Tuesday, June 10, 2008

Eastern Australian pumice


Today’s deskcrop is a dacitic lump of pumice. It isn’t exactly the flashiest rock on the planet, but it is interesting. The reason, of course, is that mainland Australia doesn’t have any dacitic volcanoes. In fact, it hasn’t since rising sea levels opened the Torres Strait and cut off Australia from New Guinea after the LGM. So where does this fresh pumice come from? It is found all up and down the East coast, on the beach, in the storm deposits above the high tide line. To understand how it got there requires knowledge of oceanography.

The following image (from csiro) shows the ocean currents around Australia. As it shows, the dominant ocean currents on the East coast of Australia are the East Australian current, which flows south from the Coral Sea. This is a continuation of the South Equatorial current, which flows west across the South Pacific Ocean.

Before crossing the Coral Sea, the South Equatorial Current flows through the Solomon Islands, Tonga, Vanuatu, and Fiji, all of which are on convergent margins (see Google Earth pic, below).

These provide an ideal environment for arc volcanism- indeed most of these islands are volcanic. Several studies have looked into the origin of this Eastern Australian pumice, and they all point to these volcanic arcs as the source. So even though Australia has no active volcanic arcs, the ocean currents deliver volcanic rocks to our shores- and these volcanic rocks in turn raft nominally sessile creatures across the ocean- note the worm casting that are growing on this sample.

In short, sedimentary transport is not confined to continental terrestrial environments- ocean currents can transport lithic material as well.

4 comments:

Anonymous said...

(An aside... obviously a long way from east coast beaches, but you can find mainland pumice at NVP volcanoes. They might not have been active for a few thousand years or more, but it's got to count for something.)

Chuck said...

I thought they were just basalt...

Anonymous said...

Sorry, my mistake.

Silver Fox said...

Chuck, I think this is a pretty interesting post, what with the pumice showing up on your eastern beaches from ocean currents. Were there ever any old stories that explained the pumice in some other way? Like before people or geologists really knew where the stuff came from? Just curious.