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What are rocks. Types of Rocks.

Introduction to the Rocks:

Rocks are composed of mineral assemblages. A few rocks consist of one mineral, such as Quartzite, which is completely quartz,but most rocks contain several minerals and these minerals have much to do with the way rocks break or bend, weather and erode.

Rock is a naturally occurring aggregate of minerals of organic or inorganic origin. The Earth lithosphere is made of rocks. Petrology is the scientific study of rocks.

Rock classification:

Rocks are classified by mineral and chemical composition, by the texture of the of the constituent particles and by the processes that formed them. These indicators separate rocks into igneous, sedimentary and metamorphic. The transformation of one rock type to another is described by the geological model called the rock cycle.

Difference Between a Rock and a Mineral:

  1. Minerals are homogeneous, naturally occurring inorganic solids. Each mineral has a definite chemical composition and characteristic crystalline structure. A mineral may be single element such as copper or gold or it may be a compound made up of a number of elements about 25000 different minerals described till to day .
  2. Rocks are made up of one are more minerals.

Types of Rocks:

01. Igneous Rocks:

Igneous rocks are formed when molten magma cools. Igneous have got their origin from word Agni which means made from heat or fire. When the rock is liquid and inside the earth, it is called magma. When the magma gets hard inside the earth’s crust,it turns into granite. It cools very slowly and is very hard.

When the magma gets up to the surface and flows out, like  what happens when a volcano erupts, then the liquid is called lava. Lava flows down the sides of the volcano. When it cools and turns hard it is called obsidian lava rock or pumice-depending on what it looks like. About 46.7% of the earth crust by volume consist of igneous rocks , making it the most plentiful category. Of these 66% are basalts and gabbros, while 16% are granite. The oceanic crust is 99% basalt, which is an igneous rock of mafic composition.

02. Sedimentary Rocks:

Sedimentary rocks are formed by deposition of either clastic sediments organic matters or chemical precipitates followed by compaction of the particulate matter and cementation. Sedimentary rocks form

 at or near the earth’s surface.

When mountains are first formed they are tall and jagged like the rocky mountains on the west coast of North America. Over time mountains become old mountains like Appalachian Mountains on the East coast of Canada and United States. When mountains are old, they are rounded and much lower. What happens in the mean time is that lots of rock get worn away erosion. Rain, freeze thaw cycle, wind and running water cause the big mountains to crumble a little bit at a time. Eventually most of the broken bits of the rocks end up in the streams and river that flow down from the mountains. These little bits or rocks and sand are called sediments. When the water slows down, enough these sediments settled to the bottom of the lake or oceans they run into. Over many years, layers of different rock bits settle at the bottom of lakes and oceans.

Think of each layer as a page a book. One piece of paper is not heavy. But a stack of telephone books is very heavy and would squash anything that was underneath. Over times the layers of sand and mud at the bottom of lakes and oceans turned into rocks. these are called sedimentary rocks.

03. Metamorphic Rocks:

Metamorphic rocks are formed by subjecting any rock type (including previously formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the earth’s surface and must be sufficiently high so as to change the original minerals. Into other mineral types are else into other forms of the same minerals (e.g by crystallization). 

Metamorphic rocks are rocks that have changed. The word comes from the Greek ”Meta” and ”Morph” which means to change form. Metamorphic Rocks were originally igneous or sedimentary, but due to movement of the earth’s crust were changed. If you squeeze your hand together very hard, you will feel heat and pressure. Marble is and example of a Sedimentary rock that has been changed into a metamorphic rock.


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