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Why is that rock black?

A cache by NOSRocks Send Message to Owner Message this owner
Hidden : 05/02/2011
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Geocache Description:

Explorers who mapped the routes through the Black Rock Desert, and the emigrants who followed them, crawled along at a two-mile an hour pace in wagons to this prominent landmark now known as Black Rock Point. This natural feature is so distinguishable due to its unique coloration, but why is that rock black?

In the Paleozoic Era (before dinosaurs roamed the land) the ancient coastline of what would eventually become North America ran east of Nevada. West of the coast was a subduction zone where the ocean floor sank into a trench and under the continental crust. The denser ocean crust melted as it sank into the upper mantle and created volcanism. These volcanoes off the coast created a volcanic island arch, similar to the Japanese Islands.

As the plates moved together the island chain eventually ran into the coast of the North American continent and expanded the western coastline. Much of the western US is what geologists term ‘accreted terrain” or exotic material that collided with the continent and added land mass. This newly-attached land contained oceanic sediments entwined with volcanic rocks.

Black Rock Point, the desert’s namesake, is part of an ancient island chain. From a distance its black color can trick you, it looks like just basalt, but the Black Rock is actually composed of fingers of volcanic rocks and black limestone. If you explore the rock, you may even find marine fossils!

Remember to Tread Lightly! by avoiding sensitive areas, which includes not disturbing historical, archeological, and paleontological sites.

The Black Rock Desert is part of the Bureau of Land Management’s Black Rock Desert – High Rock Canyon Emigrant Trails National Conservation Area. Many organizations come together to help conserve this exceptional natural place including the BLM, Friends of Black Rock High Rock, Friends of Nevada Wilderness, and Nevada Outdoor School.

E-mail NOSRocks the answers to these questions. Plus, if you would like to share a photo from your geocaching adventure add it to your log – we’d love to see it!

Take a closer look at the basalt. Find a piece of this igneous rock, it formed from the solidification of molten rock material. Looking at the rock, what is the grain size? Is the rock porous? Describe the coloring.

Black Rock Desert - High Rock Canyon Emigrant Trails National Conservation Area: Geology of the Black Rock Desert – High Rock Canyon Area. [Brochure]. Nevada: Bureau of Land Management.

Black Rock Desert - High Rock Canyon Emigrant Trails National Conservation Area: The Black Rock Playa. [Brochure]. Nevada: Bureau of Land Management.

Collard, C. (2004). Applegate-Lassen Emigrant Trail - The death route of 1849. Retrieved May 19, 2010, from (visit link)

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