Dive and Discover : Expedition 3 : Daily Updates

For the past month, we have been mapping the seafloor, collecting images of volcanic lavas, and sampling the rocks. Have you been wondering what the lavas are made of and what the interiors of the rocks look like?

Rocks are made up of minerals, and are given different names depending on the minerals in them and their chemical composition. The samples we have been collecting are a type of rock called “basalt”, and they are called Mid-Ocean Ridge Basalts, or MORB for short. Basalts are also the rock type that erupts on oceanic islands, like the Galapagos and Hawaii. But they are slightly different in chemical composition (in particular, they contain more water) and so are called Ocean Island Basalts, or OIBs for short.

When magma (the name given to molten rock) cools, minerals start to crystallize. The more the magma cools, the more it transforms into solid crystals. If cooling is slow, then the crystals have time to grow quite large. On the other hand, when magma erupts rapidly on to the seafloor and meets the very cold seawater at the bottom of the ocean, it does not have time to form many crystals. Instead, the surface of the molten rock “freezes” or “quenches” and forms a clear, brown glassy material. Because MORB always erupt underwater, they tend to have outer crusts that are made almost entirely of glass. The interior of the lava cools more slowly because the glassy outer rind insulates the hot magma from its cold surroundings. As the interior slowly cools, more crystals form, and the crystals also grow larger.

Geologists determine what minerals are in a rock by taking very thin slices of rocks that you can almost see through! They then look at them under a special microscope. Take a look at slides 3 and 4 in today’s slide show to see how pretty a black rock really looks under a microscope! Slide 3 shows the cooled interior of a pillow basalt; slide 4 shows a very thin slice of the outer rind of a MORB.

Minerals in basalts tell geologists about the chemical composition of the magma and the temperature at which the minerals crystallized. The two most common minerals in basalt are called olivine and plagioclase. Olivine is a mineral made mostly of magnesium, silicon and oxygen. Olivine crystals are olive-green, and they generally have a round shape. When these minerals are large and of high quality, they are known as the gem “peridote”. In MORB, olivine usually occurs as tiny individual crystals in clumps with plagioclase. Plagioclase is a mineral made mostly of calcium, aluminum, silicon and oxygen. Plagioclase crystals are often seen as rectangular crystals with smooth, glassy to white surfaces. Either olivine or plagioclase is the first mineral to crystallize in MORB. The other mineral that is commonly found in the more slowly cooled interior of MORB lava, is called pyroxene. It is made mostly of calcium, magnesium, silicon and oxygen, and often occurs in radiating groups with plagioclase. You can see all these minerals in today’s slide show!