Under the Volcano

June 19, 2008

Diehard stargazers no doubt had their peepers trained on the sky last night to marvel at the so-called "solstice moon": an optical illusion in which the full moon hangs so low on the horizon that it seems impossibly big. (h/t: Neurophilosophy) You might be surprised to learn that scientists are still batting around various hypotheses about why this effect occurs. It seems that oh-so-familiar moon still has some scientific secrets -- including how it came to be there in the first place.

Fortunately, scientists are hot on the trail of where the moon came from. A new paper slated to appear in the June 20 issue of Science offers further evidence in support of the current prevailing hypothesis for the moon's origin: that the moon was born from a giant collision between the Earth and another large object. And that evidence comes not from meteorites or asteroids, but from the rocky remains of lava flows (basalt) at the Kilauea Iki crater in Hawaii -- a very-much-active volcano.

Nicholas Dauphas, a geophysicist at the University of Chicago, and his co-authors based their results on a precision analysis of lava samples taken from the crater using a new type of mass spectrometer to study tiny variations in the iron isotopes typically found in basalts. Prior studies hadn't found those telltale variations, but co-author Fang-Zhen Teng (University of Arkansas) says that's because those studies only looked at the rock as a whole -- not the separate minerals individually.

Why Kilauea Iki? There's a lot of lava samples available, from many different time periods as new layers of lava flowed and cooled over the years, so it's ideal for studying the separation of minerals and elements as the magma cools, contracts, and hardens (a process called differentiation). Per Dauphas, "A good analogy is putting a bucket of salty water in your freezer and monitoring what happens to the salt concentration in the water as the ice forms." The content of the iron isotopes present in the basalt changes over time in a predictable sequence -- predictable because scientists have been measuring the isotopic composition of iron for years. What's new is the emergence of better mass spectrometry technology. Now scientists can detect and quantify even very tiny, "naturally occurring variations in the ratios of the stable isotopes of iron," according to Wikipedia.

Okay, so what does a bunch of basalt from a Hawaiian volcano tell us about how the moon might have formed? Well, basalt isn't just found on Earth. It's also on the moon, Mars, and Venus, and has even been found on the asteroid Vesta. That means that the lava samples from Kilauea actually contain "the fingerprint of planetary formation," per Dauphas et al. And their technique of analyzing iron isotopes thereby gives scientists a new tool for reconstructing planetary origins.

In particular, their findings support the prevailing scientific hypothesis that the moon ended up in Earth's orbit because of a giant impact with another object roughly the size of Mars. The theory goes that the collision blasted lots of material into orbit around the proto-Earth, and the moon then formed through accretion. Quite a lot of energy got released during this process -- hot, hot, energy -- which is why scientists believe that much of the moon was once molten. That magma ocean cooled and crystallized, resulting in a distinct (from a geochemical perspective) crust and mantle, the latter primarily containing the minerals olivine, clinopyroxene, and orthopyroxene. (I only mention the specific minerals because Dauphas & Company focused particularly on analysis of olivine crystals, more commonly known as peridot, when they studied the lava samples from Hawaii.)

So, should you happen to visit the Kilauea Iki crater in the future, take a moment to reflect on the fact that the rocky surface on which you are treading -- looking for all the world like parking lot asphalt, with the asphalt haphazardly applied -- was not only once hot, flowing magma, but also connects this unique spot on Earth to the moon.

Photos: (top) Still from the 1902 silent film Le Voyage dans la Lune by George Melies.(bottom) Basalt on Kilauea, Hawaii's Big Island. Both images from Wikimedia Commons.