Andesite, Atamo village, Bougainville Island, Papua New Guinea.
Where one tectonic plate is forced down under another plate, rocks melt to produce andesite magma.
Source: Museum Victoria
Olivine bomb, Mount Noorat, Victoria, Australia
Olivine bombs are thrown out by volcanic erruptions. The green olivine is surrounded by a layer of dark basalt. Gem-quality olivine is known as peridot.
Source: Museum Victoria
Granite, near Swan Reach, Victoria, Australia.
Granite is derived from rocks melted in the continental crust.
Source: Museum Victoria
Diorite, Saint Cloud, Stearns County, Minnesota, United States of America.
The black band cutting across this granite is an igneous rock called dolerite, which is similar in composition to basalt. When molten, the dolerite magma was forced or ‘intruded’ into the granite deep underground.
Source: Museum Victoria
The amount of silica (silicon dioxide) in magma determines how easily it moves. For example, magma that forms basalt has about 50% silica and is very fluid when it erupts. Rhyolite magma, with about 75% silica, is very sticky and does not flow easily. Andesite is about 60% silica.
Most igneous rocks contain minerals from six groups– quartz, feldspar, mica, amphibole, pyroxene and olivine. These occur in various combinations, depending on the magma composition, and are used to classify and name igneous rocks. The composition of igneus rocks provides clues to where magma forms in the Earth. Many igneous rocks contain small amounts of unusual minerals that can have useful properties.