Further work using electro
conductivity has hugely expanded and reshaped our understanding of the Yellowstone plume first imaged using seismic work. Perhaps more correctly it is mapping the zone
of influence formed by the hot plume which naturally drives fluids in the
surrounding rocks generating alteration.
Henceforth geologists will want
to find similar rocks exposed on surface to map the derivative effects on
mineralization.
Deep drilling will also teach us
a lot now that we know the effects of a super plume naturally extends far from
the zone of actual maximum heat flow.
It is interesting though just how
flat lying the plume zone appears to be, or perhaps this is a natural misread
in which rising heat is creating a larger halo effect close to the surface.
By Paul RinconScience reporter, BBC News
13 April 2011 Last updated at 09:25 ET
The underground volcanic plume at Yellowstone in the US
The volcanic hotspot below Yellowstone feeds the hot
springs , mud pots and geysers that bring millions of visitors to
the US
But the Yellowstone
"supervolcano" has erupted violently in the distant past and could do
so again at some point.
The study is set to be published in
Geophysical Research Letters journal.
In 2009, researchers used seismic waves from earthquakes to build up an
image of the hotspot beneath Yellowstone, which straddles the US states of Wyoming ,
Montana and Idaho
The authors of the latest work used variations in the electrical
conductivity of rocks to produce a new picture of the plume.
This conductivity is a property of the molten silicate rocks and the
hot briny water that is naturally present in them.
"It's like comparing ultrasound and MRI in the human body; they
are different imaging technologies," says co-author Michael Zhdanov, a
professor of geophysics at the University
of Utah in Salt Lake City
Hot envelope
The 2009 images, using seismic waves, showed the plume of hot and
molten rock dipping downward from Yellowstone
at an angle of 60 degrees. This plume extended 240km (150 miles) west-northwest
to a point at least 660km (410 miles) under the Montana-Idaho border.
This was as far as the seismic imaging could "see".
The new study, using electrical conductivity, can only see about 320km
(200 miles) below ground.
Variations in electrical conductivity reveal the volcanic plume of
partly molten rock
But it shows the conductive part of the plume dipping more gently, at
an angle of perhaps 40 degrees to the west, and extending perhaps 640 km (400
miles) from east to west.
They may look different because the two techniques image slightly
different things.
Seismic images highlight materials such as molten or partly-molten rock
that slow seismic waves, while the geoelectric technique displays briny fluids
that conduct electricity.
Co-author Robert B Smith, who is also at University of Utah
Despite differences, he says, "this body that conducts electricity
is in about the same location with similar geometry as the seismically imaged Yellowstone plume."
The more gentle tilt of the geoelectric plume could suggest that the
hot region imaged by the seismic wave technique may be enveloped by a broader,
underground envelope of partly-molten rock and liquids, the researchers say.
There have been three huge eruptions of the Yellowstone
supervolcano: 2.1 million years ago, 1.3 million years ago and 640,000 years
ago. Two of these eruptions blanketed a large area of North
America with volcanic ash.
The most recent full-scale eruption of the Yellowstone
supervolcano ejected some 1,000 cubic km (240 cubic miles) of hot ash and rock
into the atmosphere. There have been smaller eruptions in between the largest
outpourings; the most recent of these occurred 70,000 years ago.
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