A - The Grenville


 
The Grenville: A Major Mountain Building Event
 
Late Proterozoic; 1.8 - ~1.1 bya

 
Every building has a foundation, some basement on which it rests. In geology the basement refers to the igneous and metamorphic rocks that form the core or base of a continent, and underlie the younger sedimentary rocks stacked on top. Even though basement rocks underlie all of a continent they are usually only exposed in old mountain ranges where uplift and erosion has stripped away all the overlying rock. In Virginia, the basement rocks are called the Grenville, and they contain the oldest record in our history, a mountain building event of major proportions.
     
Grenville rocks are found extending along the eastern border of North America from Newfoundland to North Carolina. They are the oldest rocks of the east coast and have undergone a long complex history that culminated in the Grenville orogeny (mountain building event) of 1.1 billion years ago. The orogeny occurred when "Africa"(A2) collided and overrode the edge of North America to build a mountain range likely the size of the Himalayas (Mt. Everest is nearly 30,000 feet high).
      
In Virginia, Grenville rocks are most obvious in the Blue Ridge Province. The Blue Ridge province includes both the Blue Ridge mountains (along which Skyline Drive runs) and the strip of land just to the east of the mountain running from Galax north through Charlottesville, Culpepper, Warrenton and beyond. The rocks today are found in an Overturned Anticline (A1) whose core contains the Grenville rocks. Flanking both sides of the anticline are younger lava flows and sedimentary rocks originally deposited on top of the Grenville basement (including the Catoctin lava flows, and Lynchburg and Chilhowee sediments).  These basement rocks formed at depth, indicating that the Blue Ridge Grenville rocks exposed today were once buried far below the current day surface. Only fragments of the deep forming rocks are preserved. 
 
Conditions at the time of formation of these rocks would include very high pressures and temperatures. The pressure would be crushing; normal atmospheric pressure at sea level is 68 kilograms/cm2 (15 pounds/in^2). At only 10 km depth, pressure reaches over 30,000 atmospheres of pressure. The heat would also be intense; the Grenville igneous rocks began as magma (liquid rock), so at the time of their formation we would be surrounded by seething, glowing, red hot liquid rock, swirling and surging in large chambers intruding their way into the mountain core. Not quite the scene we see when driving along Skyline drive today. The metamorphism of the Grenville rocks took place at temperatures and pressures nearly as high as igneous rocks, but for the most part later in Virginia's history.
 
Over the next several hundred million years after the Grenville mountains formed, they underwent erosion and these deep forming rocks were eventually exposed at the surface. It was on this surface that the thick sedimentary pile of the rest of Virginia's history was to accumulate beginning in the Late Proterozoic and Cambrian about 600 million years ago (mya). Eventually this sedimentary deposit reached thousands of feet of thickness.
     
The Pre-Cambrian exposure of the Grenville rocks, however, would not be the same exposure that we see today in the Blue Ridge. The outcrops and rocks we see today required another 500 million years for exposure and included a first rifting event to open a Proto-Atlantic ocean basin, three more mountain building events to close that ocean (one Wilson Cycle), and then a second rifting event to open the Atlantic ocean (first half of a second Wilson Cycle ). During all these events, layers of sedimentary rocks accumulated horizontally. The deposited layers were then deformed into a mountain, and were then eroded down again before the rocks we see in the Blue Ridge today were exposed. 
    
Now we move to the next event in Virginia's geologic history, the rifting that opened the Proto-Atlantic ocean, Stage B or go back to see all the Stages of Geological Evolution
 
Contributed by Lynn Fichter 
Thursday, July 31, 2014
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