Extrusive igneous rocks typically have no or very small crystals such as pumice or obsidian. Intrusive rocks usually have visible grains such as granite or monzonite.

Igneous rocks make great stocking stuffers for geology fans. Shop today for your favorites.

Graphic granite is a form of granite that consists of parallel, rod-shaped quartz crystals in a matrix of feldspar. The quartz grains have an appearance of written script characters when a sample is cut perpendicular to the quartz grains, which gave the rock its name. Get your sample today!

Volcanic is composed of clay to sand-sized particles of rock and volcanic glass. This material is expelled from a volcano during a violent eruption. Steam buildup in a volcano causes violent eruptions that shred the surrounding rock into small, ash particles. Get your sample today!

Question:

Answer: The three basic rock types are Igneous, Metamorphic and Sedimentary.  Igneous rock form from hot liquid magma. The magma either erupts from a volcano, cools and hardens into extrusive igneous rocks, or forms intrusive igneous rock when the hot magma is trapped beneath the earth’s surface, cools and hardens.

Sedimentary rocks are formed when sand, small pieces or rock, or mud are pressed together to form layers of sediment. These layers of sediment are pressed together over a long period of time and form a sedimentary rock.  A metamorphic rock is formed when a sedimentary or igneous rock changes form because the temperature or pressure changes. This change typically occurs over time when the sedimentary or igneous rocks are buried beneath the earth’s surface.

Read the article titled Geology is All Around You in the “Dig Into Geology” section of our website for more information on these rock types, minerals and the geologic time scale.

Question: What are the three big groups of rocks?

Answer: The three basic rock types are Igneous, Metamorphic and Sedimentary. See the article titled Geology is All Around You in the “Dig Into Geology” section of our website for more information on these rock types.

Look at this photo.  Do you see the white rocks that look like they cut through the gray rock?  These are igneous rocks that were probably injected from a magma source underground into the gray rock above, likely through cracks in the rocks.  Rock formations can tell you a great deal of information about he history of an area.

Question:  What are some household items made out of igneous rock?

 Answer:  Many building materials are made of granite such as counter tops in homes.  Pumice, shown here, is also used in products such as pumice stones which are used to smooth rough feet and in Lava Soap which is used to remove grease from hands.

 For more great Ask-a-Geologist questions go to our website.  To ask our geologist your own question, click here.