Very “Gneiss” Rock – June 24, 2020
Last weekend, Ricardo and I ventured to the south of Colorado to climb Culebra Peak (14,047 ft.) and Red Mountain (13,908 ft.) for the Summits, Songs and Science project. Culebra is the southernmost 14er (peaks above 14,000 ft./4,267 meters) in Colorado and also in the entire Rocky Mountain range, while Red Mountain is the southernmost centennial 13er. The peaks are less than 10 miles from the Colorado/New Mexico border and, as they are on private land, permits must be obtained in order to hike them.
When climbing peaks, different rock types provide different experiences: some are so resistant to erosion that they are quite solid and trustworthy underfoot. Others can crumble and tumble. Exposure to extreme conditions like high heat and pressure can alter the mineralogy, texture, and chemical composition of rocks. Due to the action of plate tectonics, compression, stress and shearing forces over long periods of time, rocks can be essentially warped and deformed, causing them to be compacted into a smaller volume of space.
Along the way to the summit of Culebra, we saw many impressive examples of hard and deformed metamorphic (transformed) granite called “gneiss” (pronounced like “nice”). They are distinct given alternating bands of light and dark minerals, and they often have the appearance of frozen flow (think about when you squirt toothpaste out of the tube and what that looks like). While the light is more felsic (meaning minerals made of lighter elements like silicon and oxygen), the dark is more mafic (containing minerals richer in iron and magnesium). In order to form this type of metamorphic rock, temperatures needed to reach at least 550 degrees Celsius and the region needed to undergo high pressure. The presence of these rocks is testament to incredible pressures and temperatures in the region, tied to mountain-building events (orogenies) of the past. The rocks we were hiking across? They’re 1.7 billion years old and were uplifted between 80 – 35 million years ago, now soaring into the sky and topping out at just over 14,000 feet.
photo: U. Horodyskyj