English: Melt matrix impact breccia from the Precambrian of South Africa. (cut & polished surface; field of view 6.3 centimeters across)

The number one largest impact feature on Earth is the South Africa's Vredefort Impact Structure, which has a ~300 kilometer preserved diameter. The sample shown here is an impact melt rock from the Vredefort Granophyre. Such rocks have been called impact melt breccias or melt matrix breccias or impact melt rocks or tagamites. The dark-colored matrix of this rock lacks a clastic texture. The black material is glassy-textured to finely-crystalline textured, and formed by cooling & solidification of impact-generated melt.

The large, light-colored clasts in the Vredefort Granophyre are typically composed of quartzite and granite that have been recrystallized to varying degrees, but other lithologies have also been reported. Some of the clasts contain quartz having planar deformation features (PDFs), widely accepted to be diagnostic of an impact event.

The Vredefort Impact occured during the mid-Paleoproterozoic; it has been well dated to 2.023 billion years. The original crater and impact breccia that filled the crater eroded away long ago. The rocks now exposed in South Africa are materials estimated to have been ~7-10 kilometers below the original surface.