#tuff ring

Hanahuma Bay, O'ahu, Hawaii

Spattershot When the magma rising from the probable mantle plume that intersects the Mid Atlantic Ridge passed through the basaltic lump that we know as Iceland that results from this meeting and reached the surface around 2500 years before the Common Era it exploded out into the tuff ring/scoria cone in the photos. Known these days as Hverfjall, the resulting crater is around a km and a half across and is surrounded by a typical steep sided cone that is 400 metres high sitting near Lake Myvatn. Formed of layers of ash and chunks of lava that were driven out of the vent and fragmented by the pressure of expanding gases coming out of solution, it is essentially a ring of shattered debris from a lava fountain. The feature is part of a larger system known as the Krafla fissure swarm that marks an extensive weak spot in the crust that the magma forced its way through, producing a widespread set of related volcanic features from the same batch of molten rock. Loz Image credit: 1: Digital Globe 2: Orsolya and Erlend Haarberg 3:Andreas Tille https://www.icelandtravel.is/attractions/hverfjall/

I went out to Lake Pupuke today which is a crater lake and the oldest known volcano in the Auckland Volcanic Field.

References: https://bit.ly/2IOPes1

Shown above is a volcanic feature known as a tuff ring found in south-central Oregon. It's called Fort Rock for obvious reasons. It formed an estimated 50,000-100,000 years ago when basaltic magma pushed toward the surface and contacted the mud underlying an ancient lake bottom. The resulting ring is 200-300 ft (60-90 m) high and 4,460 ft (1,360 m) in diameter -- wide enough to fit the main span of San Francisco's Golden Gate Bridge inside. For scale note the farm buildings to the right of Fort Rock. Photo taken on July 8, 2015.