Introduction to the
Mojave National Preserve
& Ancient Lakes
Weathering & Erosion
Faults & Active Tectonics
Pediments & Alluvial Fans
& Older Surfaces
Sand Dunes & Dust
3D Geology Tour
Granitic Rocks and Associated Landforms
Weathering and erosion in arid regions underlain by granitic bedrock
produce unique characteristic landforms. Spheroidal weathering is a form
of chemical weathering in which concentric shells of decayed rock (ranging
from a few millimeters to a couple meters) are successively loosened and
separated from a block of rock. In the subsurface, groundwater penetrates
along fractures and causes the chemical breakdown of rock along surfaces.
Sharp corners at the intersection of fractures tend to break down first.
In this manner, blocks of granitic bedrock (or other rocks that do not
have layers or bedding) tend to become rounder as weathering proceeds.
In humid regions, spheroidal weathering of granite typically occurs in
the subsurface. In contrast, in arid regions the rate of chemical weathering
is slow relative to the rate of surface erosion. As a result, in granitic
terrains knob-shaped outcrops and spheroidal blocks accumulate on the
surface. In the Mojave region, granite typically breaks down to form fairly
uniform quartz- and feldspar-rich, coarse-sandy sediment compared to other
rock types. In addition, many of the major pediment areas in the Mojave
National Preserve have granitic bedrock. This is probably directly related
to the more-or-less uninform weathering and erosion characteristics of
large, homogeneous granitic intrustions.
|Fractured granite bedrock is exposed along Globe Wash in the Providence Mountains.
|The characteristic rounded boulders and knob-shaped outcrops throughout the Granite
Mountains formed as a result of spheroidal weathering and erosion of fractured granite.
|Large granitic intrusions of Cretaceous age are the bedrock of
Kessler Peak in the southern Ivanpah Mountains. A Joshua-tree forest
blankets a pediment surface in the foreground. This view is along
Cima Road east of the Teutonia Mine area.