By Theresa Williams
Slackjaw Sally here with more about granola bars – I mean rocks. Rocks are a mixture of naturally occurring substances, mainly minerals, and range in size from a tiny grain to an entire mountain. Even though rocks sound random, they can be grouped into three categories based on how they were formed: sedimentary, igneous, and metamorphic.
Erosion turns all types of rocks into sediment that can be carried by water and wind. Sedimentary rocks form from sediment particles settling out of water or air or by precipitation of minerals from water. The layers are compressed and cemented into rock over time (lithification). This is the kind of rock where fossils can be found.
Magma is melted rock in the mantle beneath Earth’s crust. It’s called lava when it comes out through volcanoes. Igneous rock is formed when lava or magma cools and solidifies (solidification). The more slowly it cools, the larger the mineral crystals are likely to be. This is a very common rock at areas of seafloor spreading like here at Pito Deep. The scientists on this expedition are collecting mostly gabbroic rock which is found deep in the crust where magma cooled very slowly. They can even tell how hot the magma was by looking at the mineral crystals.
The third type of rock is called metamorphic and it’s made from any type of existing rock that is changed by heat, pressure, or reactive fluids like hot mineral-laden water (metamorphism). Metamorphic rock is common where Earth’s crust has been pushed together and folded during tectonic movement and where fractured rock masses are exposed to the surface or ocean water. Around here there are tons of metamorphic rocks where fractures in the igneous rock allowed ocean water in.
The rocks Jason’s pulling from the ocean floor are mostly igneous and metamorphic. The igneous rocks formed as magma filled cracks where the plates pulled away from each other and then cooled. Metamorphic rocks are formed where fractures expose the igneous rock to ocean water. Pito Deep is a place where the ocean crust has been torn open as tectonic plates move, exposing the deeper layers. We’re grabbing bits of the gabbro layer to learn about how the deep crust cools and changes over time.