Ask a Geologist

Do you have a question about geology in Colorado, or about geology in general? Submit your question here and we will respond by email with a detailed answer from one of our professional geology guides!

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Recently Asked Questions:

What explains the distribution of hot springs around the state? Do they have similar geologic origins? - Ben

Dear Ben,

Hot springs can only form where hydrothermal fluids rise rapidly up through the rock from a depth where temperatures are high. This means the fluid needs to travel quickly and easily from a very great depth. This can only happen if there is a fault in the rocks which allows fluids to move easily through the fractured rock. So hot springs are typically associated with faults.


Dr. Rox

What's the difference between a gemstone and a regular rock? What gemstones are native to Colorado? -Kat

Dear Kat,

A rock is an aggregate of minerals. A gemstone is a precious or semiprecious rock that can be cut, polished, and used in jewelry. So gemstones are rocks, but not all rocks are gemstones. One of the most valuable gemstones found in Colorado is rhodochrosite. This beautiful red gemstone is made of manganese carbonate and is the official state mineral of Colorado. Other gemstones in the Rockies include turquoise, garnet, sapphire, topaz, amethyst, barite, opal, and aquamarine.


Dr. Rox

Are there any areas in Colorado that display the remains of ancient super volcano eruptions that happened in the earth's history? -Joe

Dear Joe,

Indeed the remnants of Yellowstone’s massive past volcanic eruptions can be found in Colorado. These eruptions occurred at 2.1 million, 1.3 million, and 630,000 years ago. Each of these eruptions deposited layers of volcanic ash in the rock record. Ash from these super eruptions has been found as far away as Missouri!


Dr. Rox

What is this stuff called black granite? What is it when it's veined into white granite, or is that something else entirely? - Jessica

Dear Jessica,

This is a great question! Technically speaking, there is no such thing as “black granite”. Granite is a rock composed primarily of quartz, feldspar, and mica and is always a light color, such as white, gray, or pink. The rock that is often referred to as black granite is actually a gabbro. Gabbro and granite are both intrusive igneous rocks, meaning a rock that forms from liquid magma that pushed or “intruded” into older rocks along fractures and then cooled slowly. Gabbro has iron and magnesium rich minerals such as pyroxene which give it a dark color. It is possible for a gabbro to intrude into an older granite, in which case you would see veins of dark rock within the lighter-colored granite.


Dr. Rox

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