Wednesday, 6 February 2013

Volcanoes of Hawaii - Looking Down instead of Up

Taken from the Wikipedia entry, this USGS 
graphic shows the elevation of the islands above 
the sea floor.  PD-USGOV-INTERIOR-USGS.
When you're lucky enough to visit Hawaii on an observing run, you can't help but be awestruck by this rather alien environment of red dusty rocks and limited vegetation.  It's been described as a lunar or martian landscape, and you can really see why.  Mauna Kea is a dormant shield volcano (i.e., one that has grown from accumulated lava flows), having pushed up from  a hotspot beneath the sea floor over hundreds of millions of years, and is the youngest in the chain of 129 volcanoes known as the Hawaiian-Emperor seamount chain, which stretches 3600 miles across the Pacific.  The Hawaiian hotspot is expected to have been stable in the Earth's mantle, with the Pacific plate simply moving over the top to create the Hawaiian islands.  So Kaua'i is the oldest, and a new volcano, the Lōʻihi Seamount, is now forming beneath the sea to our southeast.   Mauna Kea will erupt again someday, but the telescopes up here are so sensitive that there'd be plenty of warning!

The volcano is rather broad due to volcanism in the late stages of its formation, creating a large number of cinder and pumice cones around the summit (individual volcanoes in their own right), rather than one large caldera.  Mauna Kea is the fourth oldest and fourth most active in this chain, but the last eruption was over 4000 years ago, rendering this a stable site for astronomy.  The volcano and its neighbour, Mauna Loa, are so massive that they're actually depressing the sea floor beneath us.  Measured from the sea floor upwards, Mauna Kea is the tallest mountain on Earth at 10.2 km (higher than Everest).

The rocks here are a rich variety of basalts, from a mix of the magma and the subducted ocean floor, all in layers overlying one another, showing the evolution of this mountain.  As we were driving up yesterday, a region of morraine was pointed out to me, these are smaller rocks deposited as an ice-age glacier retreated over the volcano.  This makes Mauna Kea the only Hawaiian island with evidence of glaciation.
Satellite image of volcanic gases escaping from the
caldera of Kilauea in January 2012,  [the crater is known as
Halemaumau Crater] obtained from

As we've driven down the slope of Mauna Kea each morning, we've been reminded just how active this island chain is, as we've seen Kilauea erupting in the distance.  This is a forming shield volcano on the southeastern edge of the Big Island.  I hiked around the quiet caldera last time I was here, but now there's lava in the crater (check out the live webcam of a thermal camera here), and a cloud of ash (known as vog, or volcanic fog) which reflects the red glow, and is visible during the dark hours from Mauna Kea.  This current eruption has actually been happening since 1983, and a few years ago I hiked out across the lava field to see the material running into the ocean.
Screenshot of the thermal-IR view of Halemaumau crater as I write this post, the crater is filled with lava.....

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