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 common rock at areas of seafloor spreading like here at Marion Rise. The dredges are collecting a small amount of gabbroic rock which is found deep in the crust where magma cooled very slowly and lots of basalt which cooled more quickly at or near the ocean floor surface. By examining the rocks, scientists can understand the history of the rock material. 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 (metasomatized if still in the mantle). 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 ocean crust and exposed mantle rock allowed ocean water in. The scientists are happy about the variety of peridotites they’ve collected. The amount and type of change each has experienced tells them details of how this exposed mantle is changing here and will help them uncover the mystery of the Marion Rise.