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Lake Mead NRA volcanic fields

The Cenozoic

(66 million years ago to today) Chaos!

About 18 million years ago a tectonic upheaval began that profoundly marked the present Lake Mead NRA landscape.

Sometime about 18.5 million years ago an episode of major crustal extension stretching and volcanic activity initiated in the Lake Mead region. Hot magma, rising from deep in the crust built imposing stratovolcanoes. Tremendous explosive volcanic eruptions blanketed the region with ash, volcanic debris and lava flows. Deep beneath the volcanoes molten cauldrons of rock called magma chambers supplied the volcanoes above. As new magma chambers and volcanoes formed, older magma chambers cooled to form solid igneous rock masses called plutons.

volcanic dike was once a conduit for hot lava rushing to the surface
Feeder dikes in a cliff.
Bitter Ridge limestone cliffs
Bitter Ridge limestone cliffs

Mineral-rich fluids trapped in older rock were mobilized when hot magma was injected into them. These hot, hydrothermal fluids percolated up through some of the igneous rock formations, including the Paint Pots pluton, imparting their beautiful coloration!

Basin and Range province

The development of the Basin and Range province in southern Nevada began as weakened, stretched crust fractured and faulted. Massive blocks of crust slid down along faults, creating deep basins. The basins filled with sediment eroded from neighboring uplifted blocks almost as fast as they subsided. Extensive lakes formed where water was trapped by intervening mountain ranges. One lake accumulated 300 meters (over 1000 feet) of limestone that was later uplifted to form the striking cliffs of Bitter Ridge (see below). Some areas suffered as much as 15 kilometers of vertical movement in just a few million years. As great blocks of the Earth were tilted and stretched thin, massive igneous rock blobs (plutons) that cooled fairly deep beneath the surface a mere 14 million years ago were uplifted to form mountain tops!

In some areas vertical strike-slip faults developed that slash the crust horizontally, moving massive rock blocks many kilometers from their original position. One of these strike-slip faults cut through the Hamblin-Cleopatra volcano, a sizable stratovolcano, ripping it into two halves that are now kilometers apart! Tilting, uplift, down-dropping and side-by-side movement along Lake Mead NRA's innumerable faults has given its rocks the topsy-turvy appearance you see today.

Miocene gravel beds capped by lava flows
Miocene gravel beds capped by lava flows

During periods when volcanic activity waned, erosion of uplifted rocks and volcanoes created thick gravel beds that alternate with lava flows.

By 13 million years ago, the crust had been stretched and thinned so more iron and magnesium-rich magma, basalt, began to ascend from the mantle. The darker, less explosive lava produced many of the black volcanic flow-capped mesas, such as Fortification Hill and Callville Mesa, seen in many parts of Lake Mead National Recreation Area.

Although volcanic activity and faulting in the Lake Mead region continued until about 2 million years ago, by six million years ago they were declining. However, yet another geologic event was about to begin that would forever change the Lake Mead landscape.

Five million years ago the landscape was dominated by elongate ridges that had shed thick fans of sediment into adjoining valleys. The mighty Colorado river, as we know it today, did not exist. An older river, ancestor to the Colorado, flowed eastward. A small stream that flowed from the Lake Mead region into the Gulf of California had been eroding northeastward bit by bit. Finally, it cut through the cliffs at what is now the mouth of the Grand Canyon near Pierce Ferry and 'captured' the ancestral Colorado River. This is the beginning of the Colorado River as we know it today. It also marks the beginning of erosion of the extensive sediment deposits that had covered up much of the evidence for the complex geologic history of the Lake Mead area. Erosion by the Colorado River has 'opened the book of time', allowing us to probe the secrets of its 2,300,000,000 year rock record and making Lake Mead National Recreation Area a geologist's paradise!


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