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Investigating the Rio Grande Rift
Jennifer Harding
Jennifer Harding studies
processes involved in continental rifting.
A continental rift is an area where the
earth’s crust is spreading apart,
becoming thin and weak. In particular,
Harding is looking at the Rio Grande
Rift, which stretches from central
Colorado to southern New Mexico.
There are many mysteries about
the Rio Grande Rift. The surface
geology of the northern part of the rift is
characterized by minor faults that are
younger in age. The surface geology
changes to sedimentary basins and linear
mountain ranges increasing in size
moving towards southern New Mexico.
While the surface geology and
topography changes along the rift, the
rift is expanding at a constant rate of
0.12 mm/year, the reasons for which are
unknown.
MT measures the natural
electromagnetic fields in the ground to
produce an electrical conductivity map
of the subsurface. These natural
electromagnetic fields in the Earth come
from the Sun and lightning storms.
Electrical conductivity can be
determined from these electromagnetic
measurements. Electrical conductivity is
a property that represents how easily
electrons or other charged particles can
move within a material like the Earth.
For example, at 20° Celsius, silver has a
high conductivity of 6.3 x 107 S/m and
Another mystery Harding is
attempting to solve is why the upper
mantle beneath the Rio Grande Rift area
is seismically slow. A region is
seismically slow if it takes earthquakes a
longer time to travel through the Earth.
Possible causes for seismic slowness
include a change in rock composition,
increase in temperature, and increase in
water content. If these questions get
answered, larger questions like why
rifting happens in the first place could
become clearer.
To help answer questions about
the Rio Grande Rift, Harding is using a
technique called Magnetotellurics, or
MT for short. This technique involves
intensive fieldwork, but the results could
be very important in determining what is
actually beneath the Rio Grande Rift.
Figure 1: Topographic map of the Rio Grande Rift area. The red
lines represent where MT sites will are being installed with an
approximate 7.5 km spacing. The northern line is close to Denver,
CO, the middle line is close to Taos, NM, and the southern line is
close to Las Cruces, NM. It can be seen that the basins get larger
moving south.
rubber has a low conductivity of
approximately 10-14 S/m. This property
is important because it can help
distinguish between different rock types,
temperature, and water content and
better define exactly what is lying
beneath the rift.
Besides solving mysteries of the
planet we live on, Harding loves what
she does because she gets to adventure
the outdoors to collect the necessary
data. New Mexico, the land of
enchantment, is a beautiful place to
work. Ironically, that beauty all stems
from the unknown processes of rifting,
creating spectacular mountains and
canyons. The MT instruments get buried
throughout New Mexico and Colorado at
some 140 sites (Figure 1), and listen
from two days to up to three weeks –
and it should be emphasized that burying
these instruments is no trivial task. Just
to get to a single site can take hours of
off-roading and discovering that the
roads on the map have been overgrown
and washed away. Once at a site, seven
to nine holes need to be dug to thermally
insulate the equipment that will be
measuring the electromagnetic fields.
This can be especially daunting when the
soil is less like sand and more like rock.
Some of the holes even need to be four
feet deep. No easy task, but Harding
knows the fruits of her labor will be
worth it.
References:
Sheehan, A., Bedrosian, P., 2012, NSF
EAGER: Pilot study: Deep electrical
structure of the Rio Grande Rift to
constrain extent and mechanisms of rifting.
Chapin, C., 1979, Evolution of the Rio
Grande Rift – A Summary.
Berglund, H., Sheehan, A., Murray, M., Roy,
M., Lowry, A., Nerem, S., Blume, F., 2012,
Distributed deformation across the Rio
Grande Rift, Great Plains, and Colorado
Plateau
"Electrical Resistivity and Conductivity."
Wikipedia. Wikimedia Foundation, 22 July
2013. Web. 22 July 2013.