Rocks
There are many kinds of rocks. Rocks are fun to collect and study. One of the first steps to identifying rocks is to learn how they are classified. Scientists classify rocks as (1) igneous rocks, (2) sedimentary rocks, and (3) metamorphic rocks. Igneous rocks are formed when magma cools. Magma is melted, or molten, rock material. There are two types of igneous rocks: extrusive and intrusive. Extrusive rocks form when magma is pushed out onto the earth's surface. The extruded magma is called lava. The lava cools and hardens into extrusive rocks. Examples of extrusive rocks are obsidian, pumice, and basalt. Intrusive rocks form when magma cools below the earth's surface. These rocks have coarse mineral grains and crystals. Example of intrusive rocks are granites and syenites. Sedimentary rock is composed of layers which are called strata. Geologists hypothesize that these rocks were made when various sediments were deposited in layers by water, as on the ocean floor or through the activity of a flood. There are three types of sedimentary rocks: clastic sediments, chemical sediments, and organic sediments. Clastic sediments are made from rock material ranging in size from fine sand to large pebbles. For example, shale is made from clay particles, sandstone is made from sand, and conglomerate rock is made from layers of pebbles. Chemical sediments are made from deposits of minerals that were dissolved in water. When the water evaporates, the crystals of the minerals remain. Sodium chloride forms rock salt, calcium phosphate forms phosphate rocks, and calcium sulfate forms gypsum. Some deposits of limestone are formed from calcite crystals. The organic sediments are layers of material from plants and animals. Examples of organic sediments are coal, which is made from plant material, and chalky limestone, which is made from the shells of one-celled animals. Organic sediments often contain fossils which demonstrate that the layers were most likely laid rapidly, as in a flood or other catastrophe. Metamorphic rock is rock that has been changed by forces such as pressure, heat, or chemical reactions. Both igneous and sedimentary rock have been changed into metamorphic rock. For example, granite can be changed into gneiss. The calcite in limestone can be recrystallized to form marble. Sandstone becomes quartzite. Shales become slate. Rocks can be classified as igneous, sedimentary or metamorphic. It is interesting to explore rocks in nature and learn how to classify them.