CSM Rock Collection

Andesite, IG04AD01
Location: Unknown

Description

Andesite is the characteristic rock that is produced by melting in tectonic subduction zones. Andesite is rich in water and generates the characteristic stratovolcanoes that dominate volcanic island arc and continental arc terranes. Andesite volcanism is typically very explosive leading to the production of large quantities of pyroclastic and volcanoclastic rocks.

Hand specimen

Specimen size: 110 x 65 x 60 mm
The hand specimen displays white and black crystals in a grey matrix. This matrix is composed of volcanic glass that is an intermediate shade of grey indicating a composition between basalt (dark) and rhyolite (light grey). The glass should not be identified as a mineral, as represents the parts of the magma that did not have time to crystallize. The white and black crystals are called phenocrysts, and the texture of the rock is porphyritic.

Field of view: 30 x 22mm
With a hand lens, it is possible to examine the phenocrysts. The white mineral has a prismatic shape and displays two good cleavages, the mineral is twinned and harder than steel. The black mineral is difficult to determine, as the grains are so small. In the larger crystals, it is possible to determine that the hardness is greater than copper, and roughly equal to that of steel. It has two directions of cleavage that intersect at around 120/60º.

Thin section

Thin section in plane polarised light (full section)

Plane polarised light image. Geologists will be taught how to use the microscope during term 2.

Thin section in cross polarised light (full section)

Cross polarised light image. Geologists will be taught how to use the microscope during term 2.