Igneous rocks- Definition: Igneous rock is formed by magma (molten rock) being cooled and becoming solid. Formation: They may form with or without crystallization, either below the surface as intrusive(plutonic) rocks or on the surface as extrusive(volcanic) rocks. Igneous rocks make up approximately 90% of the upper part of the Earth's crust, but their great abundance is hidden on the Earth's surface by a relatively thin but widespread layer of sedimentary and metamorphic rocks. Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are coarse grained. The mineral grains in such rocks can generally be identified with the naked eye. Effects of water and Carbon Dioxide- Water lowers the solidus temperature of rocks at a given pressure. For example, at a depth of about 100 kilometers, peridotite begins to melt near 800°C in the presence of excess water, but near or above about 1500°C in the absence of water.


Sedimentary rocks-
Definition: Sedimentary rock is the type of rock that is formed by sedimentation of material at the Earth's surface.
Beginning Formation: Sedimentation is the collective name for processes that cause mineral and/or organic particles The forces of weather break rocks into small pieces that are carried away and deposited elsewhere.
Next Step of Formation: These small pieces are often deposited in shallow seas or lakes as sediments.
Last Step of Formation: As the layers of deposits pile up, perhaps over millions of years, pressure from the weight of the sediments above turns the lower layers into solid rock. Sand may turn into sandstone; silt and clay become shale.

Metamorphic rocks-


Metamorphic rocks are formed from existing rocks (sedimentary, Igneous, Metamorphic) Metamorphism – the process of forming metamorphic rocks within the lithosphere making the rocks more dense and less porous.Foliated and nonaffiliated metamorphic rocks. – Have been modified by heat, pressure and chemical process usually while buried deep below earth’s surface. Expose to these extreme conditions has altered the mineralogy, texture and the chemical compositions of the rocks. There are two basic types of metamorphic rocks: 1) foliated metamorphic rock such as gneiss, hyalite, schist and slate which have a layered appearance.



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Some examples of sedimentary rocks are;

  • Sandstone; Sandstone rocks are sedimentary rocks made from small grains of the minerals quartz and feldspar. They often form in layers as seen in this picture. They are often used as building stones.
  • Limestone; Limestone rocks are sedimentary rocks that are made from the mineral calcite which came from beds of evaporated seas and lakes and from sea animal shells. This rock is used in concrete and is an excellent building stone for humid regions
  • Shale; Shale rock is a type of sedimentary rock formed from clay that is compacted together by pressureThey are used to make bricks and other material that is fired in a kiln.
  • Gypsum; Gypsum rocks are sedimentary rocks made up of sulfate mineral and formed as the result of evaporating sea water in massive prehistoric basins. It is very soft and is used to make Plaster of Paris, casts, molds, and wallboards.

THE WAY ROCKS ARE CLASSIFIED:
  • Texture
  • May or may not react with acid.
  • Has layers
  • Flat or curved pebbles
  • composed pieces of cemented or pressed.
  • If it has pores or not.
  • If it has fossils or not.

-Volume of upper crust is 5%
-Areas of continents is 75%
-Record of geological events- e.g. Himalayas will someday be sediment


The sedimentary rock cover of the continents of the Earth's Crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 5% of the total volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks.
Sedimentary rocks are the second great rock class. Whereas igneous are made hot, sedimentary rocks are born cool at the Earth's surface, mostly under water. They usually consist of layers or strata, hence they are also called stratified rocks. Depending on what they're made of, sedimentary rocks fall into one of three types


Clastic Sedimentary Rocks
The most common set of sedimentary rocks consist of the granular materials that occur in sediment: mud and sand and gravel and clay. Sediment mostly consists of surface minerals — quartz and clays — that are made by the physical breakdown and chemical alteration of rocksThese are carried away by water or wind and laid down in a different place. Sediment may also include pieces of stones and shells and other objects, not just grains of pure minerals. Geologists use the word clasts to denote particles of all these kinds, and rocks made of clasts are called clastic rocks.

Organic Sedimentary Rocks

Another type of sediment actually forms in the sea as microscopic organisms — plankton — build shells out of dissolved calcium carbonate or silica. Dead plankton steadily shower their dust-sized shells onto the seafloor, where they accumulate in thick layers. That material turns to two more rock types, limestone (carbonate) and chert (silica). These are called organic sedimentary rocks, although they're not made of organic material as a chemist would define it.



Chemical Sedimentary Rocks

These same ancient shallow seas sometimes allowed large areas to become isolated and begin drying up. In that setting, as the seawater grows more concentrated, minerals begin to come out of solution (precipitate), starting with calcite, then gypsum, then halite. The resulting rocks are certain limestones or dolomites, gypsum rock, and rock salt respectively. These rocks, called the evaporite sequence, are also part of the sedimentary clan.

Facts about Sedimentary Rocks
1. They are composed of fragments of pre-existing rocks that have been concentrated, compacted and cemented without necessarily undergoing chemical change
2. Some are precipitated from mineral rich solutions or waters on the earth's surface
3. Composition of these rocks greatly depend on the types of rocks from which the rock fragments are derived
4. Sedimentary rocks are found to be creating on the side of pool's. And even though sedimentary rocks are small, they can be used for big things. Like houses.
5. When the Earth was first forming, about 6 billion years ago, there weren't any sedimentary rocks
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