Geology Reference
In-Depth Information
Box 7.1
The role of rock fracture in erosion.
Before rock can be eroded, intact bedrock
must be broken into smaller pieces that can
be detached from the landscape. This detach-
ment process can occur at the scale of grains,
thereby promoting grain-by-grain attrition
from the rock surface, or at scales that ena-
ble landslides. In all cases, fractures serve to
diminish overall rock strength by reducing
cohesion and potentially by lessening the
effective angle of internal friction. Fracture
production results from two major classes
of processes: geomorphic and tectonic.
Geomorphic processes encompass the broad
swath of physical, chemical, and biotic pro-
cesses that serve to break down rock masses.
In almost all cases, these processes are most
intense at the surface, and they diminish in
effectiveness with depth, typically in some
poorly known manner (see figure A).
Tectonic fracturing results from myriad
stresses within tectonic plates, but, in actively
deforming landscapes, the most obvious
cause of fracturing is the transport of rocks
above irregular fault surfaces (Molnar
et al.
,
2007). Any bend or kink or change in dip of
a fault surface causes a concentration of
stresses in the rocks of the hanging wall as
they are moved and folded above it. The spa-
tial cloud of aftershocks that follows coseis-
mic slip along a fault plane testifies to the
dispersed fracturing that occurs as accumu-
lated stress is released in the hanging wall.
Whereas recognition of these fracturing
processes and their importance is not new,
quantification of their magnitude has been a
persistent challenge. How deep is the geo-
morphically fractured layer? How does frac-
ture density vary with depth? Within any
mountain range, how diverse is the degree of
tectonic fracturing? How does the degree of
fracturing (or lack thereof) influence erosion
processes? Our inability to see into the shal-
low subsurface restricts our insights on these
questions.
Modes of
Fracturing
A
Rock Column
A. Two modes of bedrock fracturing.
Based on data from “backpack-able”
portable seismic arrays, new understanding
of the upper 10-20 m of the rock column is
now emerging. In comparison to the velocity
of compressional seismic waves (P waves) in
intact bedrock, changes in P-wave velocities
in the subsurface can be attributed to
fracturing, which impedes the transmission
of seismic waves. Recent studies on the South
Island of New Zealand (Clarke and Burbank,
2010a, 2011) reveal end-members that corre-
spond to tectonic and geomorphic fracturing
models. Geomorphic processes produce an
exponential decrease in the magnitude of
fracturing with depth (see figure B). In New
Zealand, the base of this geomorphic zone
ranges from 2 to 18 m, but averages 7 m.
Where this geomorphically fractured layer is
absent, fracture density is nearly uniform
with depth and is attributed to tectonic frac-
turing. A provocative feature of these data is
that the geomorphically fractured upper
layer tends to be present only when the rock
beneath it has been only weakly fractured by
