Download World Geography 3202

Block Mountain and Rift Valley
• When the land between two parallel faults is pushed
up, a landform called a block mountain (B) is
formed. The process can also result in a block of land
being pushed down, forming a rift valley (A).
A
B
Wearing down the land
• Just as the land is constantly being built
up; it is also constantly being worn away.
• The process of this wearing away of the
land is referred to as Denudation and it
is composed of two components:
– Weathering
– Erosion
• To begin with, we shall look at
weathering.
Wearing down the land
• The process of weathering helps wear
down the land through two methods:
– physical weathering : The process whereby
atmospheric forces and tectonic activity
physically shape the land
– Chemical weathering: The process whereby
chemical reactions of minerals, elements and
compounds causes changes in structures on
the earth.
The mechanical processes by which
physical weathering occurs.
• Frost Fracture:
– Fluctuations in temperatures above and below
freezing cause rocks to become brittle and
break.
– It also causes continual expansion and
contraction which weakens landforms and
eventually allows for breakage.
– In addition, water can get into cracks and
crevices, and contribute to frost fracture
through its own cycle of freezing and
thawing.
The mechanical processes by which
physical weathering occurs.
• Exfoliation: Sometimes, rock surfaces tend to
fracture along naturally occurring lines of
construction (This is especially true of
sedimentary rock formations).
The main interactions that result in
chemical weathering
• Solution:
– Usually rainwater is associated with this
process, absorbing carbon dioxide and
possibly other elements from the atmosphere.
– The additions of these chemicals transforms
the rain water into a weak carbonic acid,
which in turn can react with minerals in rocks
to form new soluble compounds.
The main interactions that result in
chemical weathering
• Hydrolysis:
– Carbonic acid reacts with a silicate and forms
a new soft clay mineral. The silicate’s
potassium, sodium, and magnesium are
carried away in solution. The soft clay left
behind ultimately decays into soil or mineral
deposits.
The main interactions that result in
chemical weathering
• Oxidation:
– This involves the reaction of metallic minerals
in rocks to oxygen and water.
– The result of the reaction is a new mineral
called an oxide.
– Oxidation often creates softer minerals that
weaken the rock structure which may
eventually crumble and be eroded away.
Erosion
• Another type of denudation, it includes
the breakdown of rocks and landforms,
and their transportation to other locales.
• There are several different agents, or
mediums, that contribute to the process of
erosion. One of the most important of
these is water in motion.
How running water acts
as an agent of erosion
• The earth’s running water can be thought of as a
natural plumbing system:
– Through the force of gravity, surplus water is “channeled”
from high ground to lower ground, always attempting to
reach the lowest ground, thereby being deposited in
aquifers, ponds, lakes and eventually the ocean.
• The “pipes” in this plumbing system have established
themselves over the billions of years that rain has
been has been falling on the earth.
– They are continually constructing and demolishing
themselves in an evolutionary process that is subject to the
forces that are created by the running water itself.
– Today we call these “pipes” river systems.