Weathering,
Erosion &
Deposition
6th Grade Earth Science
S6E5
Weathering vs. Erosion
Weathering is the process by which rocks and minerals
are gradually broken down.
Erosion is the movement of rocks and soil by ice,
water, wind, or gravity.
Weathering can be classified in two ways:
Mechanical and Chemical
Mechanical Weathering
Mechanical weathering breaks rock into smaller
pieces. These smaller pieces are just like the bigger
rock; they are just smaller! The rock has broken without
changing its composition. The smaller pieces have the
same minerals in the same proportions. You could use
the expression "a chip off the old block" to describe
mechanical weathering! The main agents of
mechanical weathering are water, ice, and wind.
The above diagram illustrates ice wedging.
This is how ice wedging works. When liquid water changes into
solid ice, it increases in volume. You see this when you fill an ice
cube tray with water and put it in the freezer. The ice cubes go
to a higher level in the tray than the water. You also may have
seen this if you put a can of soda into the freezer so that it cools
down quickly. If you leave the can in the freezer too long, the
liquid expands so much that it bends or pops the can.
Mechanical Weathering
(continued)
• Abrasion is another type of mechanical
weathering. With abrasion, one rock
bumps against another rock. Gravity
causes abrasion as a rock tumbles down a
slope.
Mechanical Weathering
• A plant's roots grow into a crack in rock. As the roots
grow larger, they wedge open the crack. Burrowing
animals can also cause weathering. By digging for
food or creating a hole to live in the animal may
break apart rock. Today, human beings do a lot of
mechanical weathering whenever we dig or blast
into rock. This is common when we build homes,
roads, and subways, or quarry stone for
construction or other uses.
vs.Chemical Weathering
Mechanical weathering increases the rate of
chemical weathering. As rock breaks into smaller
pieces, the surface area of the pieces increases. With
more surfaces exposed, there are more places for
chemical weathering to occur.
Make the connection…
Let's say you wanted to make some hot chocolate on
a cold day. It would be hard to get a big chunk of
chocolate to dissolve in your milk or hot water. Maybe
you could make hot chocolate from some smaller
pieces like chocolate chips, but it is much easier to
add a powder to your milk. This is because the smaller
the pieces are, the more surface area they have.
Smaller pieces dissolve more easily.
Chemical Weathering
Chemical weathering is different than mechanical
weathering. The minerals in the rock change. The rock
changes composition and becomes a different type
of rock. Chemical weathering is important. It starts the
process of changing solid rock into soil. We need soil
to grow food and create other materials we need.
Chemical weathering works through chemical
reactions that change the rock.
Chemical Weathering
agents
• There are many agents of chemical weathering.
Remember that water was a main agent of
mechanical weathering. Well, water is also an
agent of chemical weathering. That makes it a
double agent! Carbon dioxide and oxygen are also
agents of chemical weathering.
Weathering Happens at Different
Rates
Each type of rock weathers in its own way. Certain
types of rock are very resistant to weathering. Igneous
rocks tend to weather slowly because they are hard.
Water cannot easily penetrate them. Granite is a very
stable igneous rock. Other types of rock are easily
weathered because they dissolve easily in weak
acids. Limestone is a sedimentary rock that dissolves
easily. When softer rocks wear away, the more
resistant rocks form ridges or hills.
Let’s review what we
know about
weathering
Remember…
Weathering is the process by which rocks and minerals
are gradually broken down.
Erosion is the movement of rocks and soil by ice,
water, wind, or gravity.
Weathering can be classified in two ways:
Mechanical and Chemical
Let’s talk about erosion.
Erosion
Erosion happens when a bit of earth material is
separated from the surface. Once detached, a force
like moving water or wind transports the material to a
new location where it is deposited.
Erosion is responsible for some of the earth's most
remarkable natural features, like the Grand Canyon. The
term "erosion" comes from "erodere," a Latin verb meaning
"to gnaw." Erosion is what usually happens to the material
loosened by weathering. Plant roots help to hold soil
togethr. When plants and vegetation are not present,
erosion can be more dramatic. Erosion can create
sandbars, floodplains, and river deltas. Four main processes
have an eroding effect:
• gravity
• wind
• water
• ice
Gravity
Gravity is a force that will move material, once it is broken
down by weathering, downhill. Gravity is always working on
all things on Earth. Gravity pulls materials toward the center
of the Earth making rocks fall from mountain tops and sand
to settle to the bottom of oceans. It causes mass
movement, any one of several processes that move
sediment downhill. Mass movement can be rapid or it can
be slow. There are different types of mass movement.
• landslides
• mudslides
• slump
• creep
Wind
Wind is an effective agent in causing erosion in dry,
arid climates like deserts. The main way that wind
causes erosion is by deflation.
Deflation is the process by which wind removes
surface materials. Wind blows over the land and picks
up the smallest particles of sediment. Clay and silt
make up this sediment. Stronger winds can pick up
larger particles. Some materials that are somewhat
heavy might gain lift from strong winds but will appear
to almost bounce or skip along. Deflation can move
the topsoil in an area leaving only stones and rocky
materials.
