Know Your Rocks
There are three types of rocks found in the earth's crust: Sedimentary, Igneous, and Metamorphic. The type of rock is based mainly on the rock's manner of origin.
Sedmintary Rocks are formed from the compaction or cementation of sediment (sand, silt, clay, shells, etc.). This occurs at the bottom of seas and lakes.
- Limestone outcropping - Made from calcium skeletons of ancient microscopic sea creatures
- Shale - Made from mud and clay sediments
- Sandstone - Made from sand
- Coquina - Made from shell fragments
Igneous Rocks are hardened from magma or lava. Hardened magma, which flows underground, cools to form rocks with large crystals. Lava hardens in the air to form rocks with small or no crystals.
- Granite - Intrusive rock; Medium to large grains, made mostly from quartz and feldspar minerals
- Basalt - Extrusive rock; Small-grained rock made from feldspars and olivine
- Gabbro - Intrusive rock; Similar to basalt only larger grains due to slower cooling of magma
- Pumice - Extrusive rock; Volcanic glass with many air bubbles; the rock can float in water.
Metamorphic Rocks are formed by heat and pressure that changes sedimentary or igneous rocks without melting them. This usually takes many millions of years.
- Gneiss - Foliated; Metamophosed granite
- Slate - Foliated; metamorphosed shale
- Schist - Foliated; Metamorphosed slate or basalt
- Marble - Non-foliated; Metamorphosed limestone
Examples of Various Rocks
Sedimentary
 Shale is compacted mud and clay. |
 Limestone is made from the mineral calcite. |
 Sandstone is made from compacted sand particles. |
 Coquina is made from cemented shell fragments. |
Igneous
 Basalt is made mostly of iron and nickel minerals with tiny crystals. |
 Gabbro is chemically similar to basalt but with larger crystals. |
 Granite is a large-crystal rock made mainly of quartz and feldspar. |
 Pumice is a light-weight rock made from hardened lava. |
Metamorphic
 Gneiss is derived from granite. |
 Schist is a layered rock containing a lot of mica. |
 Slate comes from metamorphosed shale. |
 Marble is a white rock that was originally limestone. |