Geology of West Virginia
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West Virginia's geologic history stretches back into the Precambrian, and includes several periods of mountain building and erosion. At times, much of what is now West Virginia was covered by swamps, marshlands, and shallow seas, accounting for the wide variety of sedimentary rocks found in the state, as well as its wealth of coal and natural gas deposits. West Virginia has had no active volcanism for hundreds of millions of years, and does not experience large earthquakes, although smaller tremors are associated with the Rome Trough, which passes through the western part of the state.
The basement rock of West Virginia was formed during the Grenville orogeny, approximately 1,140 to 980 million years ago, when the land mass today known as Laurentia, the ancestral North American craton, collided with other land masses to produce the supercontinent known as Rodinia.[1] The heat and pressure of this collision melted the existing rock, some of which was pushed down into the mantle, while a high chain of mountains formed on the surface. As these mountains eroded, the underlying layers of granite and gneiss were pushed upward by heavier mantle rocks, and exposed at the surface. This Grenvillian basement rock underlies substantially all of the Appalachian region, from the Ohio valley to the sea, and extends north and south into eastern Canada and Mexico, as well as a part of the Scottish highlands.[2]
Today, the crystalline igneous rocks of the Grenvillian basement form the core of the Blue Ridge Mountains, also known as the Pedlar formation, where they are exposed. To the west, they are hidden beneath the Valley and Ridge and Appalachian Plateau, while to the east they extend under the Piedmont, coastal plain, and continental shelf. Over hundreds of millions of years, erosion formed a layer of sediment known as the Swift Run formation, extending over substantially all of West Virginia.[2][3] Although Rodinia began to break up between 850 and 750 million years ago, it was not until about 600 million years ago that the Iapetus Ocean began to open up between Laurentia and its neighbors along the region uplifted during the Grenville orogeny. At that time, the craton was rotated approximately 90° clockwise from its present orientation, and was located south of the equator. The rifting process thinned the crust and led to volcanic activity, resulting in a layer of basalt, today known as the Catoctin formation, burying the existing ridges and valleys.[4][5]
Little of West Virginia's ancient history can be seen at the surface. Only in the extreme eastern panhandle is the Precambrian core of the Blue Ridge Mountains exposed. Except for intrusives, all of the rocks occurring above the Catoctin and Swift Run formations are of sedimentary origin, and most date to the Paleozoic Era.[3]
Paleozoic Era
By the Cambrian Period, approximately 541 to 485 million years ago, a shallow sea had spread westward over most of what is now West Virginia. Marine deposition took place throughout most of this and the succeeding Ordovician Period. During this total interval of about 370 million years, most of the rocks today exposed in Jefferson and eastern Berkeley counties and in scattered areas southwestward along the boundary were deposited. Rocks of the same age are found in abundance in the deep wells throughout the State. The Taconic orogeny near the end of Ordovician time formed a high mountainous area east of West Virginia. Near the end of Devonian time, the sea was rapidly retreating westward and the continental red beds of the Hampshire Formation were being deposited over most of the State. The last significant period of marine deposition was the Greenbrier Formation, predominantly limestone, approximately 330 million years ago. About 310 million years ago, the area of West Virginia was once again above sea level, but covered by vast areas of swamp. This condition prevailed for more than fifty million years, resulting in the deposition of thousands of feet of non-marine sandstone and shale, as well as the many important coal seams that we know today. The Appalachian Orogeny began roughly 270 to 225 million years ago, building the present-day Appalachian Mountains. Most of the state was uplifted, important deposition of sediments ceased, and a gradual process of erosion commenced.