Geoscience
   
 
 
Rock is a natural solid composed of multiple crystals of one or more minerals. Although many rocks contain visible crystals of individual minerals, a rock itself does not have an overall crystalline structure. A mineral is an inorganic, naturally occurring solid with a definite chemical composition or element and a specific crystalline structure. A rock is an aggregate of one or more minerals. A rock may also include organic remains and mineraloids. The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, Igneous, Sedimentary, and Metamorphic. Click here to view list of Minerals.
 
 
Igneous Rocks

Igneous rocks are formed from the solidification of molten rock material. There are two basic types:
1) Intrusive Igneous Rocks such as diorite, gabbro, granite and pegmatite that solidify below Earth's surface; and
2) Extrusive Igneous Rocks such as andesite, basalt, obsidian, pumice, rhyolite and scoria that solidify on or above Earth's surface.
Pictures and brief descriptions of some common igneous rock types are shown below. The specimen shown is about two inches (five centimeters) across.
 

Andesite
Basalt
Diorite
Gabbro
Granite
Obsidian
 
Pegmatite
Peridotite
Pumice
Rhyolite
Scoria
Welded Tuff


 
Sedimentary Rocks

Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks:
1) Clastic Sedimentary Rocks such as breccia, conglomerate, sandstone and shale, that are formed from mechanical weathering debris;
2) Chemical Sedimentary Rocks such as rock salt and some limestones, that form when dissolved materials precipitate from solution; and,
3) Organic Dedimentary Rocks such as coal and some limestones which form from the accumulation of plant or animal debris.
Pictures and brief descriptions of some common sedimentary rock types are shown below. The specimen shown is about two inches (five centimeters) across.
 

Breccia
Chert
Lignite
Anthracite
Conglomerate
Iron Ore
 
Limestone
Rock Salt
Sandstone
Shale
Siltstone



 
Metamorphic Rocks

Metamorphic rocks have been modified by heat, pressure and chemical process usually while buried deep below Earth's surface. Exposure to these extreme conditions has altered the mineralogy, texture and chemical composition of the rocks. There are two basic types of metamorphic rocks:
1) Foliated Metamorphic Rocks such as gneiss, phyllite, schist and slate which have a layered or banded appearance that is produced by exposure to heat and directed pressure; and,
2) Non-Foliated Metamorphic Rocks such as marble and quartzite which do not have a layered or banded appearance.
Pictures and brief descriptions of some common types of metamorphic rocks are provided below. The specimen shown is about two inches (five centimeters) across.
 

Amphibolite
Gneiss
Hornfels
Marble
Phyllite
Quartzite
 
 
Schist Chlorite
Schist Garnet
Schist Muscovite
Slate
 



Click here to view list of Minerals