Transcript Weatheringx

Weathering
Weathering


Earth’s surface is always changing.
There are internal processes like
mountain building and volcanic
activity.
There are also external processes
that continually break down rock on
the surface of Earth.
 Weathering is the breaking
down of rocks at or near Earth’s
surface.
Natural arches
Mechanical Weathering


Mechanical weathering
occurs when physical forces
break rocks into smaller
pieces without changing the
rock’s mineral composition.
Each piece has the same
characteristics as the original
rock, it is just broken into
smaller pieces so the surface
area is increased.
Mechanical Weathering

In nature, three physical processes
are especially important causes of
mechanical weathering:
 Frost wedging
 Unloading
 Biological activity
Frost Wedging

When water freezes, it expands. It
enlarges the cracks in the rock.

After many cycles of this freezing and
thawing, the rock will eventually split into
pieces.

This process is called frost wedging. Frost
wedging is most common in mountainous
regions.

Rocks that break loose and tumble into
large piles are called talus.
Unloading
Large masses of rock may be exposed over time.
 Reduced pressure on the rock, called unloading,
causes the outer rock layers to expand more than the
rock layers beneath.
 This causes the top rock layers to peel off like the
layers of an onion in a process called exfoliation.

Unloading
 Exfoliation
is
especially
common in
rock masses
made of
granite.
 It often
produces
large, domeshaped rock
formations.
Half Dome in Yosemite National Park, California
Biological Activity

The activities of organisms, including
plants, burrowing animals, and
humans, can cause mechanical
weathering.
 Plant roots can grow into cracks in rocks
and apply pressure over time, splitting
rocks apart.
 Burrowing animals can move rocks to the
surface where weathering occurs more
rapidly.
 Decaying organisms can produce acids
that cause chemical weathering.
Chemical Weathering
 Chemical
weathering is the
transformation of rock into
one or more new
compounds.
 In chemical weathering, the
rock undergoes a
transformation in its
composition as a result of
the process.
Water
 Water
is the most
important agent of
chemical weathering.
 Water absorbs gases
from the atmosphere
and the ground.
These dissolved
substances then
chemically react with
various minerals.
Water
 Oxygen
dissolved in
water reacts easily
with certain
minerals, forming
oxides.
 Iron
oxide is the rust
that forms when
objects with iron are
exposed to water.
Water
Water in the atmosphere absorbs
carbon dioxide, sulfur and
nitrogen.
 Sulfur is released by the burning
of coal. Water and pollutants
cause acid rain.
 Acid rain speeds up the
chemical weathering of stone
monuments and other structures.

Chemical Weathering of Granite
 Chemical
weathering affects
rocks in different ways,
depending on their composition.
 Granite contains feldspar and
quartz. During chemical
weathering, the acid rain breaks
down the feldspar into clay and
the quartz grains into sand.
Spheroidal Weathering



Weathering can change the shape of
rock as well as its size and chemical
composition.
When water enters along the joints in
a rock, it weathers the corners and
edges most rapidly. The corners and
edges break off and the rock becomes
more rounded.
The rock takes on a spherical shape
and this process is called spheroidal
weathering.
Rate of Weathering

Mechanical weathering
affects the rate of chemical
weathering.

By breaking rock into smaller
pieces, mechanical
weathering accelerates
chemical weathering. It
increases the surface area of
rock exposed to the chemical
components.
Rate of Weathering

Two other factors
affect the rate of
weathering:
 Rock
characteristics
 Climate
Rate of Weathering

Rock Characteristics:

Physical characteristics of
rock, such as cracks, are
important because water
can get into the rock.

However, the rock’s mineral
composition makes the most
difference.
Rate of Weathering : Characteristics

Gravestones made of
granite are relatively
resistant to weathering.

In contrast, marble
gravestones undergo
much more rapid
chemical weathering
Rate of Weathering: Climate
Temperature and moisture have a
strong effect on the rate of
weathering.
 These factors control the freezethaw cycle and the amount of
frost wedging.
 Temperature and moisture affect
the growth of plants.
 Chemical processes happen
faster at higher temperatures.

Differential Weathering

Different parts of rock
weather at different rates.
This process is called
differential weathering.

Caused by differences in
mineral composition.

More resistant rocks
protrude as high peaks
since they break down more
slowly.