Explore the Deep Sea

Volcanoes & Vents

Hot Rocks

Overview of how mineral deposits and igneous rocks are formed at mid-ocean ridges and hydrothermal vents.

Crystals depend on chemicals & cooling

Basalt from the seafloor that has cooled so quickly it appears almost glassy. Image courtesy of K. Kusek

As molten rock cools, different sorts of crystal form in it. The type of crystals depend on the chemical composition of the rock. For example, magnesium-rich rocks often contain many crystals of a greenish mineral called "olivine." The size of the crystals depends on the rate of cooling: the slower it cools, the larger the crystals can grow before the rock solidifies. Molten rocks that cool very rapidly contain only tiny crystals, or none at all (e.g. obsidian). Granite is an example of a rock that has cooled fairly slowly: many crystals are big enough to see with the naked eye. Basalt has a different chemical composition than than granite (it contains less silica), and has cooled faster. The above picture is of basalt which has cooled so quickly it appears almost glassy. Some famous natural landmarks are made of basalt, such as the Giant's Causeway in Ireland, the Organ Pipes in Victoria (Australia), and the Devil's Tower in Wyoming (USA).

Bright colors

Thin section of ferroandesitic rock under cross-polarized light. Image courtesy of M. Perfit

If you cut rocks into very thin slices (thin sections) and shine particular sorts of light ("polarized light") though them under a microscope, you will often see brilliant colors. The behavior of the light depends on the structure and chemistry of the crystal. So looking at rocks under the microscope can help to work out the type of rock (more on this).

Researchers also carry out chemical analyses to determine the elements present in rocks.

Mineral-rich deposits: precious metals

Rock from seafloor deposit. Image courtesy of K. Kusek

Hot fluids circulate in the crustal rocks in volcanic zones. Many different minerals are dissolved in these fluids because they are so hot. As they near the surface and mix with seawater, minerals precipitate out of solution. Some of the minerals are valued by humans: for instance, gold, copper, nickel, cobalt and manganese are sometimes found in large quantities near mid-ocean ridges. But mining them is not straightforward when they are found below several miles of overlying ocean, far from land.


Lava flows (and drips)

Stalactite-like spines protruding from the underside of a mid-ocean ridge basalt. Image courtesy of M. Perfit

As hot lava flows rapidly across the uneven seafloor, it can vaporize the seawater trapped in pockets under it. If the lava is sufficiently runny (low viscosity), it can then drip into these vapor cavities. The photo to the left shows some drips on a basaltic rock collected from the Juan de Fuca mid-ocean ridge in the north-east Pacific Ocean. Each drip is around 1-5cm (0.5-2 inches) long.

Next: thin sections