What this lesson teaches
I can explain the chemical and physical weathering processes and what controls them.
Syllabus 9173, Topic 2.2, 1. Geomorphic Processes. What influences the weathering of rocks?
Make a guess
Freeze-thaw breaks rock where water in cracks freezes and thaws. Could it happen in the tropics?
- No. It is too hot in the tropics.
- Yes, across the tropics.
- No, because tropical rock has no cracks.
- Yes, on high mountains that freeze at night.
Show the answer
Yes, on high mountains that freeze at night.
Freeze-thaw needs temperatures that cross freezing. Warm tropical lowlands lack this, but high tropical mountains can have it.
Physical weathering breaks rock into pieces without changing its minerals, and each process needs particular conditions.
Freeze-thaw needs water in cracks and temperatures that go above and below freezing. Ice expands and pushes the crack wider. Warm tropical lowlands lack these conditions, but high tropical mountains can have them.
Pressure release happens when overlying rock is removed by erosion or quarrying. The rock expands, and cracks form parallel to the surface. Sheets of rock then peel off, especially from granite. It is not caused by falling air pressure during a storm.
Salt weathering happens when salty water enters cracks and evaporates. Salt crystals grow and force the grains apart. It is common in dry places and on coasts. Repeated wetting and drying helps it along.
Thermal weathering comes from rock heating and cooling. Different minerals, and the surface and inside of the rock, expand at different rates. This creates stress and cracks. Large daily temperature ranges help, but rock type, moisture and exposure also matter.
Physical and chemical weathering work together to change the surface. Breaking rock exposes fresh surfaces for chemical reactions. Chemical weathering weakens rock, making it easier to break. Unlike erosion, both happen in place.
Sort these
Is each process chemical weathering (minerals change) or physical weathering (rock breaks without changing)?
Groups: Chemical weathering, Physical weathering.
- Daily heating and cooling cracks a rock surface
- Feldspar in granite turns into clay
- Rock salt simply dissolves in water
- Curved sheets peel off granite after quarrying
- Ice in a crack expands and pushes it wider
- Carbonic acid dissolves limestone
- Salt crystals grow in cracks on a dry coast
- Iron minerals in rock turn red-brown
Show the answer
Chemical weathering
- Feldspar in granite turns into clay Water reacts with the silicate mineral to form a new mineral, which is hydrolysis.
- Rock salt simply dissolves in water The mineral goes into solution, which is a chemical process, not a break-up.
- Carbonic acid dissolves limestone The acid reacts with calcium carbonate, so the rock's minerals change and dissolve.
- Iron minerals in rock turn red-brown Iron reacts with oxygen to form iron oxides, a chemical change called oxidation.
Physical weathering
- Daily heating and cooling cracks a rock surface Uneven expansion creates stress and cracks, which is thermal weathering.
- Curved sheets peel off granite after quarrying Removing the rock above lets it expand and crack, which is pressure release.
- Ice in a crack expands and pushes it wider Freezing water forces the rock apart without changing its minerals.
- Salt crystals grow in cracks on a dry coast Growing crystals force grains apart, breaking the rock without a chemical change.
- Freeze-thaw
- Ice widens cracks; needs freezing, so only high up in the tropics.
- Pressure release
- Rock expands and peels after overlying rock is removed.
- Salt and thermal
- Salt crystals or heating and cooling crack rock.
Step by step
Check the condition
Freezing, unloading, salt or heating?
Explain the stress
What expands or presses on the crack?
Find the setting
Altitude, moisture, salt and exposure.
Worked example: A tropical coast and a mountain top
On a tropical coast, salt spray and drying can break up porous rock, even though it never freezes. On a high tropical summit, water in cracks may freeze at night. Latitude alone does not decide the process.
Watch out for this
Every crack in tropical rock comes from heating and cooling.
Cracks may also come from unloading, salt, freezing at altitude or chemical weakening.
Check your understanding
After quarrying removes overlying rock, curved sheets start peeling off granite. Which process is this?
- Carbonation of the granite surface.
- A daily fall in the air pressure.
- Pressure release after unloading.
Show the answer
Pressure release after unloading.
Yes. The rock expands once the weight is removed.