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Rocks and Minerals

Rocks and Minerals 

Rocks are naturally formed and are simply composed of crystals or particles of one or more minerals. There are many kinds of rock, and they can be classified in a number of ways. However, geologists classify rocks based on how the rocks were formed. The three classes are igneous rocks (formed directly from liquid rock), metamorphic rocks (formed by direct alteration of existing rocks), and sedimentary rocks (formed by eroded materials from other rocks). Rocks and Minerals

1. Igneous Rocks:

Igneous rocks solidify from a liquid magma as it cools.

When magma cools rapidly, mineral crystals do not have time to grow very large. On the other hand when magma cools slowly crystals grow to several millimeters or more in size. Granite is a slow cooled quartz rich (felsic ) rock, whereas basalt is a rapidly cooled magnesium rich (mafic) rock.

Igneous rocks are classified as:

Extrusive Rocks

Extrusive igneous rocks solidify from molten material that flows over the earth’s surface (lava). Common extrusive rocks are basalt, andesite, and rhyolite.

  • Basalt:  Basalt is characteristically a dense, black, massive rock, high in calcium and iron-magnesium-bearing minerals and low in quartz content. Great examples of basaltic lava flows can be found in the Deccan plateau of India.
  • Andesite:  Andesite has higher quartz content than basalt and is usually lighter in color. Andesites are practically found all are the andes Mountain Chain in South America.
  • Rhyolite: Rhyolite is typically a fine-grained, white pink, or gray rock, high in quartz and feldspar content with some amphibole and biotite. A well-known example is the Topaz Mountain rhyolite in the Thomas Range, Juab County.

Intrusive Rocks

Intrusive rocks are made of molten material (magma) that flows and solidifies underground. These rocks usually have a coarse texture (individual .minerals are visible without magnification), because the magma cools, slowly .underground, allowing crystal growth. Common rock types within the intrusive category are granite and diorite.

  • Granite: Granite is the intrusive equivalent of rhyolite but has a coarser texture. Granite temples can be found in Mohebali puram, India.
  • Diorite: Diorite has the same texture as granite but has the mineral composition of an andesite, which is diorite’s extrusive equivalent. Diorite forms the large batholiths beneath the Andes mountains.

2.  Sedimentary Rocks :

Sedimentary rocks are types of rocks created from deposition of layers upon layers of sediments over time. These types of rocks are formed on the Earth’s surface, as well as underwater. Wherever sedimentation goes on, sedimentary rocks are formed over time. The sediments that compose these rocks may be of organic, chemical or mineral origin.

Although sedimentary rocks constitute only 5% of the total crust volume, they extensively cover most continental surfaces. Most of the natural energy resources like coal and the fossil fuels are contained within the layers of sedimentary rock.

3.Metamorphic Rocks:

Metamorphic rocks are any rock type that has been altered by heat, pressure, and / or the chemical action of fluids and gases. The meaning of the term ‘metamorphic’, in fact, is ‘changed’. When igneous rocks, or sedimentary rocks, or, even metamorphic rocks get buried very deep under the earth’s surface, a process that takes millions of years, they get changed into something, else by the enormous pressure and heat inside the earth.

 Some examples of metamorphic rocks are:

  • Limestone being changed into marble
  • Shale turning into slate
  • Granite being changed into gneiss
  • Sandstone turning into quartzite

The Rock Cycle

The rock cycle consists of a series of constant processes through which earth materials change from one form to another over time. As within the water cycle and the carbon cycle, some processes in the rock cycle occur over millions of years and others occur much more rapidly. There is no real beginning or end to the rock cycle, but it is convenient to begin exploring  with magma.            Rocks and Minerals

              Magma, or molten rock, forms only at certain locations within the earth mostly along plate boundaries. When magma is allowed to cool, it crystallizes. Rocks that are formed out of cooled magma are called igneous rocks; intrusive igneous rocks if they cool below the surface (like gabbro) extrusive igneous rocks if they cool above (like basalt).

              Rocks like basalt are immediately exposed to the atmosphere and weather. Rocks that form below the earth’s surface, like gabbro, must be uplifted and all of the overlying . Rocks and Minerals

Material must be removed through erosion in order for them to be exposed. In either case, as soon as rocks are exposed at the earth’s surface, the weathering process begins. Physical and chemical reactions caused by interaction with air, water, and biological organisms cause the rocks to break down. Once rocks are broken dowry wind, moving water, and glaciers carry pieces of the rocks away through a process called erosion.

Moving water is the most common agent of erosion, all of these rivers carry tons of sediment weathered and eroded from the mountains of their headwaters to the ocean every year. The sediment carried by these rivers is deposited and continually buried in floodplains and deltas.

If sedimentary rocks or intrusive igneous rocks are not brought to the earth’s surface by uplift and erosion, they may experience even deeper burial and be exposed to high temperatures and pressures. As a result, the rocks begin to change. Rocks that have changed below the earth’s surface due- to exposure to heat, pressure, and hot fluids are called metamorphic rocks.

Some of the processes within the rock cycle, like volcanic eruptions, happen very rapidly, while others happen very slowly, Like the uplift of mountain ranges and weathering of

Igneous  rocks. Importantly, there are multiple pathways through the rock cycle. Any kind of rock can be uplifted and exposed to weathering and erosion; any kind of rock can be buried and metamorphosed. As Hutton correctly theorized, these processes have been occurring for millions and billions of years to create the earth as we see it: a dynamic planet.  Rocks and Minerals



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