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Physical Weathering

- is physical. I.e. does not involve any chemicals,
- block disintegration
o Prevalent in hot and arid regions where there is a great temperature range that is, very high day temp and very low night temp.
o Rocks exposed are alternately cooled and heated, hence they contract and expand alternately.
o This causes fractures to form.
o Overtime, the rocks break into angular blocks
- Frost shattering / freeze thaw action
o Prevalent in cold temperate regions and mountain tops.
o Also at height latitudes and altitudes where temp fluctuate below and above freezing point.
o Esp. in rocks with many joints or cracks
o On warm days, ice melts and water seeps into the joints in the rocks.
o Temperatures drop, the ice freezes into ice again.
o As ice expands when cooled, it exerts pressure on the walls of the joints and causes them to enlarge or deepen.
o Throughout the day, this process repeats.
o More water may enter cracks.
o Over time, the joints will become wider and deeper and eventually the rock will split into two
o Rocks collect at foot of the mountain known as scree.
- Unloading
o Rocks under the surface of earth are under a lot of pressure and hence, are “compressed”
o Over time, the rocks above are eroded away and this relieves the underlying rock of the pressure.
o Hence, it expands in volume. [Imagine squeezing a sponge, when u exert pressure/squeeze, it compress. But when u release/ reduce pressure, it returns to original state/expands]
o As a result of the underlying rock’s expansion, joints or cracks appear
o Large sheets of rock up to 15m thick break away from parent rock.
o This peeling away of the rock exposes a dome like feature known as exfoliation dome
o E.g., sugarloaf mountain, Rio de Janeiro.
- Salt wedging/ crystallization
o Prevalent in places with irregular and little rainfall like deserts or areas experiencing droughts.
o Underground water which contain dissolved salts seep upwards into cracks and joints
o When the water evaporates, salt crystals are left behind [halite, calcite, gypsum]
o Over time, more water is drawn up and the process repeats.
o Existing salt crystals grow and newer crystals from.
o Crystals exert pressure on sides of joints [ similar to frost shattering] and causes the rocks to break into wedges/ disintegrate.