Water
Summary of Water
Erosion
Splash Erosion
The force of falling irrigation or rainwater displaces soil particles.
Sheet Erosion
Impermeable surfaces, compacted soil, or bare soil lets water run across it, washing
away disturbed surface particles.
Rill Erosion
Sheet erosion wears down soil to establish a definite path, forming rivulets in the soil
referred to as rills. Rill erosion is much more visible to humans than splash or sheet
erosion.
Gully Erosion
Over time, rills widen and deepen into a gully, accelerating the effects of erosion by
creating more and more surface area susceptible to disturbance.
Bank Erosion
Fast water flows (often caused by influx of stormwater from impermeable surfaces)
wear away stream sides at an accelerated pace, often causing bank failure.
Ice
• Ice erosion is the process of large chunks of ice, known as
glaciers, eroding an area over a long period of time with the
help of gravity.
• Ice erosion is caused by movement of ice, typically as
glaciers. Glaciers can scrape and break up rock and then
transport it, leaving moraines, drumlins, and glacial erratics in
its wake typically at the terminus or during glacial retreat. Ice
wedging is the weathering process where water trapped in tiny
rock cracks freezes and expands, causing the breakup of the
rock. This can lead to gravity erosion on steep slopes.
The scree which form at the bottom of a steep mountainside is
mostly formed from pieces of rock broken away by this
means. It is a common engineering problem wherever rock
cliffs are alongside roads and morning thaws can drop
hazardous rock pieces onto the road.
Deposition
The erosional transport of material through the
landscape is rarely continuous. Instead, we find that
particles may undergo repeated cycles
of entrainment, transport, and deposition. The process
of erosion stops when the transported particles fall out
of the transporting medium and settle on a surface.
This is called deposition.
Deposition can also be caused by
particle precipitation and flocculation. Both of these
processes are active only in water. Precipitation is a
process where dissolved ions become solid because
of changes in the temperature or chemistry of the
water. Flocculation is a chemical process where salt
causes the aggregation of minute clay particles into
larger masses that are too heavy to remain
suspended.
Soil
Four Major Components of Soil
• Once you step out onto a piece of ground, you step
out onto something that is alive. Soil is not just a
piece of dirt. Soil is made up of living and nonliving
material spread as a very thin layer over the entire
surface of the planet we call earth.
• Soil must provide nutrients, water, and air and helps
to support the plant.
Soil Composition
• Another material is called organic matter. It is made
up of decaying plant and animal matter.
Soil Types
• Sand is the largest particle in the soil. When you rub
it, it feels rough. This is because it has sharp edges.
Sand doesn't hold many nutrients.
• Silt is a soil particle whose size is between sand and
clay. Silt feels smooth and powdery. When wet it
feels smooth but not sticky.
• Clay is the smallest of particles. Clay is smooth
when dry and sticky when wet. Soils high in clay
content are called heavy soils. Clay also can hold a
lot of nutrients, but doesn't let air and water through
it well.
Soil Erosion and Humans
• Soil erosion is a natural process. It becomes a
problem when human activity causes it to occur
much faster than under natural conditions
CAUSES OF SOIL
EROSION
Wind and water are the main agents of soil erosion.
The amount of soil they can carry away is influenced
by two related factors:
• speed - the faster either moves, the more soil it can
erode;
• plant cover - plants protect the soil and in their
absence wind and water can do much more
damage.
POLITICS, ECONOMICS
AND SOIL EROSION
•
•
•
To understand soil erosion we must be aware of the political and
economic factors affecting land users.
In South Africa apartheid policies ensured that 42% of the people lived on
13 % of the land (the "homelands"). This overcrowding has resulted in
severe erosion. As the land became increasingly degraded and thus less
productive, subsistence farmers were forced to further overuse the land.
The intensive agriculture and overgrazing that followed caused greater
degradation. Soil erosion can be seen as both a symptom of
underdevelopment (i.e. poverty, inequality and exploitation), and as a
cause of underdevelopment. A reduced ability to produce, invest one's
profit and increase productivity, contributes to increasing poverty, and
can lead to desertification, drought, floods, and famine.
On commercial farm lands, overstocking, mono-cropping, and the
plowing of marginal lands unsuitable for cultivation has led to soil erosion
and desertification. Frequently these practices have been unwittingly
encouraged by the state offering subsidies which made it profitable to
exploit the land in the short-term.
PREVENTING SOIL
EROSION
Political and economic changes need to address the distribution of land as well as
the possibility of incentives to encourage farmers to manage their land sustainably.
Aspects of technical changes include:
• the use of contour plowing and wind breaks;
• leaving unplowed grass strips between ploughed land;
• making sure that there are always plants growing on the soil, and that the soil is
rich in humus (decaying plant and animal remains). This organic matter is the
"glue" that binds the soil particles together and plays an important part in
preventing erosion;
• avoiding overgrazing and the over-use of crop lands;
• allowing indigenous plants to grow along the river banks instead of plowing
and planting crops right up to the water's edge;
• encouraging biological diversity by planting several different types of plants
together;
• conservation of wetlands