Basalt Day, or Geology of Georgia, Continued (Part of a Very Occasional Series)

Basalt is both one of the most common rocks on planet Earth and at the same time one of the most unworldly rocks imaginable. The Universal Solar Calendar identifies January 6 as Basalt Day. 

Basalt, my friends, is basically solidified lava. As a gross oversimplification, the in-ground, unerupted portion of the lava mass is called magma and solidifies as granite. The above-ground, extrusive portion after eruption is the actual lava and solidifies as basalt.

The cool thing about basalt is that when it solidifies, due to its mineral content, it can form into distinctive hexagonal columns. In some cases, cliffs, mountains, and fields of basalt columns can look almost sculpted or architectural, as if they were designed or built by some higher intelligence (pro tip: they weren't - it's just columnar jointing, not evidence of the existence of gods or aliens). The basalt columns of Devils Tower in Wyoming were used as a symbol of extraterrestrial intelligence in the 1970s' film Close Encounters of the Third Kind.       

While much of the Pacific Northwest is covered by basalt, it is rare in Georgia. However, a form of basalt known as diabase occurs here in the form of dikes and sills. Before you start snickering over the terms, "dikes and sills" are basically solidified lava that filled in cracks and fissures in the country rock as the below-ground magma moved toward the surface.  Since the filling in the cracks and fissures is obviously younger than the rock that was cracked, the diabase in dikes and sills can be used to estimate the age of the surrounding rock.

In Georgia, a geologic feature known as the Brevard Zone cuts diagonally across the northern part of the state. It would take a whole other blog post (or posts) to describe what the Brevard is or isn't, but suffice it to say for now that it is a fault-like structure of uncertain origin.  The rocks within the Brevard are so highly deformed and metamorphized that it's almost impossible to say what exactly went on in there. But the zone is cross-cut by diabase dikes and sills, so we know that whatever happened, happened before the appearance of  the magma body that formed the diabase dikes and sills.

Isotopic age-dating of the diabase tells us that the diabase is of Triassic age.  Cool, dinosaurs.  Whatever happened within the Brevard Zone was long since over before the appearance of Triassic dinosaurs.  And we know that because of basalt.

I know it sounds contradictory because I said that basalt is rare here in Georgia, but the largest body of basalt rock in the United States occurs here in Georgia. The problem is, it's in South Georgia beneath some 2,500 feet of Coastal Plain sediment. The South Georgia Rift basin is a massive feature extending from southern Alabama, across the State of Georgia, and into the southernmost past of South Carolina and on into the Atlantic basin.  Geophysical measurements and the rare deep wells have confirmed that the South Georgia Rift basin is full of basalt. The basin is probably a remnant of the initial opening of the Atlantic Ocean as North America began to separate from Africa during the Triassic, and those diabase sills and dikes that cut across the Brevard were likely fed by the same upwelling mantle that forced the basin to open.

It's Basalt Day. I pause for a moment to reflect on its presence here in my home state and contemplate what it tells us about the most ancient of histories of this state.  The mountains can tell stories to those who know how to read the rocks, and the shy, reticent basalt mostly hidden in the subsurface of Georgia has important things to add to the story.