Scratch it, drip acid on it, even lick it: how geologists identify minerals
A geologist's toolkit is surprisingly hands-on. Rubbing a mineral across unglazed porcelain leaves a coloured streak that betrays its identity. A drop of hydrochloric acid makes some fizz. A magnet picks out others. And halite gives itself away the simplest way of all: it tastes exactly like table salt, because it is.
Geology, from Greek words for earth and study, examines the rocks that make up Earth and other bodies, including meteorites, and the processes that change them. A mineral is a natural element or compound with a fixed chemical makeup and an orderly arrangement of atoms. Colour helps identify one but can mislead, since impurities tint many specimens. Hardness measures resistance to scratching, lustre describes how a surface reflects light, from metallic to waxy, and breakage either follows neat parallel planes, called cleavage, or leaves ragged fracture.
Rocks are aggregates of minerals, and they come in three families linked by a cycle. Igneous rock hardens from magma or lava. Weathering breaks rock into grains that settle and harden into sedimentary layers such as sandstone, shale, carbonates and evaporites, which host most coal, oil and gas. Heat and pressure transform either kind into metamorphic rock with a new internal fabric, and anything can melt and start over.
The great breakthrough came in the 1960s, when researchers established that the rigid outer shell is cut into plates gliding over a softer but still solid layer of mantle. Seafloor spreading and the global pattern of mountains and earthquakes supported the idea. Ridges on the ocean floor mark plates pulling apart, arcs of volcanoes mark one plate diving beneath another, and faults like San Andreas mark plates grinding sideways. The theory finally supplied a mechanism for Alfred Wegener's continental drift and is often called geology's grand unifying theory.
Nobody can dig to the core, so seismologists read earthquake waves instead. Because shear waves cannot pass through liquid, early studies revealed a molten outer core around a dense solid inner one, with mantle boundaries at 410 and 660 kilometres down. Since the 1970s, full-waveform inversion has produced images of the interior much as a CT scanner images a body, and laboratory experiments recreate deep pressures to explain what the waves detect.
Source: Geology
