First explanations for boundary within Earth's mantle
Observed physical transition hundreds of miles below Earth's surface
Sinking slabs of ocean crust and rising
plumes of hot rock in Earth's mantle are observed to behave differently
below one megameter (1,000 kilometers) depth. Two explanations for this
behavior were published on Dec. 11, 2015. At left, Rudolph et al.
(Science, 2015) propose a viscosity increase (dark blue) below the
megameter boundary. At right, Ballmer et al. (Science Advances, 2015)
propose a density increase due to accumulated ocean crust (dark
squiggles) below the boundary.
Credit: Nicholas Schmerr/Vedran Lekic/UMD
Earth's mantle, the large zone of
slow-flowing rock that lies between the crust and the planet's core,
powers every earthquake and volcanic eruption on the planet's surface.
Evidence suggests that the mantle behaves differently below 1 megameter
(1,000 kilometers, or 621 miles) in depth, but so far seismologists have
not been able to explain why this boundary exists.
Two new studies co-authored by University of Maryland geologists
provide different, though not necessarily incompatible, explanations.
One study suggests that the mantle below 1 megameter is more
viscous--meaning it flows more slowly--than the section above the
boundary. The other study proposes that the section below the boundary
is denser--meaning its molecules are more tightly packed--than the
section above it, due to a shift in rock composition.
Taken together, the studies provide the first detailed look at why
large-scale geologic features within the mantle behave differently on
either side of the megameter divide. The papers were published on
December 11, 2015, in the journals Science and Science Advances.
"The existence of the megameter boundary has been suspected and
inferred for a while," said Vedran Lekic, an assistant professor of
geology at UMD and co-author of the Science paper that
addresses mantle viscosity. "These papers are the first published
attempts at a detailed explanation and it's possible that both
explanations are correct."
Although the mantle is mostly solid, it flows very slowly in the
context of geologic time. Two main sources of evidence suggest the
existence of the megameter boundary and thus inspired the current
studies.
First, many huge slabs of ocean crust that have been dragged down, or
subducted, into the mantle can still be seen in the deep Earth.
These
slabs slowly sink downward toward the bottom of the mantle. A large
number of these slabs have stalled out and appear to float just above
the megameter boundary, indicating a notable change in physical
properties below the boundary.
Second, large plumes of hot rock rise from the deepest reaches of the
mantle, and the outlines of these structures can be seen in the deep
Earth as well. As the rock in these mantle plumes flows upward, many of
the plumes are deflected sideways as they pass the megameter boundary.
This, too, indicates a fundamental difference in physical properties on
either side of the boundary.
"Learning about the anatomy of the mantle tells us more about how the
deep interior of Earth works and what mechanisms are behind mantle
convection," said Nicholas Schmerr, an assistant professor of geology at
UMD and co-author of the Science Advances paper that addresses
mantle density and composition. "Mantle convection is the heat engine
that drives plate tectonics at the surface and ultimately leads to
things like volcanoes and earthquakes that affect people living on the
surface."
The physics of the deep Earth are complicated, so establishing the
mantle's basic physical properties, such as density and viscosity, is an
important step. Density refers to the packing of molecules within any
substance (gas, liquid or solid), while viscosity is commonly described
as the thickness of a fluid or semi-solid. Sometimes density and
viscosity correlate with each other, while sometimes they are at odds.
For example, honey is both more viscous and dense than water. Oil, on
the other hand, is more viscous than water but less dense.
In their study, Schmerr, lead author Maxim Ballmer (Tokyo Institute
of Technology and the University of Hawaii at Manoa) and two colleagues
used a computer model of a simplified Earth. Each run of the model began
with a slightly different chemical composition--and thus a different
range of densities--in the mantle at various depths. The researchers
then used the model to investigate how slabs of ocean crust would behave
as they travel down toward the lower mantle.
In the real world, slabs are observed to behave in one of three ways:
The slabs either stall at around 600 kilometers, stall out at the
megameter boundary, or continue sinking all the way to the lower mantle.
Of the many scenarios for mantle chemical composition the researchers
tested, one most closely resembled the real world and included the
possibility that slabs can stall at the megameter boundary.
This
scenario included an increased amount of dense, silicon-rich basalt rock
in the lower mantle, below the megameter boundary.
Lekic, lead author Max Rudolph (Portland State University) and
another colleague took a different approach, starting instead with
whole-Earth satellite measurements. The team then subtracted surface
features--such as mountain ranges and valleys--to better see slight
differences in Earth's basic shape caused by local differences in
gravity. (Imagine a slightly misshapen basketball with its outer cover
removed.)
The team mapped these slight differences in Earth's idealized shape
onto known shapes and locations of mantle plumes and integrated the data
into a model that helped them relate the idealized shape to differences
in viscosity between the layers of the mantle. Their results pointed to
less viscous, more free-flowing mantle rock above the megameter
boundary, transitioning to highly viscous rock below the boundary. Their
results help to explain why mantle plumes are frequently deflected
sideways as they extend upward beyond the megameter boundary.
"While explaining one mystery--the behavior of rising plumes and
sinking slabs--our results lead to a new conundrum," Lekic said. "What
causes the rocks below the megameter boundary to become more resistant
to flow? There are no obvious candidates for what is causing this
change, so there is a potential for learning something fundamentally new
about the materials that make up Earth."
Lekic and Schmerr plan to collaborate to see if the results of both
studies are consistent with one another--in effect, whether the lower
mantle is both dense and viscous, like honey, when compared with the
mantle above the megameter boundary.
"This work can tell us a lot about where Earth has been and where it
is going, in terms of heat and tectonics," Schmerr said. "When we look
around our solar system, we see lots of planets at various stages of
evolution. But Earth is unique, so learning what is going on deep inside
its mantle is very important."
