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USGS Geology in the Parks

Evidence from the oldest rocks

basement rock

The Precambrian (4,550,000,000-570 million years ago)

If you could step back in time 1.8 billion years, you would not recognize our planet. At that time much of the western part of the North American continent did not exist! The western edge of the continent was located just to the west of what is now Lake Mead NRA. Offshore, an active volcanic island chain erupted sporadically, spewing ash and volcanic debris. Ash and sediment shed from the eroding volcanoes collected in thick layers on the sea bottom. Carried along on the moving veneer of rock that geologists call plates, this island chain collided with the continental margin of North America. The force of the collision welded the volcanic islands and associated debris onto North America. Intense heat and pressure changed the volcanic rock and sedimentary debris into metamorphic rocks; hard, crystalline rocks that form part of the core or basement rock of our continent (see rocks to left)

basement rock

There is little evidence preserved to tell us what happened during the 400 million years following this early continent-building collision. The next major event recorded in the rocks occurred when hot granitic magma intruded into the older metamorphic rocks about 1.4 billion years ago. The hot magma eventually cooling to form solid granitic igneous rock enclosed within the surrounding metamorphic basement.

Over the next 900 million years, the deeply buried metamorphic rocks were uplifted. As they rose, the rocks on top were eroded away, so we have no rock record of that time. This nine hundred million year gap is called the 'Great Unconformity'. This is the same Great Unconformity you may have seen in the Grand Canyon.

Precambrian world

By 650 million years ago, most of North America had been planed off to a nearly flat surface with some areas of gently rolling hills. Picture the environment at Lake Mead NRA at about 650 million years ago. North America was situated south of the equator, surrounded by mostly temperate seas. What will become Lake Mead NRA lies along the shore of this ancient ocean. The shoreline would have appeared stark and desolate because land plants and animals had yet to evolve! Only a narrow fringe of marine algae would have adorned the weathered rock surfaces along the shore.

On to the Early Paleozoic...

To learn more about this time period at Lake Mead NRA click here.
To learn more about the life on Earth during this time period click here.
View of the continents 650 million years ago.
earth 650 million years ago


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