5. Igneous rocks (or primary rocks) are of two types - intrusive and extrusive. When molten lava from inside the Earth
comes to the surface, it rapidly cools down and becomes solid. Rocks made in this way are called extrusive igneous
rocks, and they have a very fine-grained structure. Which of the following is an extrusive igneous rock?
Diamond
Marble
Anato
Answers
Answer:
Of the three principal rock types (igneous, sedimentary, and metamorphic), igneous rocks can be thought of as "primary" rocks because they crystallize from a liquid. Sedimentary and metamorphic rocks, which we will study later, may be thought of as derivative rocks.
Igneous rocks are rocks formed from the crystallization of a liquid (molten rock). Igneous rocks may be divided into two categories. Intrusive or plutonic rocks crystallize from magma beneath the earth's surface. Extrusive or volcanic rocks crystallize from lava at the earth's surface.
The texture of an igneous rock (fine-grained vs coarse-grained) is dependent on the rate of cooling of the melt: slow cooling allows large crystals to form, fast cooling yields small crystals. Magmas and their resultant plutonic rock bodies cool and crystallize slowly and are characterized by coarse-grained texture, in which the mineral crystals are visible to the unaided eye. On the other hand, lavas cool quickly at the earth's surface and are characterized by fine-grained texture, in which the crystals are too small to be seen by the unaided eye.
Very quickly cooled lavas, typically those quenched in water, will have a glassy texture. They cool too quickly to form crystals. Glasses do not have an orderly arrangement of atoms and there are therefore no minerals, in the strict sense, in them. Volcanic glass is called obsidian.
In addition to texture, igneous rocks may are classified according to their chemical composition. The most general classification is based on the relative abundance in a rock of felsic (feldspar and silica-quartz) minerals vs mafic (magnesium and ferrum or iron) minerals. Felsic minerals (quartz, K feldspar, etc) are light colored while mafic minerals (hornblende, pyroxenes) are normally dark colored. Felsic minerals have the lowest melting points (600 to 750 °C) and mafic minerals have higher melting points (1000 to 1200 °C).