weathering – freeze-thaw


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–thawfreeze–thaw process is repeated and cracks spread through the rock. Eventually small pieces of rock (called screescree) 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”.


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.


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.


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

pH876543


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.


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


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


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


Sedimentary Rock – sedimentation

layers of sedimentsgetting older


Where in the world?

weathering – freeze-thaw

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