Determining Rock Types

March 21, 2014

Uncovering the Past

by Kathleen H. Robbins 

When is a rock just a rock? It’s no riddle. As earth science educators, we know that the answer is never. Rocks are indicators of the landscape and its environment. Rocks tell a story and provide the first hint of what lies beneath. But successfully identifying rocks takes a little time to master. Rock identification helps in understanding their place, purpose, and influence on all that exists in your environment. Identifying rocks lends itself to the discovery of local rock formations. Identifying formations, and placing them in the chronological pattern of local geology, can help in understanding how past geological forces have influenced the evolution of present flora and fauna species.Students will need a foundation to begin the identification process, and understand the difference between crystals, crystalline, and granular.

Download the Activity PDF

Students will need to:

  • Identify crystals
  • Identify clasts
  • Identify crystalline texture

Through this activity, students will understand that there are certain properties common within rock types that will help identify them. Solid background reference materials and examples are needed as you teach the concepts and execute successful identification strategies.


Igneous rocks form when molten rock (magma) from deep within the Earth solidifies. The chemical composition of the magma and its cooling rate determine the final igneous rock type.

Intrusive (Plutonic)

Intrusive igneous rocks are formed from magma that cools and solidifies deep beneath the Earth’s surface. The insulating effect of the surrounding rock allows the magna to solidify very slowly. Slow cooling means the individual mineral grains have a long time to grow, so they grow to a relatively large size. Intrusive rocks have a characteristically coarse grain size.

Extrusive (Volcanic)

Extrusive igneous rocks are formed from magma that cools and solidifies at or near the Earth’s surface. Exposure to the relatively cool temperature of the atmosphere or water make the erupted magma solidify very quickly. Rapid cooling means the individual mineral grains have only a short time to grow, so their final size is very tiny, or fine-grained. Sometimes the magma is quenched so rapidly that individual minerals have no time to grow. This is how volcanic glass forms.

Igneous Rock Samples

Photos courtesy of American Educational Products, LLC, Fort Collins, Colorado.

Banded Rhyolite
‍Pegmatite Coarse


Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth’s surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock.Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Pieces of rock are loosened by weathering, then transported to some basin or depression where sediment is trapped. If the sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock. Clastic sedimentary rocks may have particles ranging in size from microscopic clay to huge boulders. Their names are based on their grain size.Chemical sedimentary rocks are formed by chemical precipitation. This process begins when water, traveling through rock, dissolves some of the minerals carrying them away from their source. Eventually these minerals are re-deposited when the water evaporates or when the rock becomes over-saturated.Biologic sedimentary rocks are formed from the remains of dead organisms. They may accumulate from carbon-rich plant material or from deposits of animal shells.

Sedimentary Rock Samples

Photos courtesy of American Educational Products, LLC, Fort Collins, Colorado.

‍Bituminous Coal
Fossil Limestone
Sandstone Grouping
Grey Shale

Metamorphic rocks are rocks that have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids, or a combination of these factors.Foliated metamorphic rocks are formed when pressure squeezes the flat or elongate minerals within a rock so they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.Non-foliated metamorphic rocks do not have a platy or sheet-like structure. There are several ways that non-foliated rocks can be produced. Some rocks, such as limestone, are made of minerals that are not flat or elongate – no matter how much pressure is applied, the grains will not align.Another type of metamorphism, Contact Metamorphism, occurs when hot igneous rock intrudes into some pre-existing rock. The pre-existing rock is essentially baked by the heat, changing the mineral structure of the rock without addition of pressure.

Metamorphic Rock Samples

 Photos courtesy of American Educational Products, LLC, Fort Collins, Colorado.

White Marble

There are exceptions to every rule of identification. A dichotomous key flow chart serves as a decision tree to assist in eliminating those exceptions. Students can determine the identity of a rock by elimination based on characteristics that the specimen does not possess.

Kathleen H. Robbins

Kathleen is an Earth Science and Astronomy teacher at Harry S. Truman High School in Bronx, New York, and a graduate student in the Environmental Technology program at New York Institute of Technology. Kathleen’s free time is spent fly fishing, hiking, camping, and traveling to explore unique and interesting geologic formations. Kathleen’s teaching practice focuses on hands-on-learning and practical experiences to help students understand the natural world around them.