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Rock is broken down where it lies, by ice, roots, salt, heat and cold, acid rain and rust; and breaking it down is not the same as carrying it away.
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By the end of this lesson you will be able to look at what has happened to a rock and say whether it was physical or chemical weathering, name the agent that did it, and say what was left. You will be able to count the freeze-thaw cycles a winter delivers, and — the point the whole unit rests on — say why a rock split by frost on a summit has been weathered and not eroded.
You know from the rock cycle that any rock at the surface is broken into grains, which are carried away and laid down as sediment. This lesson takes the first of those steps on its own — the breaking — and gives it a name, two kinds and a list of agents. The one thing to hold onto from the start is that breaking and carrying are two different things.
Weathering is the breaking down of rock where it lies, by the weather and by living things. Physical weathering breaks rock into pieces without changing what it is made of. Chemical weathering changes the rock's substance — dissolves it, or turns a mineral into a new one. Freeze-thaw is water freezing in a crack, expanding, and pushing the crack wider, over and over. Erosion is the carrying away of the pieces, and it is a different word for a different thing.
Weathering breaks a rock down where it is. Erosion carries the pieces away. A boulder split in two by ice on a mountain top has been weathered and not moved an inch; the same fragments rolling down a stream bed are being eroded. The two words name two different things, and a landscape needs both to change.
There are two kinds of weathering, and the way to tell them apart is to ask whether the rock's substance changed.
Physical weathering breaks a rock into smaller pieces, and the pieces are the same stuff as before. Its agents push:
Chemical weathering changes what the rock is made of. Its agents react:
Both kinds usually work together. Chemical weathering weakens a surface; physical weathering breaks the weakened surface off; and the fresh surface underneath is exposed for the chemistry to start on again.
Another way: picture
Two panels. On the left, a boulder with a crack running through it, drawn three times: water in the crack, then ice with the crack a little wider, then the boulder in two halves lying where they fell. On the right, a gravestone with crisp lettering beside the same gravestone a century on, the letters shallow and blurred, and rain falling on both.
Another way: steps
To name what happened to a rock:
| Kind | What it does | Agents | What is left |
|---|---|---|---|
| physical | breaks rock into pieces | ice, roots, salt crystals, heat and cold | the same rock, in smaller pieces |
| chemical | changes the rock's substance | slightly acidic rain, oxygen and water | a solution, or a new mineral such as rust |
The last column is the test. Pick up a piece of the weathered rock: if it is the same stone as the parent, the weathering was physical; if it is something new — rust, clay — or if the stone has simply gone into the water, the weathering was chemical.
Weathering and erosion are used as one word. They are two. A rock split by frost on a summit has been weathered and not eroded; the halves rolling down a stream are being eroded. A landscape needs both: weathering makes the pieces, and erosion takes them away.
Freezing is called a chemical change. The water changes state, but the rock is pushed apart and stays the same stone. Weathering is named by what happens to the rock.
Weathering is thought to need rain. Ice, roots, salt and heat all work without a drop of rain falling, and a desert boulder flaking in the sun is being weathered as surely as a gravestone in the wet.
A worn stone is thought to be an old stone. Weathering depends on the stone and the weather, not on age. Marble and granite of the same age in the same rain come out quite differently.
Below a mountain crag is a slope of sharp-edged rock fragments, none of them rounded.
Sharp edges: not carried far.
The crag is cracked, and water in the cracks freezes on winter nights. Each freeze pushes a crack wider; eventually a slab falls.
Physical weathering by ice.
The fall is the first metre of erosion, but the breaking that made the slab was weathering, and it happened in place.
The two words, one after the other.
A flat surface of limestone is cut into blocks by deep, widened cracks with rounded edges.
Rainwater is slightly acidic and runs into the cracks, dissolving the limestone along them a little with every shower over thousands of years.
Chemical weathering: the rock goes into solution.
Nothing was pushed and nothing was carried; the missing limestone left as dissolved rock in the water.
The crust is orange and crumbly, and the rock underneath is grey: the surface is not the same stuff as the rock…
Substance changed, so it is chemical.
…so this is chemical weathering, and the agents are the damp and the air: oxygen and water turning the iron minerals into rust…
…which crumbles and falls off, exposing fresh rock for the same thing to start on again.
Each observation on the left was caused by one of the agents of weathering on the right. Match them.
| water freezing and expanding in a crack | a root growing and pushing outwards | slightly acidic rainwater dissolving the stone | oxygen and water turning iron into rust | salt crystals growing in the pores | |
|---|---|---|---|---|---|
| a crack in a crag widened over the winter until a slab fell away | |||||
| a joint in a rock has been levered open, with a tree growing out of it | |||||
| the lettering on a limestone gravestone has gone blurred and shallow | |||||
| an iron-rich rock has a crumbling orange-brown crust | |||||
| the surface of a seaside wall is crumbling to grains |
Three things were seen on a field trip. For each, say whether it is physical or chemical weathering, and what happened to the rock. First: a tree root has grown into a joint in a rock and levered the two sides apart. Second: a cave has opened up inside a limestone hill, with no river running into it. Third: the lettering on a limestone gravestone has become blurred and shallow.
| kind of weathering | what happened to the rock | |
|---|---|---|
| first | ||
| second | ||
| third |
On a mountain top, water in a crack freezes and splits a boulder in two. The two halves lie exactly where they fell. Which word describes what has happened?
On a crag, water in the cracks freezes at night and thaws by day. In a typical winter the temperature crosses freezing on $4$ nights each week, and the winter lasts $14$ weeks. How many freeze-thaw cycles does a crack go through in one winter?
Answer: cycles
Each observation on the left was caused by one of the agents of weathering on the right. Match them.
| water freezing and expanding in a crack | a root growing and pushing outwards | slightly acidic rainwater dissolving the stone | oxygen and water turning iron into rust | salt crystals growing in the pores | |
|---|---|---|---|---|---|
| a crack in a crag widened over the winter until a slab fell away | |||||
| a joint in a rock has been levered open, with a tree growing out of it | |||||
| the lettering on a limestone gravestone has gone blurred and shallow | |||||
| an iron-rich rock has a crumbling orange-brown crust | |||||
| the surface of a seaside wall is crumbling to grains |
In a city square a marble statue stands on a granite plinth. After a century in the same rain, the statue's face has lost its features and its inscription is blurred, while the plinth's edges are as sharp as the day it was cut. What is happening, and why the difference?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Three things were seen on a field trip. For each, say whether it is physical or chemical weathering, and what happened to the rock. First: a crack in a mountain crag has widened over the winter until a slab has fallen away. Second: a cave has opened up inside a limestone hill, with no river running into it. Third: flakes have peeled off a desert boulder that is baked by day and cold at night.
| kind of weathering | what happened to the rock | |
|---|---|---|
| first | ||
| second | ||
| third |
You can name the kind of weathering from what was seen and the agent that did it. Say out loud the difference between weathering and erosion, using a boulder split by ice as the example, and name two agents that work without any rain. Next: erosion and deposition, which is what happens to the pieces once something carries them.
9. Your turn: an iron-rich rock in a damp wood has a crumbling orange crust that comes away in your hand., step 3