Volcanic glass

Magma rich in silica and poor in dissolved water is most easily cooled rapidly enough to form volcanic glass.

As a result, rhyolite magmas, which are high in silica, can produce tephra composed entirely of volcanic glass and may also form glassy lava flows.

[2] Ash-flow tuffs typically consist of countless microscopic shards of volcanic glass.

Tachylite is opaque to transmitted light because of the abundance of tiny oxide mineral crystals suspended in the glass.

Alteration of volcanic glass at mid-ocean ridges may have contributed significantly to the formation of massive sulfide deposits, and alteration of volcanic ash beds formed economically important zeolite and bentonite deposits.

A sand grain of volcanic glass under the petrographic microscope . Its amorphous nature makes it disappear in cross-polarized light (bottom frame). The scale box is in millimeters.