Lecture Twenty One - Weathering: fluid rock reaction and making sedimentary rocks from igneous and metamorphic rocks Flashcards
What is weathering?
The breakdown of rocks (and their mineral components) by physical or chemical processes at the interface between the planets crust and atmosphere.
On earth, weathering is driven by interactions between the crystalline Earth, the atmosphere, hydrosphere and life.
Weathering breaks down igneous and metamorphic rocks.
It is a vitally important process that transfers matter and chemical energy between the curst, atmosphere, hydrosphere and biosphere.
All of the chemical energy that dives life on earth, all soil, biomass (living and non-living), dissolved ions in water were originally sourced from igneous and sedimentary rocks.
What is physical weathering?
Physical disintegration of rocks without mineralogical transformation - no chemical change.
Caused by:
1) Rock on rock abrasion by wind, water and ice - When rocks are carried through water, wind or ice and eroded from hitting one another.
2) Thermal expansion/contraction - When temperature oscillates between increases and decreases, causing mineral crystal structures to expand and contract, eventually causing the rock to split up.
3) Pressure release - When there is a large layer of rock over another layer of rock in the crust. When the top layer erodes away, exposing the underlaying layer, the pressure released from the underlying layer causes the the top layer of this rock to fracture off.
4) Crystallisation pressure (frost/salt wedging).
5) Biological activities - E.g. when a tree grows through a sedimentary rock.
Explain chemical weathering.
Chemical alteration of rocks resulting in mineralogical transformation.
A change in elemental composition and crystal structure of naturally occurring chemical compounds.
Typical mechanisms of mineralogical transformation at Earth’s surface:
- Dissolution in water.
- Dissolution mediated by acids (can be organic).
- Oxidation/reduction (can be catalysed by microbes).
- Dehydration/hydration reactions.
Chemical weathering can be mediated by biological processes.
Major factors that control the rate of chemical weathering:
- Temperature (chemical reactions occur more quickly at higher temperatures).
- The strength of chemical bonding in a minerals crystal structure.
The composition of solutions to which minerals are exposed (pH, dissolved ions, dissolved oxygen etc).
The surface area exposed on mineral grains that is abatable to react with water, air and accessible to organisms.
What is spheroidal weathering?
More surface area available for geochemical reaction - dissolution of minerals faster at corners.
Affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock that is known as saprolite.
When saprolite is exposed by physical erosion, these concentric layers peel (spall) off as concentric shells much like the layers of a peeled onion.
It occurs as the result of the chemical alteration of such rocks along intersecting joints and from water seeping into joints and pooling in irregularities in the rocks surface.
The alteration by weathering of the bedrock will be greatest along the corners of each block, followed by the edges, and finally the faces of the cube. The differences in weathering rates between the corners, edges, and faces of a bedrock block will result in the formation of spheroidal layers of altered rock that surround an unaltered rounded boulder-size core of relatively unaltered rock known as a corestone or woolsack.
