The Explorer's Guide to Impact Craters -- Shocked Gneiss

Gneiss (pronounced “nice”) is a foliated or banded metamorphic rock, which forms when igneous or sedimentary rocks are buried deep (up to several kilometers!) in the Earth's crust and become altered by the extreme heat and pressure in the crust.

On Devon Island, where the Haughton impact structure formed, gneiss was present 2 kilometers below the surface. These over 3 billion years old rocks were then excavated by the impact of an asteroid or comet and brought to the surface where we see them today. The gneiss fragments are actually found as fragments within the pale gray impact melt breccias that line the interior of the Haughton impact structure. Gneiss is normally a dark dense rock, but at Haughton the gneiss resembles pumice stone — an ash-white, porous and very lightweight rock. In fact, some of these fragments float in water! The reason why the gneiss is so light is due to the air spaces or bubbles that formed as the gneiss was compressed by the shock wave, and then released. Certain minerals in the rock were also vaporized, leaving behind a porous "ghost" of a rock, much different from its original state as a dark dense gneiss.

See shocked gneiss in the Haughton impact structure