The origins of the lavas pouring out of Earth's largest ongoing volcanic eruptions are being challenged in this week's Nature. The eruptions are along the mid-ocean ridges that run for tens of thousands of miles though the deep sea. These are spreading centers where the Earth's crust is being
pulled apart and partially melted rock wells up to continuously fill
the voids – building vast amounts of new oceanic crust. But no one
is arguing that part of the story.
What is in question is just how well we
are interpreting the lavas from those eruptions and what they tell us
about the rocks down in the Earth's mantle. This is a big deal,
because the rocks at ocean spreading centers are among the very few
ways we can study the Earth's mantle. The trick has always been to
figure out what changes the deep sea lavas have undergone between the
mantle and the time they erupt. Geochemists read the mineral structure of lava rocks to figure what the molten material was like when it was in the mantle.
For a long time geologists centered
this reverse engineering around a process called fractional
crystallization. I remember this process initially giving me a little headache
when I was a geology undergraduate, but it's really pretty intuitive
if you spend any time in a kitchen or chemistry class. You take a pot
full of hot magma and start to cool it so that some minerals solidify
into crystals and sink to the bottom. Then keep cooling it so that
more crystals of various kinds of minerals form and drop out and
there is less and less melted rock.
What happens during this process
is that the still melted part of the mix loses a lot of elements that
are easy to make crystals with. What's left in the melt are a bunch
of elements that are lousy crystal makers: aptly called “incompatible
elements.” These tend to be rare elements like strontium, neodymium
and hafnium, and they provide clues to the origins of the magma in
the mantle.
When the magma is entirely erupted and
cooled, what you end up with in the pot – or making up the oceanic
crust – depends a lot on what mix of elements you started with (the
rocks it came from in the mantle) and how much time you took cooling
it down, and whether you added a little fresh magma to the mix
during the process. So what you get out of the pot should reflect
what you put in – which should reflect regional variations in the
mantle. Right?
This is where a new paper in Nature by geoscientists Hugh O'Neill and Frances Jenner comes into
play. They have discovered an unexpected worldwide pattern in those
incompatible elements that hints at a larger uniform process
producing the magmas that make up the oceanic crust of the Earth; a
“cycling of magma through the global ensemble of magma chambers,”
as they describe it.
And why does this matter? Because, as
geoscientist Albrecht Hofmann explained in a Nature
commentary, it means we have to re-examine the process behind the
Earth's most voluminous eruptions. That kind of fundamental change of
course doesn't happen every day.
IMAGE: Glowing molten volcanic rock of Eyjafjallajokull Fimmvorduhals, Iceland
(Martin Rietze, Corbis)
