Transcript Mechanical weathering

Weathering
The break down of rocks on the
Earth’s surface.
Two Types
The two types of weathering are:
1. Mechanical weathering – rocks are
physically broken into smaller pieces,
but there is no chemical change in the
rock.
2. Chemical weathering – the chemical
composition of rocks is altered.
Types of Mechanical Weathering
1. Frost Wedging – occurs when water is
trapped inside the cracks of a rock and
then freezes. The ice expands and
exerts a force strong enough to
fracture the rock.
2. Crystal Growth – occurs when
saltwater enters a crack in a rock and
then evaporates. It leaves behind salt
crystals which can exert force and
enlarge the cracks.
Types of Mechanical Weathering
3. Thermal Expansion – occurs when
rocks are exposed to severe heat.
The crystal structure of the minerals in
the rock expand, causing the outer
layers of the rock to peel off.
4. Root Penetration – occurs when plants
and trees take root in the crevices of
rocks. The growing root can expand
the crevice.
Root
Penetration
Types of Mechanical Weathering
5. Mechanical Exfoliation – occurs after
an intrusive igneous rock has been
exposed at the surface by erosion of
overlying rock. The overlying rock no
longer exerts pressure on the igneous
rock and so it expands outward,
fracturing into sheets parallel to its
exposed surface.
6. Abrasion – occurs when rocks and
particles collide and grind each other.
Mechanical Exfoliation
Types of Chemical Weathering
1. Dissolution – water removes
chemicals (ions) from a rock and
carries them away. Halite (rock salt)
dissolves like table salt. Limestone is
dissolved when water (H2O) and
carbon dioxide (CO2) in the
atmosphere combine to form carbonic
acid (H2CO3). Sulfuric acid (H2SO4)
may also be formed and it dissolves
limestone quite rapidly.
Dissolution of Limestone
Types of Chemical Weathering
2. Oxidation – this is another word for
rust. The surfaces of iron-rich rocks
will rust when they come in contact
with water. Copper develops a green
surface.
Oxidation
Oxidation
Types of Chemical Weathering
3. Hydrolysis – occurs when water ions
displace another element in the
mineral’s structure, forming a different
mineral. Aluminum-rich rocks are
weathered by hydrolysis forming clays.
Factors That Affect Weathering
Climate – warm, moist climates (tropics)
have the highest rates of chemical
weathering.
Living Organisms – burrowing animals
(earthworms, groundhogs, ants)
transport unweathered rocks from below
ground to the surface where they can
be exposed to weathering agents.
Factors That Affect Weathering
Time – the longer a rock is exposed to
weathering agents, the more it will
decompose.
Mineral composition – rocks composed
of silica strongly resist weathering, while
those containing iron, halite, and
limestone are more susceptible to
weathering.