8G Rocks and weathering

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21 Φεβ 2014 (πριν από 3 χρόνια και 6 μήνες)

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© Boardworks Ltd 2003

© Boardworks Ltd 2003

Weathering
-

chemicals

Firstly, there is carbon dioxide gas
which dissolves in rain to form
weak carbonic acid. This very
slowly eats away certain rocks.

There are also acids in the rain
that can chemically eat away at
rocks


especially rocks consisting
of metal carbonates (such as
chalk, limestone and marble).


Secondly, there are nitrogen and
sulphur oxides which produce much
more acidic rain that can rapidly,
chemically dissolve the rocks.

© Boardworks Ltd 2003

Weathering


freeze
-
thaw

You may have heard of the
saying,

This is because as water
freezes it expands
.

This creates powerful forces
that can enlarge the cracks.

As this
freeze

thaw

process is
repeated and cracks spread
through the rock. Eventually
small pieces of rock (called
scree
) break off altogether.

Most rocks are hard, but
despite this they can be broken
by just a small amount of water
getting into cracks in the rock.

“Hard as rocks”.

© Boardworks Ltd 2003

Freeze
-
Thaw!



Colin forgot to chill the wine so he put it in the
freezer to quickly make it cold


but then forgot it
was there!


Next time he went to the freezer he found it totally
shattered.


Explain what has happened.

The water expanded as it
froze creating huge forces.

These shattered the glass
bottle.

© Boardworks Ltd 2003

Weathering
-

expansion of rock

Freeze thaw is the not the only
cause of weathering.


In places with large daily
changes in temperature (e.g.
deserts) expansion and
contraction of the rock itself
occurs. The surface gets the
hottest and so expands the most.
This may cause it to “peel off.”

Additionally, some rocks contain
crystals that expand by very
different amounts. This too can
cause cracks.

© Boardworks Ltd 2003

Weathering
-

Plants and lichens

Plant roots can get into tiny cracks and
can physically open them up further.
In addition, decaying plant roots also
produce acid which can chemically eat
away at the rock.

Similarly lichens produce acids
which weather the rocks upon
which the lichens are growing.

Plant roots can
cause cracks in
rocks

pH

8

7

6

5

4

3

© Boardworks Ltd 2003

Transportation

The weathered rock is broken down further
by the action of wind, rain or ice. These
small pieces of rock are then moved away
by weather and gravity into rivers where
they are transported to the sea.


This process is called erosion.


At the sea the now tiny fragments
of rock sink to form a sediment.

Rock

Weathering

Erosion

Transportation

Sedimentation

© Boardworks Ltd 2003

Sedimentary Rock


Deposition

Small particles of rock formed by
weathering are transported into the sea
where they are deposited (sink) and
form a sediment.

At this stage dead creatures may
become trapped within the sediment
giving rise to fossils.

Over millions of years, the pressure of
the layers formed above and the
effects of salts cement the sediment
together to give sedimentary rocks like
sandstone and mudstone.

Fast flowing water

Slow water

Pressure

sedimentation

cementation

© Boardworks Ltd 2003

Sedimentary Rock


sedimentation

layers of sediments

getting older

© Boardworks Ltd 2003

1.
Arrange these into the correct order:

2.
Name two physical causes of weathering.

Weathering

Erosion

Transportation

Sedimentation

Freeze


thaw

Expansion
-

contraction

3.
Name two chemicals responsible for chemical weathering.

carbon dioxide nitrogen oxides sulphur oxides

Sedimentation, weathering, transportation, erosion