Rocks & weathering lesson 4

PHYSICALWEATHERIN

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Weathering

The decomposition and disintegration of rocks in situ.

Decomposition

Refers to chemical weathering and creates changes in rock substances

For Example: Granite decompose into clay

Disintegration

• Produces smaller, angular fragments of the same rocks, such as scree.

• No change in chemical composition.

• For example: Granite disintegrates into smaller fragment but rock type is still granite

Erosion• Is the breaking

down and removal of rocks

• agents of erosion such as rivers, glaciers and the sea

Denudation Weathering

and erosion working together causing landscape to be worn down

Regolith

• a layer of loose material covering solid rock.

• It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.

• Weathering and erosion depend on each other

• One could not exist very effectively without the other

• Weathering break down a rock surface to produce a pile of loose debris (Regolith)

• Erosion then removes the broken down rock fragments often using the ‘weathered fragments’ as ‘tools’ for erosion.

• This exposes a fresh rock surface to the processes of weathering and so the two processes continue

Three Types of Weathering

1. Physical Weathering

2. Chemical Weathering

3. Biological Weathering

Physical Weathering

• Disintegration of rock into smaller particles by mechanical processes but without any change in the chemical composition of the rock

• Where is it likely to occur?- devoid of vegetation

- deserts, high mountains, arctic regions

• End product- sands

Four main types of Physical / Mechanical Weathering

1. Freeze-thaw action / Frost Shattering / Ice crystal Growth

2. Salt crystalisation

3. Granular disintegration /Exfoliation

4. Pressure release

1. When water within

the cracks freezes to

ice, its volume

expands as much as 9%.

2. This expansion creates a powerful force called frost action or freeze-thaw action, which can exceed the tensional

strength of rock.

Continuation of alternate freezing and thawing will cause joints and pores

enlarge and shatter.

Freeze thaw action

Factors encouraging Frost Shattering / Freeze Thaw action

• Rapid freezing with a minimum temperature of -50C

• Frequent cycle of freeze thaw actions

• High degree of porosity or density of cracks in a rock

• Presence of water

Salt-crystal growth (haloclasty)

• causes disintegration of rocks

• saline solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind.

• salt crystals expand as they are heated up, exerting pressure on the confining rock.

• may also take place when solutions decompose rocks

• Example: limestone and chalk to form salt solutions of sodium sulfate or Sodium carbonate, when the moisture evaporates to form salt crystals.

Salt weathering of sandstone near Qobustan, Azerbaijan.

Salt weathering of building stone on the island of Gozo, Malta

Insolation weathering / heating and cooling

• Expansion and contraction of rock particles resulting from extreme variations in temperature

• Significant in desert area where diurnal temperature range is high (400C – 500C)

Pressure release

• It is not caused by element of the weather. However, it does occur in situ

• Involve the disintegration of rocks to expand

• Pressure release can be caused:i. Erosion of overlying rockii. When huge ice sheets melt at the end of a

glacial period

Pressure release:

• common in intrusive rocks that were formed deep under ground.

• E.g. granite batholiths.

• When this rock is exposed to the surface by uplift and erosion the rock expands and sheet joints form parallel to the rock surface.

WWhat type of weathering?

What type of weathering?

What type of physical weathering?

Classwork

• Ross pg 51 – Noting activity