The document is a transcript for a homework assignment on weathering and erosion. It includes slides covering the key topics of physical weathering, chemical weathering, biological weathering, and erosion. For physical weathering, it discusses how temperature changes and freezing/thawing can cause rocks to expand and contract, forming cracks. Wind and waves also wear away rocks. Chemical weathering involves acid from rainwater and more acidic acid rain reacting with minerals in rocks. Limestone weathers faster than granite from acid. Biological weathering occurs when plant roots grow in cracks and break rocks, and some animals burrow and scratch rocks. Erosion is the movement of weathered rock particles from their source, with larger rocks transported by faster

1. 7w – Homework assignment: Weathering and Erosion
Just like last week, you have a video and this transcript. Use both together or just one/the other (as you
please!). Write summaries after each section and bring these to class on Tuesday 19th March, 2013.
Video: http://www.youtube.com/watch?v=UoBEAv2uq3Y
Transcript: www.slideshare.net/mh392/7w-homework-2-transcript
Slide 1 (7w – Weathering and Erosion): Hey! Today’s homework is going to continue from last week’s
homework and is all about weathering and erosion.
Slide 2 (Homework): The procedure here is the same as last week. Write summaries at the end of each
section using the video or the transcript or both. If you’ve misplaced your transcript you can find it
under the following website: www.slideshare.net/mh392/7w-homework-2-transcript. Again, bring the
summaries in next Tuesday.
Slide 3 (Contents): The topics covered today will be weathering, the three types of weathering, and
erosion.
Slide 4 (What is weathering?): So, what is weathering? Well, weathering is a fancy term for the
process by which rocks break down into smaller particles. Nothing lasts forever and rocks aren’t an
exception. In fact, rocks are wearing away all the time. We just can’t see it happening because it’s a very
slow process that takes place over thousands of years. There are three types of weathering depending
on the weathering process. These are called physical weathering, chemical weathering, and biological
weathering.
Slide 5 (Physical weathering): Our first type of weathering is called physical weathering and, as the
name suggests, is caused by physical changes. So this can be anything from temperature changes,
freezing and thawing, as well as wind and waves.
Slide 6 (Temperature): When it’s hot outside, rocks expand a little, and when it gets cold again, rocks
will contract. If you keep on heating and cooling a rock, cracks will start appearing and little pieces of
rock will start falling off. A good example of this is in deserts, for example the Namib Desert in Africa.
During the day, the temperature will rise to above 40°C and then cool down at night to 20°C. Do this
over years and years and you’ll start getting cracks in your rock like the ones in the picture.
Slide 7 (Freezing and thawing): After cracks have appeared in the rocks, water can get trapped in the
cracks. When the temperature outside is cold enough, the water in the cracks will freeze. Freezing liquid
water into ice causes the volume of the water to increase by almost 10%. This means that it’s expanded
and this expansion can force the rock apart and cause the crack to get even bigger. Once the
temperature rises again, the ice will melt back and the rock contracts. Do this over a long period of time
and you’ll get huge cracks. Your rock might even completely break apart!
Slide 8 (Wind and waves): Now if we move towards the ocean, we’ll see another kind of weathering.
Here, the wind can pick up grains of sand and blow them against rocks. Waves can also hit rocks and
wear them away.
Slide 9 (Quick Re-cap 1): Now, take a few minutes to write a summary on physical weathering. Make
sure to mention the effects of temperature, freezing and thawing, and wind and waves. Include the key
terms ‘expansion’ and ‘contraction’!
Slide 10 (Chemical weathering): The second type of weathering we’ll look at… It’s chemical
weathering! Chemical weathering is all about chemicals and how chemicals can change rocks.

2. Slide 11 (Rainwater): Rainwater is naturally slightly acidic because it contains dissolved gases,
including carbon dioxide, from the air. So, when it rains, the chemicals react with the minerals in the
rocks. Some rocks react faster and are therefore weathered faster. For example, limestone and chalk are
both made of a mineral called calcium carbonate. Calcium carbonate reacts with acid to form new
substances, so when it rains, a chemical reaction will occur on the rock and any new substances that are
formed are washed away. This leaves the rocks weathered.
An example of a rock that is not weathered as fast is granite. Granite reacts very slowly with acid, so any
weathering will happen over a long period of time. The chemical reactions that happen with granite will
make the original minerals in granite into weaker minerals, leaving bits to crumble off.
Slide 12 (Acid rain): Acid rain is different from rainwater. Rainwater is naturally slightly acidic but
acid rain is even more acidic! Acid rain happens when fossil fuels are burned, releasing sulphur dioxide
and more carbon dioxide into the air. This will then dissolve and come back down to earth as acid rain.
Acid rain can weather rocks very quickly in comparison with rainwater. Many statues have been
damaged by acid rain. For example, the Statue of Liberty is actually made out of copper and would have
once looked like the top image. But, over time, acid rain has caused the brown colour to change to green
and people are constantly needed to renovate the statue from damage.
Slide 13 (Quick Re-cap 2): Once again, take a few minutes to write a summary about chemical
weathering. This time, make sure to mention how acid rain is different from normal rain and why it’s
more acidic. When writing your summary, give an example of rocks that weather fast and rocks that
weather slowly.
Slide 14 (Biological weathering): The third type of weathering is biological weathering. Biological
weathering is all about how plants and animals can break up rocks and lead to weathering.
Slide 15 (Plants): Plants can cause weathering of rocks when roots grow inside cracks. The roots start
to grow bigger and bigger and at the same time, the crack widens further and further. When the crack is
too big, the rock breaks apart. This is happening all the time underneath the soil but when roots grow
big enough, they can even break rocks on pavements.
Slide 16 (Animals): Animals can also cause weathering. Some animals burrow under the ground and
scratch rocks in the process. Animals that do this include rabbits and prairie dogs. Furthermore, acids in
animal waste can cause weathering, too.
Slide 17 (Quick Re-cap 3): Again, write a summary but this time on biological weathering. This one
probably won’t be so long. Maybe a few sentences!
Slide 18 (Erosion): We’ve almost reached the end. Our last topic is erosion. Erosion is the process that
occurs after weathering. So, if step 1 is weathering, the wearing away of rocks, then step 2 is erosion,
the movement of the pieces of rock from the site of weathering. For instance, weathering will lead to
pieces of rock falling into a river and getting transported away. If a river is flowing fast enough, large,
heavy rocks can be transported. But if the river is flowing very slowly, it can only transport small and
light rocks.
During erosion and transportation, pieces of rock can bash against each other causing more pieces of
rock to fall off. This is called abrasion and is what leads to sediment, which is the little pieces of rock or
sand that you find at the bottom of the river.
Slide 19 (Quick Re-cap 4):
Your last summary for this homework should be on the difference between weathering and erosion.
How are they related? Make sure to mention transport of large and small rocks, as well as abrasion.
Once you’ve done that, you’re finished! I’ll see you next Tuesday!