Igneous, Metamorphic, Sedimentary Rocks

Geologists group rocks into three categories based on their origin: igneous, sedimentary and metamorphic.

Igneous rocks solidify from a molten or partially molten state.  These rocks are characterized as either extrusive or intrusive.  Extrusive igneous rocks solidify from molten material that flows over the earth’s surface (lava).  Extrusive igneous rocks typically have a fine-grained texture (individual minerals are not visible unless magnified) because the lava cools rapidly when exposed to the atmosphere, preventing crystal growth.  Common extrusive rocks are basalt, andesite and rhyolite.  Intrusive rocks form from molten material (magma) that flows and solidifies underground.  These rocks usually have a coarse texture (individual minerals are visible without magnification) because the magma cools slowly underground, allowing crystal growth. Common rock types within the intrusive category are granite and diorite.

Erosion and deposition play a key part in the formation of sedimentary rocks.  Wind, water, ice, and chemicals break down existing rock into sediment that is then transported and deposited by wind, water, and glaciers.  As sediment accumulates with time (thousands of years) it becomes compacted and cemented (lithified), eventually forming rock.  Some common sedimentary rocks are shale, sandstone and limestone.


Metamorphic rocks are any rock type that has been altered by heat, pressure, and/or the chemical action of fluids and gases.  Metamorphic rocks are classified by their structure and their dominant minerals.  Metamorphic rock structure is either foliated (has a definite planar structure) or nonfoliated (massive, without structure).  Common foliated metamorphic rocks are slate, phyllite, schist, and gneiss.  Common nonfoliated metamorphic rocks are quartzite and marble.