Rocks and Geology Around
Las
Vegas
Rocks and Geology Around Las Vegas is intended to give a hiker's-level overview of the
mountains and rocks around Las Vegas, giving hikers and botanists enough information to recognize the main types of rocks and basic
information on how they formed. Most people look at rocks as unchanging, something that always has been and always will be. The
rocks we see today, however, are part of a process where things change over time, and when we understand rocks as part of a process,
our appreciation of the world around us -- how it got that way and how we fit with it -- is so much greater.
There are only three basic types of rock: Sedimentary, Volcanic, and Metamorphic.
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Sedimentary Rocks, the most common rocks around Las Vegas, are formed by two processes. First,
sedimentary rocks are formed by cementing together particles that eroded from pre-existing rocks. Particles are washed
or blown away to accumulate somewhere else, and if left in place long enough, can become cemented to form rocks such as
conglomerate, sandstone, and mudstone, depending on the size of the particles and the degree of sorting of the eroded materials.
Second, sedimentary rocks form on the bottom of the ocean when particles "rain" down from the surface. These particles can
become compressed and cemented to form limestone. Fossilized sea creatures are often found in these rocks.
Most of the mountains around Las Vegas are composed of sedimentary rocks.
Red Rock Canyon (photo) provides spectacular examples of both types: the gray
mountains are limestone, and the red-and-white hills are sandstone. |
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Igneous Rocks, less common around Las Vegas, are formed when magma (hot, molten material from
deep within the earth) cools. When magma pours out of a volcano and cools at the surface, it typically forms lava. When magma
explodes out of a volcano, it can form fragmented (pyroclastic) rock, ash-flow tuffs, and volcanic glass (obsidian). When
magma cools slowly deep underground, it forms granite.
Fortification Hill (photo) on the east side of
Lake Mead provides a spectacular view of igneous rocks. The black hill
is lava from an ancient volcano (background), the top of which eroded away to reveal the light-colored granitic core. Out
at Lake Mead, the dark-colored mountains along the Colorado River are volcanic. Granite is uncommon in southern Nevada, but
occurs out in the Gold Butte Region (north of Lake Mead along the Nevada-Arizona border). |
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Metamorphic Rocks, uncommon around Las Vegas,
are formed by changing the structure of pre-existing rock. This usually is due to high pressure and temperatures deep within
the crust of earth, but rocks can be chemically altered. Metamorphic rocks are never melted in the process, just heated and
squeezed such that the minerals in the original rock are changed.
The type of metamorphic rock that results from the process depends on the original rock and the degree and type of
metamorphism. For example, sandstone changes to quartzite, clay-rich sedimentary rocks change to schist, and granite changes
to gneiss. Limestone is chemically changed to dolomite when some of the calcium carbonate is replaced with magnesium
carbonate. |
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