Hey! A Sedimentary Rock Looks Like an Igneous Rock!

Q: I encountered a sedimentary rock in lab called Greywacke and it reminded me of an igneous rock, Gabbro. How does one differentiate these 2 apart? Or basically igneous from sedimentary and vice versa.

- Feiruz R

A: Igneous rocks were formed when the material was a melt, so the individual mineral grains are tightly fused and intergrown, and the porosity in the rock is very low. A greywacke was formed by accumulation of cold, weathered detrital material. It might superficially LOOK like an igneous rock, but a porosity test would give it away immediately. A closer examination with a hand-lens will show angular grains in a greywacke that do not interlock seamlessly; the word "greywacke" means that this rock also includes very fine silt, so this tends to fill those inter-grain boundaries between the larger crystals - but with a hand-lens you can see this. A petrographic microscope makes it even more obvious that a greywacke is really a garbage can term, representing an accumulation of weathered material of all different grain sizes.