Tuesday, June 18, 2013

Le'Ahi, Oahu, Hawaii

Diamond head state monument, near Waikiki beach and Honolulu, is one of the archipelago's best known geowonders. Born in a single eruption of olivine rich lava 300000 years ago, this 232 metre high monogenetic tuff cone is younger than the two 2-3 million year old basaltic shield volcanoes that form the bulk of the island. A tuff cone is a ring of ash and cinder around a vent, formed when ground or sea water meets rising magma in a steamy phreatomagmatic explosion (this lava erupted through a coral reef and chunks of it are intermixed with the basalt). Frozen spatters of lava sometimes cement the ash and cinder particles together, making the feature more resistant to erosion. It reputedly enjoys a splendid summit view.

It is part of a complex of cones and vents, called a volcanic field, all of which erupted later than the main shield volcanoes. Several other Oahu landmarks were born from these eruptions. Monogenetic eruptions are usually quite short, ranging from a few days to a few months. Such late stage eruptions are often olivine rich in basaltic volcanoes, as they sample the bottom of the magma chamber which includes many olivine crystals that have settled to the bottom after precipitating from the magma. It was therefore famed in its time as the source of superb gem quality peridot, as transparent green to brown olivine is known.

Its proper name comes from the indigenous Hawaiian words for promontory and tuna, because from a certain angle it resembles a dorsal fin. It was a sacred site for the original Hawaiians, who practised rituals, including holua sledding and human sacrifice there. Later, a military base, called fort Ruger was built within the cone, and at one stage it was an artillery firing point in Hawaii's coastal defence system. An FAA air traffic control centre also shared the premises between 1963 and 2001. Nowadays a National Guard facility is still in residence. The English name comes from 19th century English sailors, who mistook the calcite crystals in the rock (precipitated from circulating hot groundwater) for precious stones.

A 1970's heavy rock band also named themselves after the feature, as far as I know, the only geologically named rock band other than Magma. If anyone knows of any others, please let us know.

Image credit: Provelt/Wikimedia Commons.

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