Hot Spot Volcanism

We understand Convergence zone volcanism but what about hot spots?  What are they? How do they form?

Hot spots are mantel plumes that allows very hot materials from as deep as the outer core to rise up and literally burn their way through the lithosphere. It can happen anywhere under a tectonic plate, deep under an ocean or even a continental crust. 

No one is really sure what these hot spots are, what causes them or why they can last for millions of years. It has been suggested that they are flaws in the mantle that allows convective forces to focus the movement of liquified rock on these points. These hot spots last so long in geological terms that as they stay fixed at one point. Tectonic plates are dragged across the hot spots slowly creating features like the Hawaiian Island chain or the Snake River caldera complex or the Yellowstone hot spot.

US Geological Survey Article: Hot Spots or Thermal Mantle Plumes

The basaltic lava that comes from hot spot volcanoes is very different from the kind produced by convergence zone volcanism. In many ways it is the chemical opposite of silica. rich Rhyolite lavas. It comes from much deeper inside the earth. It flows much more like iike a true liquid than a silica based lava which behaves much more like wet lumpy clay. Rhyolite has to be blasted out of volcanoes. Basaltic lava cools slowly and is content to pool and run like any other liquid. This liquid is somewhere in excess of 1200 degrees centigrade and will destroy almost anything it encounters.

Basaltic lava flows can be enormous. The largest known volcanic event in the last 500 million years created the Siberian Traps and may have caused the Permian-Triassic (PT) extinction event from out gassing so huge a volume of magmatic gas that it changed the chemistry of the atmosphere. On top of the massive outgassing event, the Siberian Traps supervolcano deposited 1 to 4 million cubic kilometers of basaltic lava (240,000–960,000 cubic miles).  

The Columbia River basin in Washington State is a basaltic lava flow that occurred about 16 million years ago in Eastern Washington and Idaho and ran all the way to the sea. In some parts of Washington these lava flows are three miles deep.

This picture comes from a Hawaiian volcano that has been erupting constantly for forty years.  This type of lava cools unevenly. The top can cool and harden while inside its still molten. This type behavior is how lava tubes are formed.

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As I've run out of allowed images, that's all for today. we'll take it up there tomorrow.