Weathering
Mechanical weathering is the physical breakdown of rock into smaller particles. This can
happen through several processes
Frost action- this occurs when water seeps into cracks within a rock. When the water freezes, it
expands causing the rock to fracture into ever smaller pieces.
Exfoliation- layers of rock can peel off of a feature over time, much like unwrapping an onion.
Roots- as vegetation grows, its' roots can penetrate weaknesses and cracks in rock. As the root
grows and expands, it can exert tremendous forces and break up the rock like a wedge.
Chemical Weathering is the chemical change or dissolving of rock material. The most relevant
example in the Rockies involves limestone. Rain becomes slightly acidic when it absorbs
carbon dioxide in the atmosphere and then falls to earth. This mild acid rain reacts chemically
with limestone, and breaks down the rock. Water flowing through soil picks up natural acids that
can also react with limestone. Rillenkarren and karst features are examples of chemical
weathering.
The rate of both mechanical and chemical weathering is affected by climate. Moisture and
temperature variations determine the rate of weathering.
Mass Wasting
When the weight of an unsupported feature exceeds its' strength, the feature collapses. This
can be a bank along the edge of a steep stream channel, a rock slide, an avalanche, or a
serac falling off a glacier. The driving force here is gravity. Mass wasting has nothing to do with
heavy drinking.
Stream Erosion
The name says it all. Flowing water removes and carries away sediments. Water however does
not usually wear down bedrock, the sediments carried in the water slowly grind away at the
rock. Think of liquid sand paper.
Glaciation
Geologically speaking extensive glaciation is not a common occurrence. During their relatively
brief appearance large scale glaciations are one of the most powerful forces to shape a
landscape. Like stream erosion, glacial ice can not significantly affect bedrock. It is the rocks
embedded in the ice, the forward movement of the glacier, and its tremendous mass that
grinds and transports enormous amounts rock material. Imagine a belt sander with an
elephant standing on top, being dragged across a hardwood floor.