17 August 2008

Shield Volcanoes

Posted by Ryan Anderson

I recently returned from a really awesome trip to Hawaii, where I participated in the 2008 NASA Planetary Volcanology workshop! The idea behind the workshop is that planetary scientists who are trying to interpret orbital data of other planets should have some experience doing the same on Earth. Now that I’m back, I have a lot to tell you about volcanoes. Today we will start by talking about shield volcanoes, starting with Hawaii.

First of all, what exactly is going in Hawaii? Why are there volcanoes there? After all, it’s smack in the middle of a tectonic plate, and most volcanoes form in fault zones at the edges of plates. It turns out that Hawaii is a textbook example of “hotspot volcanism”. The basic idea is that there is a “plume” of hot rock welling up from deep inside the earth right beneath Hawaii. (sort of the same as the hot “lava” welling up inside a lava lamp) When the plume hits the lower crust, the crust partially melts and the liquid rock works its way to the surface where it erupts as a volcano.

The cool thing is that the hotspot has remained pretty much stationary, but plate tectonics keeps moving the crust. That means that a volcano will form over the hotspot, but then the plate moves so that volcano goes dormant and a new one springs up nearby. In fact, if you look at a map of the pacific ocean floor, you can track the chain of islands and seamounts from the Big Island of Hawaii all the way up to the Aleutian trench, where the pacific plate gets subducted back into the mantle.

The Hawaiian volcanoes are known as “shield volcanoes”. Shield volcanoes are so named because they are very broad and convex, like a giant Greek warrior’s shield. They get this shape because the lava from a shield volcano is very fluid, and flows a long way before solidifying. As the lava oozes out over the surface, it continually builds up the “shield-shaped” profile. The Big Island of Hawaii is actually made up of several shield volcanoes. Here is a panorama that I took of the active Mauna Kea shield, as seen from the slopes of Mauna Loa.

Earth is not the only place in the solar system to have shield volcanoes. Venus and Jupiter’s moon Io have tons of them. But the best example of a shield volcano that we know of is Olympus Mons on Mars. Olympus Mons is a monster volcano: 27 km high and 550 km across. This image shows the outline of the entire state of Hawaii, superimposed on the enormous Olympus Mons.

Here’s another view, comparing the heights of Mt. Everest on the Earth, Maxwell Montes on Venus and Olympus Mons. (The horizontal scale has been drastically squashed; the slopes of Olympus mons should only be a few degrees.)

Why is it so big? Two reasons. First, plate tectonics doesn’t seem to work on Mars, so the hotspot underneath Olympus Mons just keeps feeding the same volcano, rather than constantly creating new ones. Second, the gravity on Mars is only about 1/3 that on Earth and the crust is much thicker and stronger. On Earth, when mountains get too big, the crust actually flexes and sinks. Much like an iceberg in the ocean, a mountain on earth is just the tip of a huge mass of rock that extends deep down. Mt. Everest is about as tall as a mountain can be on the Earth. Add any more rock to Everest, and it will just sink a little more. On Mars, with lower gravity and a stronger crust, mountains can be built up to a much greater height. That’s why Olympus Mons is about three times as tall as Mount Everest.