Sunday, 16 August 2009

A volcanic graveyard

Looking north-east from the Canada France Hawaii Telescope (CFHT) on Mauna Kea one will see a gently sloping plateau that looks as though a collection of cinder cones have been scattered on the surface by some greater power. The contrast between the brown volcanic basalt rock and the red cinder cones can be very dramatic, especially at sunset when the cinder cones are side-lit by the setting sun.

Incidentally, in many of my posts you will have seen that I use the words "pu`u" and "mauna", almost always as part of the name of a volcanic feature or mountain. "Pu`u" means "hill" in Hawaiian and "mauna" means "mountain", hence the distinction between Mauna Kea, a mountain, and Pu'u Wekiu, a cinder cone atop Mauna Kea which actually forms the highest spot on the mountain and hence the summit.

Mauna Kea is a dormant volcano. It last erupted about 4,500 years ago and will likely erupt again although no one knows when. For the time being it's one of the best, if not the best, astronomical observing site on the planet. Volcanoes like Mauna Kea typically erupt from fissure vents surrounding the summit rather than the summit itself and these eruptions lead to the formation of cinder cones of which there are a countless number on the mountain. Many have names and I don't know if they all do, I suspect not. The ones in the picture are close to the summit and were likely formed during the most recent eruptions thousands of years ago, but they are now dead.

Cinder cones hardly ever grow more than a thousand feet high from their base, they tend to collapse before that point and then the lava flow will burst through the weakest point on the side of the cone which is why many of the cones are horseshoe-shaped. Once the eruption has stopped though, it's extremely rare for an eruption to reoccur at that pu`u but if volcanic activity is high, it's likely an eruption will occur nearby forming a new cinder cone and so on. Hence the cluster of cones in the picture.

The reason the cinder cones are so red is that after an eruption has ceased there is still an enormous amount of heat generated by the magma just below the surface. Water trapped within the rock turns to steam and escapes via weak points in the cinder, sometimes for months or even years. This steam oxidizes the iron in the cinder chemically transforming it into iron oxide (i.e., turning it to rust).

When the sun sets and the atmosphere turns red, these cinder cones can take on a most dramatic look.




Rob P. said...

Beautiful images. mahalo.

Tom said...

Mahalo to you too, Rob, for those kind words.

Keera Ann Fox said...

Once again, the place looks like the set for a sci-fi movie.

Tom said...

Keera - I have a photo of the CSO that I'll likely post in the next couple of days - definitely a sci-fi look to that one! People often say, when visiting the summit for the first time, that it looks like another planet!


Keera Ann Fox said...

It looks like what we think another planet looks like. How weird (or ironic) that it is actually our own. :-)