Landscapes & Weathering
Geography year 7
Try to explain what has happened in each of the pictures above.
What has caused these features (apart from the sheep) to occur?
Exposed rocks can be broken down by the processes of
weathering.
What is a weathering
Weathering is the break-up and decomposition of rocks in-situ (in
their place of origin).
Weathering does not involve the movement of material and this
makes it different to erosion.
Weathering can be broken into either three categories:
1. Chemical Weathering: The break down of rocks caused by a
change in their chemical make-up.
2. Physical or Mechanical Weathering: The break down of
rocks caused by physical processes with no change in the
rocks chemical make up.
3. Biological Weathering: Biological is sometimes included
within physical weathering. Biological weathering is when flora
and fauna break down the rock e.g. growing roots systems or
burrowing animals.
Chemical Weathering
•
Example of chemical weathering
Chemical Weathering
Chemical weathering slowly decomposes or decays rocks
and minerals. It is caused by water reacting with the
mineral grains in rocks, to form new minerals and soluble
salts.
Physical Weathering
There are two main types of physical weathering:
•
Freeze-thaw occurs when water continually seeps into
cracks, freezes and expands, eventually breaking the rock
apart.
•
Exfoliation occurs as cracks develop parallel to the land
surface a consequence of the reduction in pressure during
uplift and erosion.
A freeze-thaw cycle (FTC) occurs when air temperature drops
low enough to freeze water (0°C, 32°F), then increases enough
for it to thaw again.
Thermal Factor
Changes in temperature contribute to expansion and contraction of the
rocks.
When the temperature of the rock rises, the rock expands and when the
temperature of the rocks decreases, the rock contracts.
Because the outer surface is more exposed than the inner surface,
there is uneven contraction and expansion of the rock’s constituent
minerals. This process results in physical stress, also termed as thermal
pressure, which can crack or break the rock apart.
Over time, sheets of rock separate from the stripped rocks and break
into smaller pieces along the fractures through a process termed as
exfoliation. Exfoliation happens when cracks form parallel to the land
surface
Cross section of exfoliation process that happen to a rock
Salt-crystal Growth or Salt Weathering
Salt-crystal growth is also referred to as salt weathering or
haloclasty. It occurs when saline solutions enter the rock pores
or cracks and evaporates, leaving behind salt crystals. The
accumulated salts crystals are heated up when environmental
temperatures are high, and they expand thus releasing pressure
on the rock, causing the rocks to disintegrate.
Biological Weathering
Biological weathering is the weakening and subsequent
disintegration of rock by plants, animals and microbes.
Biological Weathering
Two examples of lichen: Left, circular lichen in Great Falls, MD, and
right, a lichen covered rock at Lake Superior. (Images by C. Geiss.)