Pahoehoe lava in Hawaii
PHOTO BY MARTIN SAAR
Martin Saar collecting groundwater samples from springs
in the Oregon Cascades for noble gas analyses
PHOTO SUPPLIED BY MARTIN SAAR
PHOTO BY BRIAN LIEB
groundwater, glaciers, and noble gasses
For Saar, volcanology is just one avenue for examining geologic
fluid mechanics. Other recent studies include:
Seasonal groundwater recharge and seismic activity: Saar and
a colleague analyzed the relationship between seasonal groundwater
recharge (the process wherein surface water continually replaces
groundwater flowing towards discharge sites such as springs,
lakes, or rivers) in the Oregon Cascades and increased seismic
activity. The process of groundwater recharge is re-initiated
with each spring's snow melt, and slowly creates a signal
of fluid pressure migrating downwards from the surface. Their
results showed that the spring recharge process can trigger
increased seismic activity in the summer months.
Glacier dynamics over the last 400,000 years: Saar and two
colleagues performed numerical analyses on the relationship
between the size of glaciers and the frequency of the volcanic
eruptions that took place in eastern California over the past
400,000 years. Their results suggested that the rate of change
in the ice volume, as well as the ice volume itself, as glaciers
grew and retreated could be correlated to patterns of volcanic
eruption. Their study also provided insight into the viscosity
of the Earth's crust, which can have fluid-like properties,
over a long geologic timescale.
Noble gasses as a new yardstick for measuring groundwater
flow: Saar and three colleagues analyzed all the naturally
occurring stable noble gasses (helium, neon, argon, krypton,
and xenon) dissolved in groundwater samples collected in springs
of the Oregon Cascades. By combining results obtained from
measuring multiple noble gasses, inferences of groundwater
flow patterns previously based on analysis of groundwater
temperature and helium only were considerably improved.
For a complete description of Martin Saar's and the hydrogeology
and geofluids research group's interests, visit www.umn.edu/~saar.
When Mount St. Helens in southwestern Washington state began to
demonstrate signs of volcanic unrest in the fall of 2004, scientists
watched and waited to see if an eruption as devastating as the one
that occurred there in 1980 would result. During that event, after
two months of increased seismic activity, a magnitude 5.1 earthquake
occurred, spawning an explosive eruption. Within 10 minutes, debris
from an avalanche spread more than 15 miles west of the volcano.
The eruption also felled trees and killed most wildlife in a 200-square-mile
area, ejected debris and magma in a column several miles high, and
distributed ash as far as 900 miles away.
Ultimately, Mount St. Helens did erupt again in 2004. Fortunately,
that eruption was an effusive eruption, meaning it was dominated
by the outpouring of lava onto the ground, instead of an explosive
eruption, characterized by the violent fragmentation of molten
rock. Although scientists cannot yet predict whether a particular
eruption from the same volcano will be explosive or effusive, studying
the geologic fluid mechanics of the volcano's magma (the
term for molten rock located beneath the Earth's surface) increases
our understanding of the problem. In the University's Department
of Geology and Geophysics, newly arrived assistant professor Martin
Saar is studying the physics of fluid flow in such a geologic context.
Saar, the George and Orpha Gibson Chair in Hydrogeology and Geofluids,
uses field work, laboratory experiments, and computer simulations
to examine geologic fluid mechanics in a wide range of models. The
research he performs has implications for geothermal energy exploration,
volcanic and seismic hazard assessment, water management, construction
of reservoirs, and remediation of environmental contamination.
Two kinds of volcanoes and two kinds
In one aspect of his current research, Saar studies the characteristics
of magma and lava (the term for molten rock when it erupts
onto the Earth's surface). These two types of molten rock typically
contain crystals and bubbles of suspended gasses. Saar's studies
of magma rheology, the characteristics of deformation when
stresses are applied to a material, inform scientists about the
dynamics of volcanic eruptions--if they will be explosive or effusive--and
about lava flow velocity and distance, known as emplacement.
For example, the permeability of the liquid phase of magma for the
gas bubbles suspended within it affects the ability of the magma
to release its gasses. "If magma can lose bubbles while ascending
within the volcano, gas can escape, decreasing the magma pressure
similarly to a shaken soda bottle that has been opened slowly. This
[degassing phenomenon] may be one of the most important factors
for determining if an eruption will be explosive or effusive,"
stated Saar. When the bubble and crystal content of magma change,
its fluid characteristics (such as viscosity and permeability) change.
As a result, its likely association with an explosive eruption or
its lava's emplacement characteristics will also change.
Saar is also interested in the transition between two well-characterized
types of lava, commonly known by their Hawaiian names, pahoehoe
(which has a smooth or ropy surface) and a'a (which has a rough,
blocky surface). Understanding lava transition events and the emplacement
characteristics of these two types of lava is important in regions
like Hawaii, where learning more about the distance that lava can
be expected to travel is critical for protecting life and property.
Saar has collected (now solidified) lava samples along a pahoehoe
to a'a transition from the 1974 Kilauea volcanic eruption in Hawaii
and studied the relationship between their fluid mechanical properties
and emplacement type. Related to this work, Saar and his students
have developed a computer simulation to reconstruct, long after
the partially molten rock has solidified, the fluid dynamics that
did occur within magmas and lavas.
Much of Saar's work focuses on large-scale, complex systems that
are inaccessible to standard measurements. "When dealing with
the actual Earth itself rather than with simplified lab experiments,
it is difficult to reduce the number of variables possibly influencing
any given process of interest," noted Saar. "As a result,
we don't know all the parameters when working on such large scales
of time and space, but we can try to determine the parameters that
appear to be most critical." In situations where all the variables
cannot be controlled, referred to as underconstrained models,
geologists routinely make multiple direct and indirect observations
to improve, or further constrain, their results.
For Saar, the aim of using such multiple measures is to simplify
a conceptual model of a real-world process to the point where an
analytic mathematical solution can be found. However, sometimes
conceptual models cannot be reduced to find analytic solutions.
For these situations, Saar is developing and using large-scale computer
models through a multi-processor Linux computer cluster. The goal
of his computer analyses is to minimize the error between what the
model predicts and what the data show. Having more data (from multiple
measures) allows a model to be further constrained, providing a
better-fitting model. "It is impossible to find one unique
model for such underconstrained systems, but we can determine a
best estimate," said Saar.
Just as such large-scale models present challenges, studying the
rheology of magma and lava poses its own unique set of obstacles.
Magma and lava are difficult to study both in the natural setting
and in the laboratory due to the high melting temperature of rocks
(magma exists between 650 and 1200 degrees Celsius). To simulate
lava in the laboratory, Saar typically uses analog fluids.
Starting with corn syrup, he mixes other materials in, such as bubbles
and particles (poppyseeds). In doing so, he creates a multi-component
system that exhibits complex rheological behavior that may be similar
to lava and magma. To test some of the findings derived from experiments
with analog fluids and computer simulations, Saar currently has
a proposal pending to study rheology and permeability measurements
on actual (re)molten rock samples.
Laying the groundwork
As part of his new post, Saar is forming a new hydrogeology and
geofluids research group within the Department of Geology and Geophysics
that is funded in part from an endowment from George and Orpha Gibson.
"It's important to conduct fundamental research at the University,"
noted Saar. "It's also important to put numbers on things to
look at phenomena in and on the Earth, and possibly other planets,
in a quantitative way." The applied value of Saar's research,
however, should not be overlooked. "We hope that what comes
out of our labs will be picked up by people eventually and applied
in the fields of groundwater resource management, geothermal energy
exploration, or volcanic hazard assessment."
WRITTEN BY LINDA RAAB