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The Changing Face of Idaho

Paleogeologic Maps of Idaho

By Marty Godchaux (Bio)

This set of sketch maps [below; click for larger images] illustrates what geologists think Idaho might have looked like during various time intervals in the past two billion years of Earth history. Needless to say, our knowledge is less precise for the most ancient configurations than for the most recent ones.

The first four maps (numbered 1, 2, 3 and 3a) include speculative global reconstructions that illustrate the probable tectonic settings of "proto-Idaho" and its relationship to times of formation and breakup of supercontinents Rodinia (Late Precambrian) and Pangaea (Early Mesozoic).

The final four maps (numbered 4, 5, 6 and 7) concentrate on Idaho alone. By the Late Mesozoic, roughly 100 million years ago (m.y.a.), proto-Oregon and proto-Washington had already been added to North America, and Idaho no longer had a Pacific coast. Accretion of the Wallowa and Seven Devils terranes caused folding and thickening of the older rocks and triggered large-scale melting in the middle and lower crust and emplacement of the Idaho Batholith. Continued subduction of oceanic plates to the west fueled the explosive eruptions along the Challis Arc, beginning around 50 m.y.a. in the north and getting progressively younger, to less than 40 m.y.a., near the Nevada border. The rise of the Yellowstone mantle plume triggered the Columbia River Basalt eruptions and the immediately following rhyolitic explosions and subsequent basaltic effusions along the Snake River Plain - Yellowstone trend.

These also are time-transgressive, beginning about 16 m.y.a. near the Idaho-Oregon-Nevada junction and continuing to present-day Yellowstone as North America moved slowly westward over the plume. Continental-scale extension occurred in about the same pattern at the same time, and may also be related to the plume. It produced the Basin-Range Province, which extends all the way from central and eastern Idaho to the trans-Mexican Volcanic Belt. The final set of landscape-modifying events included the incision by major rivers of deep canyons and the inundation of the northern and southern regions of the state by the enormous post-glacial Missoula and Bonneville Floods.

Map 1

Map 2

Map 3

Map 3a

Map 4

Map 5

Map 6

Map 7