the Latin igneus, meaning "of fire".

Magma origination
The Earth's crust averages about 35 kilometers thick under the continents, but averages only some 7-10 kilometers beneath the oceans. The continental crust is composed primarily of sedimentary rocks resting on crystalline basement formed of a great variety of metamorphic and igneous rocks including granulite and granite. Oceanic crust is composed primarily of basalt and gabbro. Both continental and oceanic crust rest on peridotite of the Earth's mantle.

The Earth's mantle immediately below the crust typically is relatively rigid, and this relatively rigid mantle and the overlying crust comprise the lithosphere. Because of the typical increase of temperature with depth, the mantle becomes less rigid and convects even though it is completely or mostly solid; the convecting mantle immediately below the lithosphere makes up the asthenospheric mantle. The mantle, which extends to a depth of nearly 3,000 kilometers, is the source of much magma. Most magmas are generated at temperatures between 600 to 1600 °C. Most of the higher-temperature magmas, those with temperatures between about 900° and 1600°C, are formed in the upper two hundred kilometers or so of the mantle.

Rocks may melt in response to a decrease in pressure, to a change in composition such as an addition of water, to an increase in temperature, or to a combination of these processes. Other mechanisms, such as melting from impact of a meteorite, are less important today, but impacts during accretion of the Earth led to extensive melting, and the outer several hundred kilometers of our early Earth probably was an ocean of magma. Impacts of large meteorites in last few hundred million years have been proposed as one mechanism responsible for the extensive basalt magmatism of several large igneous provinces.

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Answer 3

These are primary rocks. They are formed when molten lava solidifies. They are least porous compared to sedimentary and metamorphic rocks.(which are the other two types). They can bear maximum compressive stress and considered very strong building material.You will find these rocks in areas where there were volcanic activity some time or other.

Answer 4

THEY LIE IN THE LOWEST STRATA, CLOSE TO THE MAGMA.INGENIOUS ROCKS ARE HARD N LUSTROUS LIKE GRANITE.

Answer 5

they are hard rocks

Answer 6

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