(a) List three characteristics of metamorphic rocks.
(b) Describe three modes of formation of metamorphic rock
(c) Outline three economic benefits of rocks.
Explanation
(a)
- Heat and Metamorphism
- Pressure and Metamorphism
- Chemical Action of Fluids
(c) There are three main types of rocks: sedimentary, igneous, and metamorphic. These rocks are formed by physical changes—such as melting, cooling, eroding, compacting, or deforming—that are part of the rock cycle.
Sedimentary Rocks
Sedimentary rocks are formed from pieces of other existing rock or organic material. Three types of sedimentary rocks exist clastic, organic (biological), and chemical. Clastic sedimentary rocks, like sandstone, form from clasts or pieces of other rock. Organic sedimentary rocks, like coal, form from hard, biological materials like plants, shells, and bones that are compressed into rock.
Clastic and organic rocks form with the weathering, or breaking down, of the exposed rock into small fragments. Through the process of erosion, these fragments are removed from their source and transported by wind, water, ice, or biological activity to a new location. Once the sediment settles somewhere, and enough of it collects, the lowest layers become compacted so tightly that they form solid rock.
Chemical sedimentary rocks, like limestone, halite, and flint, form from chemical precipitation. A chemical precipitate is a chemical compound—for instance, calcium carbonate, salt, and silica—that forms when the solution is dissolved in, usually, water, evaporates and leaves the compound behind. This occurs as water travels through Earth’s crust, weathering the rock, dissolving some of its minerals, and transporting it elsewhere. These dissolved minerals are precipitated when the water evaporates.
Metamorphic Rocks
Metamorphic rocks are rocks that have been changed from their original form by immense heat or pressure. Metamorphic rocks have two classes: foliated and non-foliated. When a rock with flat or elongated minerals is put under immense pressure, the minerals line up in layers, creating foliation. Foliation is the aligning of elongated or platy minerals, like hornblende or mica, perpendicular to the applied pressure direction. An example of this transformation can be seen with granite, an igneous rock. Granite contains long and platy minerals that are not initially aligned, but when enough pressure is added, those minerals shift to all points in the same direction while getting squeezed into flat sheets. When granite undergoes this process, like at a tectonic plate boundary, it turns into gneiss (pronounced “nice”).
Nonfoliated rocks are formed the same way but do not contain the minerals that tend to line up under pressure and, thus, do not have the layered appearance of foliated rocks. Given enough heat and pressure, sedimentary rocks like bituminous coal, limestone, and sandstone can turn into non-foliated metamorphic rocks like anthracite coal, marble, and quartzite. Nonfoliated rocks can also form by metamorphism, which happens when magma comes in contact with the surrounding rock.
Igneous Rocks
Igneous rocks (derived from the Latin word for fire) are formed when molten hot material cools and solidifies. Igneous rocks can also be made in a couple of different ways. They are called intrusive or plutonic, igneous rocks when formed inside the earth. They are called extrusive, or volcanic, igneous rocks if they are formed outside or on top of Earth’s crust.
Granite and diorite are examples of common intrusive rocks. They have a coarse texture with large mineral grains, indicating that they spent thousands or millions of years cooling down inside the earth, a time course that allowed large mineral crystals to grow.
Alternatively, rocks like basalt and obsidian have very small grains and a relatively fine texture. This happens because when magma erupts into lava, it cools more quickly than it would if it stayed inside the earth, giving crystals less time to form. Obsidian cools into the volcanic glass so quickly when ejected that the grains are impossible to see with the naked eye.
Extrusive igneous rocks can also have a vesicular or “holey” texture. This happens when the ejected magma still has gases inside of it, so when it cools, the gas bubbles are trapped, giving the rock a bubbly texture. An example of this would be pumice.
(c)
- The rocks are the major source of minerals such as gold, diamond, limestone, and petroleum that can be exported to the south.
- Source of fuel and metasedimentary and local sources of the fuel for India and domestic use.
- Some rocks like granite and sandstone for the road and bridges are used for the construction.