Geo 365: March 6, Day 65: Palagonite Tuff

When a basaltic melt encounters water at or near the earth's surface, several things may happen: if the flow is gentle and gas poor, pillow basalt may form. Likewise, if the flow is under high confining pressure, such as an ocean ridge, under thousands of feet of water, that pressure prevents brecciation, and again pillows will form.

With lower pressures, though, and especially if the magma interacts with water below the surface, and erupts with a very high gas content, breccias of one type or another will form. In the case of Table Rock, the basalt was likely interacting with water in the ground, as well as erupting into a shallow lake. The basalt shattered and quenched upon eruption, but remained hot- at least warm- for some period of time. In these conditions, the very fine-grained or glassy volcanic rock altered quickly- you can think of it as accelerated weathering- to form the clay-like material, palagonite.

There are a large number of volcanic features around the Fort Rock-Christmas Lake Valley formed from water/lava interactions, and are largely composed of palagonite tuff such as that shown above. Fort Rock is the most famous, but none of the others I've seen have the variety that Table Rock does.

Photo unmodified. August 20, 2011. FlashEarth Location.