Hobbies And Interests
Types of Rock
All the rocks on the planet can be grouped into one of three primary types. Igneous rocks are formed by the cooling of magma or lava. The second type is called metamorphic rock. These rocks are formed deep within the Earth by extreme pressure, high temperature, hot fluids or a combination of these factors. The final group is called sedimentary rock. Rocks comprised of smaller pieces that are bonded together fall within the sedimentary rock group.
Sedimentary Rock
Sedimentary rocks are formed from pieces of pre-existing rock or once living organisms that accumulate in layers on the planet's surface. There are three types of sedimentary rocks. Biological sedimentary rocks are comprised of a large amount of organic material. Chemical sedimentary rocks are formed when water dissolves and removes certain minerals. The final type is called clastic sedimentary rock. This type of sedimentary rock is comprised of many pieces, or clasts, of pre-existing rock, called detritus that have become bonded together.
Clastic Sedimentary Rock
Rocks comprised of stuck together rock pieces are called clastic sedimentary rock. Clastic rocks are formed when detritus accumulates in a location, typically transported there by water flows. This accumulation becomes buried, and the growing pressure compacts the pieces together into a rock layer. Clastic rocks are named according to the size of their clast: clay, silt, sand, pebbles, cobbles and boulders.
Clay Clastic Rock
Clay represents the smallest grains, or clasts, of detritus. These grains are less than 0.0039 millimeters. Examples of clay clastic rock include claystone, mudstone and shale.
Silt Clastic Rock
Silt represents the next smallest grains of detritus. Silt is comprised of grains that are between 0.0039 and 0.0625 millimeters. Clastic rocks formed from silt are called siltstone. At times, clay and silt clastic rocks are combined into one group, called mud clastic rocks.
Sand Clastic Rock
Sand represents the next largest grains of detritus. Sand clastic rocks are comprised of grains that are between 0.0625 and 2.0 millimeters. Clastic rocks formed from sand are called sandstone.
Gravel Clastic Rock
The three largest clasts are generally considered gravel clastic rocks. Within this group are rocks formed from pebbles, cobbles and boulders. Pebbles range in size from 2.0 to 64 millimeters, cobbles from 64 to 256 millimeters, and boulders are pieces of rock larger than 256 millimeters. Rocks comprised of clasts of these sizes form conglomerate or breccias. The large pieces are held together by a sand or mud matrix, which serves as a kind of cement to bond them together.