Transcript this presentation and discussion.

Weathering
True or False: The Earth’s surface has
stayed the same for thousands of years
True or False: The Earth’s surface has stayed the
same for thousands of years
The Earth’s surface is always
changing!
Weathering
takes place as rocks are broken down into smaller
pieces over time by the effects of weather
What is Weathering?
•First step to forming soil and sedimentary
rock.
•It is the breakdown of rocks into smaller
pieces by physical and chemical
processes.
•No movement is involved in weathering.
•The smaller pieces do not move to a new
location until erosion carries them
away.
What is Mechanical Weathering?
• Breakdown of rock into smaller pieces
without any change in the chemical
composition of its minerals
– Sometimes called “physical” weathering
– Rock is torn apart by physical force, rather
than by chemical breakdown.
– breakdown of rock by
physical means
Mechanical Weathering
Ice wedging – water seeps into cracks
in rocks, freezes and expands, and
eventually cracks the rock
Ice Wedging
– Water water seeps into cracks in rocks and
freezes.
– Water expands 10% when it freezes, pushing
rock apart.
– Repeated
freeze and
thaw cycles
over the
years causes
rock to break
Ice causes weathering
Abrasion – grinding and wearing down of
rock surfaces by other rocks
Example: rock bump against each
other at the beach
Example: pebbles bump against each
other in a river bottom
Wind causes weathering
Why wasn’t
this mass of
land
weathered
away?
What evidence of
weathering do
you see in this
picture?
Mechanical - Biotic
Biotic – means life
– Weathering caused by living
organisms
– Plant roots act as a wedge
and widen cracks.
– Other causes of biotic
weathering:
•
•
•
•
•
burrowing animals
microscopic plants
animals
algae
fungi
http://commons.wikimedia.org/wiki/File:Wedge_Rock_Turkey_Run_SP,_IN.jpg
Plants –
roots can crack
rock
Mechanical Weathering –
breakdown of rock by physical means
Animals – human traffic or worms and other
animals digging
Exfoliation
• Exfoliation or unloading
-Rock breaks off into
sheets along joints which
are parallel to the surface.
-Caused by expansion of
rock due to uplift and
removal of surface material
that originally buried the
rock
http://commons.wikimedia.org/wiki/File:Exfoliation_and_mass
_movement.jpg
Thermal Expansion and Contraction
– Repeated daily heating and cooling of rock
– Heat causes expansion; cooling causes
contraction.
– Different minerals expand and contract at
different rates causing the rock to split.
http://commons.wikimedia.org/wiki/File:Penmaen_Dewi-St_David%27s_Head_-_geograph.org.uk_-_1097950.jpg
What is Chemical Weathering?
• Chemical reactions break down the bonds
holding the rocks together, causing them to
fall apart.
– Chemical weathering occurs in all types of
rock.
– Rock reacts with water, gases. and
solutions.
Chemical Weathering
•Chemical Reactions change rock composition.
They break down rock and minerals into
new substances.
•Chemical reactions happen
faster in warm,
wet conditions
Chemical Weathering -
breaks down rock and
. minerals into new
substances
4 Big Players
1. Water : will dissolve rock, but it takes
1000’s of years.
Big Bend National
Park, West Texas
Grand Canyon National Park, Arizona.
Water causes weathering
What evidence of
weathering do
you see in this
picture?
Chemical Weathering
2. Carbon Dioxide:
Dissolves in water
to form a weak
acid that reacts
with calcite,
limestone, and
marble
Ex. Acid precipitation
(acid rain) breaks down rock.
Carbonation – Carbon dioxide
(CO2) is dissolved in water making
carbonic acid
breaks down rock and
minerals into new
substances
Chemical Weathering
breaks down rock and minerals
into new substances
There are acids
in ground water
that dissolve rock
underground,
creating caves.
Limestone being eroded by water that is high in carbonic acid
(formed by carbon dioxide). Natural Bridge Caverns, San Antonio, TX.
Chemical Weathering
3. Organic matter decays,
makes water more
acidic and reactive.
Ex. Living things like algae, lichen, and
humans dissolve rock with the acids
they contain.
Chemical Weathering
4. Oxygen: reacts with iron and makes rust
(oxidation)
Oxidation – chemical reaction in which an element (iron)
combines with oxygen to form an oxide (rust),
rust = iron oxide
Palo Duro Canyon, Texas
Monument Valley, Utah.