RocksGroup7

=  __Igneous Rock__ __s__  - form when lava or magma cools and minerals crystallize.= = = = = = -Magma is molten rock below earth's surface while lava is magma that flows out onto earth's surface. = Intrusive or Extrusive The type of igneous rock that forms depends on the composistion of the magma. Igeous are broadly classified as **intrusive** or **extrusive.** When magma cools and cry//s//tallizes below Earth's surface the rock is classified as **intrusive.** Crystals of intrusive igneous rocks are generally large enough to see without magnification the magma cools slowly for these large crystals to form.
 * =__What is a rock?__ a lump or mass of hard consolidated mineral matter. =

Magma that cools and crystallizes on Earth's surface forms ** extrusive rocks. ** Extrusive rocks generally have smaller crystals due to the fact that they go through a faster cooling process.

Igneous rocks are classified according to their mineral compositions. Basaltic rocks, such as gabbro are dark colored, have lower silica contents, and contain mostly plagioclase and pyroxene. Granite rocks, such as granite, are light colored, have high silica contents, and contain mostly quartz, potassium feldspar, and plagioclase feldspar.

A Basaltic Rock A Granite Rock ||  || Rocks that have a composition of minerals that is somewhere in between basaltic and granitic are called intermediate rocks.

Igneous rocks differ in the sizes of their grains or crystals. Texture refers to the size, shape, and distribution of the crystals or grains that make up a rock.

The texture of rhyolite can be described as fine-grained, while granite can be described as coarse grained.





__Crystal size and Cooling Rates__

When lava flows on Earth's surface, it cools quickly and there is not enough time for larghe crystals t form. The result is an extrusive rock with crystals so small that they are difficult to see without magnification. These extrusive rocks with small crystals include rhyolite and obsidian, which is a rock cooled so rapidly it looks almost like glass.

Obsidian

In contrast, when magma cools slowly beneath Earth's surface, the crystals have a longer amount of time to cool. This results in intrusive rocks with much larger crystals. Some examples of intrusive igneous rocks with fairly larger crystals include granite, diorite, and gabbro. They all have crystals larger than 1 centimeter.

Gabbro   A rock can also have a spongy appearance, which is called a vesicular texture. This is a result of when magma contains dissolved gases that escape when the pressure on the magma lessens. If the lava is thick enough to prevent the gas bubbles from escaping, holes called vesicles are left behind, leaving behind a spongy and pourous texture.

Pumice a website on how to use the reference tables to identify igneous rocks : [] ** __SEDIMENTARY ROCKS-__ a rock formed by the compaction and cementation of sediments **

The formation of sedimetnary rocks beings when weathering and erosion produce sediments.



__Sediments__ - small pieces of rock that are moved when deposited by water, wind, glaciers, and gravity.

//Weathering// is the breaking down of particles. This results in the sediments that create sedimentary rocks. **Chemical weathering** occurs when thee minerals in a rock are dissolbed or otherwise cemically changed. During **physical weathering**, minerals remain chemiclly unchanged but change physically.

After something has been weathered it goe through erosion. **Erosion** is the removal and transport of sediments. The our main agents of erosion are wind, moving water, gravity, and glaciers. During erosion, these sediments are transported to different locations. When sediments become glued together, they form **sedimentary** **rocks**. This begins when weathering and erosion produce sediments.


 * __//Depostition//__**: When transported sediments are deposited on the ground or sink to the bottom of a body of water, deposition occurs. Sediments are deposited in layers with the largest grains at the bottom and the smallest grains on top.

__Lithification__ - As more sediment is deposited in an area, the bottom layers are subjected to increasing pressure and temperature. These conditions cause lithification, the physical and chemical processes tht transform sediments into sedimentary rocks.

- Lithification begins with compaction. **Compaction** is the weight of overlying sediments forcing the sediment grains closer together, causing physical changes. **Cementation** also occurs when mineral growwth glues sediment grains together into solid rock. 

