basalt - tholeiite basalt aa from the flow that cut Mauna Loa Road in November 2022 - large hand/display specimen

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 yellowish-green 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 Mauna Loa that cut the road to the NOAA observatory at 8,000' elevation in November 2022. The project to reopen the road will start in 2025. In the meantime, the observatory, at an elevation just above 11,000 feet, is without access or power and is inactive.This aa is extremely loose and scratchy in the flow and these examples will impress any student that walking on an aa flow would be a challenge. The field photo, taken on the north slope of Mauna Loa, shows dark gray pahoehoe - smooth and ropy lava, over older brown aa, with an area of black aa behind.

For the QAPF diagram and other images, select and click again to enlarge. The pencil is 5 1/2" long, for scale.

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