2011 - Cooperative Institute for Research in Environmental Sciences ...

Greg Tucker
Is Climate Change Etched
into Mountain Hillslopes?
FUNDING: NATIONAL SCIENCE FOUNDATION
far flung as
the Grand
Tetons,
Lake Baikal and the East African Rift Valley—are
fault scarps created by extensional
tectonics. Geologic dating reveals that the
Apennine fault scarps are young, created by
a series of earthquakes over roughly the last
10,000 years. Yet in Italy, as in many other
extensional tectonic settings, earthquakes
have been slowly building mountains for
far longer—up to 3 million years in the case
of the Apennines. So why then aren’t the
fault scarps far higher and older?
A recent study by our team suggests
that the answer lies in erosion and climate
change. When an earthquake brings
fresh rock to the surface along a fault line,
weathering breaks down the exposed rock.
The weathering rate depends partly on
rock type and partly on climate. A new
mathematical model predicts that the rate
is, surprisingly, recorded in the shape of the
mountain front. Analysis of a well-known
scarp in Italy suggests that weathering rates
were about 30 times higher in the last ice
age than they are today. The high glacialage
rate was probably driven by frost
shattering of rocks, a process that requires
sustained subzero temperatures. The Italian
fault scarps not only shed light on the
role of climate in shaping mountains, but
also provide valuable data on rates of rock
breakdown and erosion.
54 CIRES Annual Report 2011
If you sit down at a
café in central Italy and
gaze out at the surrounding
scenery, you’ll notice
a mysterious white-gray
band that runs along the
foot of each mountainside.
At first glance, it
looks like the work of an
ambitious but unimaginative
graffiti artist. In
fact, what you’re seeing
is the legacy of thousands
of years’ worth of earthquakes.
The white bands
in Italy—which have
equivalents in places as
(a)
(b)
VF
3 km
CIRES graduate student Scott McCoy studies an Italian fault scarp.
Mt. Velino
VFE
MF
study
region
Italy
Rome
active
faults offsetting
base of Holocene
extension
Tyrrhenian Sea
Venice
200 km
N
Adriatic Sea
Top: Satellite image of Velino-Magnola Mountains, central Italy, showing the Velino and
Magnola faults (yellow lines). Bottom: The Magnola mountain front. Arrow indicates fault scarp.
Source: Tucker, GE, SW McCoy, AC Whittaker, GP Roberts, ST Lancaster, and R Phillips (2011),
Geomorphic significance of postglacial bedrock scarps on normal-fault footwalls, J. Geophys.
Res., 116, F01022, doi:10.1029/2010JF001861.