Metamorphic Rock Gneiss

Mineral Sciences

Catalog Number:
401463
Object/Specimen Description:

These metamorphic rocks are garnet gneiss. The garnet fragments are the round dark brown mineral in the rocks. The rocks measure around 12 cm x 10 cm x 6 cm in size or less.

Specimen Count:
2
Collector:
A. Blankenbicker
Precise Locality:

Morrison Quarry

Locality:
US High Plains and Rocky Mountains (ID, MT, ND, SD, WY, CO)
Collecting Date:
May 12-16, 2014
Collecting Locality:
North America, United States, Colorado, Jefferson County
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High pressures and temperatures within the Earth's crust can change rocks from one type to another. Scientists call this process metamorphism, which comes from the Greek words for "after" and "form." At the junctions where the Earth's crustal plates collide, pushing up mountain ranges, or where one plate slides underneath another, the original rocks, or protoliths (from the Greek words meaning "first" and "rock") undergo reactions that change the chemical or crystal structure of the rocks with little or no actual melting. The grains of minerals within many metamorphic rocks are aligned in parallel due to the forces pushing on them. The pressure-temperature combination causes some sedimentary rocks to lose water molecules or become anhydrous.

Artist interpretation of the Laramide Orogeny, which led to the creation of the Rocky Mountains in the Western United States.
Cross-section and geologic map of the geology near Morrison, CO.
Looking down into a rock quarry in Morrison, CO. All of the rock removed is gneiss to be used in roads, buildings, and other construction projects.
Magma rich in silica was injected into cracks in the older gneiss, creating this silicic intrusion, mainly consisting of pink feldspar and white quartz at its core. Location: Morrison, CO.
Touch a cliff face with rocks separated by 1.4 billion years in age, at a quarry in Morrison, CO. Sandstone on the left (south), schist on the right (north).
Detailed geologic time scale.
Graphic representation of geologic time, with some well-known fossils

The continental crust in Colorado was constructed from several island chains that collided and merged together about 1.75 billion years ago. It was then metamorphosed at depth under heat and pressure, and magma was injected through cracks in the rock. After several million years of erosion the rocks of the Morrison Quarry became exposed at the surface. Layers of sandstone were deposited directly on top of the exposed rocks forming what is known as the "Great Unconformity," a 1.4-billion-year gap missing between one rock layer and the next. These rocks were likely uplifted during the formation of the Ancestral Rockies between 320 and 270 million years ago. The Ancestral Rockies were eventually eroded away, buried by sediments and flooded by shallow oceans. They were brought to the surface once again during the growth of the current Rocky Mountains.

Artist interpretation of the Laramide Orogeny, which led to the creation of the Rocky Mountains in the Western United States.
Cross-section and geologic map of the geology near Morrison, CO.
Touch a cliff face with rocks separated by 1.4 billion years in age, at a quarry in Morrison, CO. Sandstone on the left (south), schist on the right (north).

Rocks record earth's history throughout time, and typically younger rocks are found above older rocks as sediments are deposited year after year. Occasionally erosion will erase rocks away and along with them a history of when they were formed. The Great Unconformity is a feature observed in Colorado where 300-million-year-old sandstone rests on top of gneiss that is about 1.7 billion years old. What happened to the 1.4 billion years in between? A record of that time was washed away, so no one can say for sure what happened in this area during that time.