Rock Classification Sedimentary, Igneous and Metamorphic

All rocks are classified by their origin. Sedimentary rocks are classified in three types, which are inorganic, organic and chemical. All sedimentary rocks form from compaction and cementation. When rain falls down and breaks down existing rocks, this forms sediments which eventually are compacted and cemented together to form a clastic sedimentary rock.

In the rock cycle, sedimentary rocks are created after compaction and cementation. After sediment forms from weathering, erosion and deposition, sediment is created and compacted into a sedimentary rock.

Siltstone is a sedimentary rock composed of silt and has a grain size of 0.006 to 0.004cm. Siltstone is often found in stream deposits and lake beds in Kansas. This rock has a color of grey, brown or reddish brown. Siltstone can form fossils by burial and cementation.


Conglomerate is another sedimentary rock. Conglomerate has rounded pebbles and has a beige, black or brown color to it. These rocks are found in streams and beaches with strong waves. This rock has coarse-grained sediments within fine grained.


Limestone is a sedimentary rock composed of mainly of calcium carbonate in the form of mineral calcite. Skeletal fragments of marine organisms such as coral can be found in this rock. Limestone can be found in shallow water areas.

The map above shows where limestone can be found.
The map above shows where sedimentary rocks can be found in the U.S..

Igneous rocks are another main rock type. Igneous rocks are formed by the cooling or solidification of magma or lava. The two types of igneous rocks are intrusive and extrusive. On the rock cycle, igneous rocks can be found after the crystallization of magma.

The red on the map represents where you can find igneous rocks in the U.S..

Intrusive rocks are formed when magma cools slowly below Earth's surface. Intrusive rocks have large, well-formed crystals. Some examples of intrusive rocks are granite, gabbro and diorite.

Extrusive rocks are formed when magma cools quickly above the Earth's surface. Most extrusive rocks have small crystals. Some examples of extrusive rocks are basalt, andesite and obsidian.

Granite is a light colored igneous rock with large grains. This rock is composed of quartz and feldspar. Granite can be found in the continental plate of Earth's crust because it forms underground.


Gabbro is a coarse grained intrusive igneous rock. Gabbro is dark colored and consists of plagioclase and pyroxene. This rock can be found along mid-ocean ridges.


Obsidian is an extrusive igneous rock formed on the surface of Earth from cooled magma. This rock is a dark colored volcanic glass. Obsidian is composed of felsic lava.


Metamorphic rocks are formed by extreme heat and pressure. Magma can cause any rock to form into a metamorphic rock. These rocks are classified by the texture which is foliated or nonfoliated. Some examples of metamorphic rocks are marble, schist and slate. In the rock cycle, metamorphic rocks are formed from heat and pressure of igneous or sedimentary rocks.

Marble rock forms when limestone is exposed to heat and pressure. Marble is non-foliated and made up of very fine-grained mica.


Schist is a foliated, course-grained metamorphic rock. This rock is composed of layers of different minerals.


Slate is a fine-grained, foliated metamorphic rock composed of clay and volcanic ash. Slate is found where the continental crush is compacted.

The map above shows where you can find metamorphic rocks in the U.S..

Geologists use rock hammers, hand lens and chisel-tip hammers. Rock hammers are used to split and break rocks. Hand lenses are used to help geologists identify rocks and see details. Chisel tip hammers are most often used for sedimentary rocks and fossils.

Rock Hammer
Hand Lens
Chisel-Tip Hammer
Created By
Lucia Toledo


Created with images by epicioci - "stone romania mountain" • James St. John - "Siltstone"

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