The Earth Story

Ancient Impact

Over 2 billion years ago, a piece of space debris slammed into what today we know as South Africa. That impact created the Vredefort structure, a 250 to 300 kilometer wide crater that dominates the geology of that area. Much of the Vredefort structure has been destroyed by tectonic processes since that impact, but about 1/3 the rim and part of the interior remains.

Today that impact structure is drained by the Vaal River, but the area still bears the scars of that impact right down to the smallest scale. During an impact, a shock wave propagates through the target minerals and can alter or deform their structure. In the mineral quartz, one of the most common minerals in continental crust, offset planes called “planar deformation features” are developed: finding this structure can be diagnostic for finding an ancient impact structure.

Planar Deformation Features show up in these microscope images of quartz; the planar offset patterns are highlighted by arrows. The interesting thing about these grains is they’re not found in the crater itself, they’re found in the Vaal River. These scientists went far downstream of the crater and took scoops of the sediment to see if shock features survive sedimentary transport, even if they’re 2 billion years old, and found that quartz and other minerals do in fact preserve evidence of the ancient impact even when transported downstream. Finding similar shocked grains in other river basins around the world could therefore be used as evidence of ancient impacts somewhere in the river basin being sampled.

-JBB

Image credit: Cavosie et al. (2010) http://gsabulletin.gsapubs.org/content/122/11-12/1968.abstract All scale bars are 250 μm

Shocked Quartz

This is a grain of quartz, one of the most common minerals on Earth, viewed under a microscope. The grain is sand-sized, about 0.13 mm across.

The color variation in the grain occurs because the grain is a bit round, it is thinner at the edges and so the edges interact less with light that travels through than happens at the thicker core of the grain. But that’s not the shocking part of this shot.Those lines are called “planar deformation features” or “pdfs” (we had that abbreviation before Adobe, I promise). They form when intense shock waves travel through a quartz grain and partially disrupt the mineral structure. In other words, PDFs in quartz indicate that the grain was exposed to a large impact.

This quartz grain was found in a drill core that traveled through the Chesapeake Bay impact crater. Finding features like these in quartz grains can be close to diagnostic for verifying that an impact took place in some area, even with buried or eroded craters.

-JBB

Image credit: USGS http://geology.er.usgs.gov/eespteam/crater/shockquartz.html

Read more: http://www.sciencedirect.com/science/article/pii/0012821X9190066Q