basalt - tholeiite basalt aa from the flow that destroyed Kapoho, Hawaii in 2018 - teaching hand specimen
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Tholeiite basalt, rising from partial melts of peridotite in the mantle, is the most common igneous rock on earth and makes up the oceanic crust. It is the type of basalt found on the moon. Peridotite is composed primarily of olivine and pyroxine. Olivine crystallizes at a high temperature and is one of the first minerals to form crystals in the rising magma. It is common in Hawaiian basalts, weathering out of a Mauna Loa cinder cone near South Point, where it is concentrated in a cove to form a famous green sand beach.
Aa forms when lava cools quickly and moves fast, with the surface breaking up into a clinkery mass of loose jagged pieces. Aa flows tend to be thicker than pahoehoe and are almost impossible to walk on. This aa was formed on the flow from Kilauea's east rift zone that consumed the community of Kapoho in 2018 and completely filled in Kapoho Bay, forming a lava delta.
This aa is extremely scratchy and will impress any student that walking on an aa flow would be a challenge. The field photos are views of this flow.
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