Rock of the Month 3

Similar in color to its magnetic counterpart, but lacking magnetite.  This rock broke along a fracture, allowing water to seep in and oxidize the rock to a brown color.

Magma can start crystallizing slowly underground, and then erupt as lava and cool quickly.  The rock formed has phenocrysts (large crystals) in a matrix of microscopic crystals.

Chemically similar to the other rhyolites, with small white crystals of quartz and feldspar.  A distinctive erratic on Cape Cod beaches.

Typically gray or pale brown rock, this specimen has large white feldspar phenocrysts.

Obsidian forms as magma is ejected explosively from a volcano, and chills into a rock instantly.  The black color is due to ferrous iron in the magma

This gray example has a gray groundmass with abundant white feldspar phenocrysts.

Fine-grained dark volcanic rock.  This specimen is rich in magnetite (iron ore), which makes the rock magnetic.

Microscopic quartz crystals make these pebbles hard enough to scratch glass. The bands are different lava flows.  Red color signifies oxidized iron.