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Weathering & Erosion
Weathering and Erosion
• Weathering is the break down of rocks
that have been exposed to the atmosphere
• Once the rocks are broken down, the
pieces are transported from one place to
another. This process is called erosion
• Erosion is caused by wind, moving water
(streams, waves, ocean currents), ice
(glaciers), and by gravity.
• *Most erosion that takes place on Earth is
caused by moving water
Types of Weathering
Mechanical weathering – rocks are broken
down into smaller pieces without
changing their composition (doesn’t
change what they’re made of)
Chemical weathering - rocks break down as
their minerals change in composition
(they become different substances)
Types of Mechanical Weathering
• Ice wedging/frost
action
– water enters small
cracks in the rock
– When it freezes, it
expands and forces the
crack to open more
– The process repeats
over again until the
rock breaks apart
Eventually, the rock splits from
repeated frost action!
Types of Mechanical Weathering
• Wetting and drying
– breaks up rocks that are
made from clay
– When they are wet they
expand, and they shrink as
they dry
– As this repeats over and
over, the clay becomes
weak and cracks (think of
all the projects you have
made out of clay…they all
crack and fall apart)!
Types of Mechanical Weathering
• Exfoliation
– soil and rock is
removed (glaciers or
uplifting), exposing
rock found deep
underground
– This releases the
pressure causing the
surface of the rock to
expand and
eventually crack
Types of Mechanical Weathering
• Plant Roots
– The deep roots
of trees often
break apart
rocks
• Lichen & Moss
– These low to the
ground plants
can grow on
rocks and
eventually break
them apart
Types of Chemical Weathering
• Hydrolysis
– water (hydro)
reacts with
minerals
such as
feldspar and
form clay
Types of Chemical Weathering
• Oxidation
– oxygen reacts
with some
minerals,
especially those
containing iron
(magnetite) to
form rust (called
iron oxide)
– This occurs faster
with water!
Chemical
weathering of
basalt
Iron Banding in
Basalt
Oxidative weathering of mineral
deposits (new deposits are
white/yellow, weathered deposits are
reddish-brown)
Types of Chemical Weathering
• Carbonic acid
– carbon dioxide
(CO²) dissolves in
water to form this
acid
– can cause
minerals to
dissolve,
especially those
containing calcite
“Gnarled Rock” – a formation of limestone
chemically weathered by acid rain
Factors which affect rates of
weathering:
• Rock’s resistance to weathering
(HARDNESS: Rock’s composition:
What is it made of ? )
• Amount of surface area
• Climate:
– Chemical weathering occurs faster in
warm, wet climates
– Mechanical weathering occurs faster in
cold or dry climates
The central area of rock was less affected
by weathering…thus the “arch” was
formed!
Surface Area
• Greater surface area increases the rate
of weathering
• Surface area is the amount of rock
surfaces exposed to the atmosphere
• Weathering creates more surface area
#1
#3
#2
#4
Dry Climate = Mechanical Weathering!
Devil’s Marbles,
Australia – Mechanical
weathering from wind
and sand!
Warm Climate = Chemical Weathering
Parthenon – Athens, Greece