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An Idaho Timeline: 200 million years to the present

By Bill Bonnichsen (Bio)

Although geologic events that have been part of Idaho's development have gone on for at least 3 billion years, as evidenced by the oldest rocks in the state, most of the action leading to Idaho's magnificent landscape occurred only in the last 200 million years, or so. Some of the main things that happened are included in the accompanying time line. The events shown are by no means all that has happened in Idaho's history; these are only some of the really main events, and can help the reader by being a framework against which other events can be compared.

There are tectonic events in which parts of continents move around and during which mountains are formed, such as the impingement of the Wallowa microcontinent against the western side of North America, and the crustal stretching that occurred throughout western North America that led to the formation of numerous basin-and-range mountain ranges.

The melting of large volumes of rock deep within the earth, time after time, led to the formation of enormous quantities of magma that rose up towards the surface. Those magmas that didn't quite get that far crystallized in the earth's crust, forming large plutons, of which the Idaho batholith is Idaho's prime example, while those magmas that got to the surface exploded or flowed out to cover large areas, such as the Snake River Plain, with lava flows and ignimbrites (volcanic rock layers resulting from explosive eruptions).

Erosion and deposition processes that occur on the earth's surface because of water or ice have always had a major effect on landscape development. In Idaho, these include the deposition of many marine sediments layers before 200 million years ago (so these aren't in the time line), the development of Lake Idaho in the western Snake River Plain, a tectonically-induced depression, and later erosion from ice flowage and the Bonneville and Missoula megafloods that cut some of Idaho's major valleys and canyons.

Finally, just to give some perspective, some of the life events are shown in the time line, including the last really major extinction event, the loss of the dinosaurs, and the eventual rise of man. In the time line, one should note that the scale changes from place to place. This is because more and more space was needed to show all of the events in the more recent history of Idaho's development.