Resource: Plate Tectonics: Lake Mead, Nevada

Features on the surface of Earth provide evidence of past events as well as ongoing processes in our planet's geologic history. Sometimes the ongoing processes uncover evidence of events in the more distant past. For example, as the Colorado River carved out the Grand Canyon, it uncovered 400-million-year-old rocks and exposed evidence of ocean advances and retreats, as well as the history of plant and animal evolution during that time. However, ongoing processes may also destroy evidence of past events, eroding or otherwise transforming geological features and making the task of unraveling geologic history a difficult one.

Like countless regions of the world, the area surrounding Lake Mead, Nevada, has been the scene of a multitude of geologic events. For example, there is evidence that about 2 billion years ago this area formed the western coast of North America. Subsequently, the continent collided with and connected to a volcanic island chain, was flooded several times by inland seas, experienced numerous periods of uplift and erosion, and joined with other continents to form Pangaea, before breaking apart again.

Despite all these events, the geological events of the past 18 million years are the ones that have most clearly defined the region. During this time, movement of Earth's tectonic plates caused explosive volcanic eruptions. At times, this activity blanketed the area with ash, debris, and lava flows. The movement of tectonic plates also stretched the continental crust, causing fault lines to open and the land to break into tipped block formations with high peaks and low valleys. What's more, scientific evidence suggests that this area of stretched crust, called a rift zone, continues to pull apart today. If so, the area around Lake Mead, a human-made reservoir, may one day be the site of a natural inland sea and the boundary between two diverging continents.

To learn more about how the movement of tectonic plates creates rift valleys, check out Plate Tectonics: An Introduction.

To learn more about the rich geologic history of America's Southwest, check out The Grand Canyon: How It Formed.

To see where Earth's many tectonic plates are diverging and converging, check out Tectonic Plates and Plate Boundaries.

• What is happening to the North American continent near the longitude of Lake Mead?
• In the past 20 million years, what has happened to the distance between Los Angeles and Las Vegas?
• What feature of geological significance does Lake Mead sit atop?
• Dr. McNutt refers to 20 million years as being a very short time. What time frame is she using?
• What does Dr. McNutt mean when she says that the continent is falling apart?
• Using simple materials, such as dominoes and a stretchable fabric, model the creation of slanted layered rock formations.