** Cross bedding- is formed as inclined layers of sediment are deposited across a horizontal surface.

There are 3 different types of sedimentary rocks such as:** __Clastic__- formed from the abundant deposits of loosee sediments that accumulate on Earth's surface

The Clastic Rock - Sandstone __Chemical__- they are not made of pieces of sediment They have mineral crystals made from elements that are dissolved in water.

A Chemical Rock- Rock Salt

__Organic__- form from the remains of animals and plants.

An Organic Rock- Coal

__Stratification__- the arrangement of visible layers __Fossils__-the remains, impressions, or any other evidence of plants and animals preserved in rock. __Ripple marks__- formed by the action of winds, streams, waves, and currents on sand, and form on the surface of bedding planes. __Mud crack__s-develop when deposits of wet clay dry up and contract. __Nodules__-lumps of fine-grained silica. They're found in limestones. __Concretions__-round masses of calcium carbonate occurring in layers of shale. __Geode__s-hollow spheres in which ground water has deposited quartz or calcite crystals.
 * Sedimentary rocks have many different features such as:**

 how to use to reference table to identify a sedimentary rock: [|**http://www.schooltube.com/video/59007/Sedimentary-Rocks-Reference-Table**] __**Metamorphic rocks**__ **-** form when pre exsisting rockss undergo heat and pressure. Geologists know this happened because pressure and temperature increase with depth. When temperature or pressure becomes high enough rocks melt and form magma. When high temperature and pressure combine and change the texture, mineral composition, or chemical composition of a rock without melting it, it forms a metamorphic rock. || ||

Metamorphic Rock is the result of the transformation of an existing rock type, the protolith, in a proces called metamorphism. Metamorphic rocks are classified into two textural groups: foliated and nonfoliated. Mineral composition is also used to identify metamorphic rocks.

Layers and bands of minerals characterize **foliated** metamorphic rocks. High pressure during metamorphism causes minerals with flat or needlelike crystals to form with the long axes perpendicular to the pressure. These cause the layers observed in foliated metamorphic rocks.


 * Non foliated** metamorphic rocks are composed mainly of minerals that form with blocky crystal shapes.

An example of a foliated rock - Slate



A non foliated rock - Quartzite




 * Grades of Metamorphism

Different combinations of temperature and pressure result in different grades of metamorphism. Grade of metamorphism basically just means how much heat and pressure the rock goes through.


 * Low-grade** metamorphism is associated with low temperatures and pressure and a particular suit of minerals and textures. High-grade metamorphism is associated with higher temperatures and pressures, and different types of minerals and textures within the rock.

// Types of Metamorphism // The effects of metamorphism can be the result of contact metamorphism, regional metamorphism, or hydrothermal metamorphism.


 * Regional Metamorphism**- when high temperatures and pressure affect large regions of Earth's crust, they produce large belts of regional metamorphism. Results of regional metamorphism include changes in minerals and rock types, plus folding and deforming of the rock layers that make up the area.


 * Contact Metamorphism**- When molten material, such as that in an igneous intrusion, comes in contact with solid rock, a local effect called contact metamorphism occurs. Because laval cools too quickly for the heat to penetrate far into surface rocks, contact metamorphism extrusive rocks limited to thin zones.



**
 * Hydrothermal Metamorphism**- when very hot water reacts with rock and alters its chemical and mineral composition, hydro thermal metamorphism occurs. As hot fluids migrate in ad out of the rock during metamorphism, the original mineral composition and texture of the rock change.



**THE ROCK CYCLE**


 * // Any rock can be changed into any other type of rock. //

The different rocks, igneous, metamorphic, and sedimentary are grouped according to how they form. The rock most likely will change into a different type of rock.

Heat and pressure an change an igneous rock into a metamorphic rock. A metamorphic rock can then be changed into another metamorphic rock or melted to form an igneous rock. The metamorphic rock can be weathered and eroded into sediments that could become cemented into sedimentary rock. This continuous changing and remaking of rocks makes up the Rock Cycle. **

interactive rock cycle website -http://www.learner.org/interactives/rockcycle/diagram2.html