Weathering

Weathering is a general term describing all of the changes that result from the exposure of rock materials to the atmosphere. It is also the breakdown and alteration of rocks at Earth’s surface through physical and chemical reactions with the atmosphere and the hydrosphere.

Types of weathering processes

Physical weathering

Physical weathering is the mechanical fragmentation of rocks from stressacting on them. Ice wedging may be the most important type. Physical weathering is caused by physical changes in the environment.

Insolation Weathering/ Thermal expansion (exfoliation)

The Process: A rock expands when heated and contracts when cool.Outer layers of the rock heat up and cool down more rapidly than the inner ones, resulting in stresses set up in the layers. Repeated differential heating and cooling sets up more stress within the boulder, causing the outer layers to peel off like the layers of an onion(Exfoliation: Sheeting).

Conditions: It usually happens in deserts, for the day is intensely hot and the nights very cold. There is also minimal cloud and vegetation cover there, exposing the rocks to the elements. The diurnal range of temperatures can exceed 50 degrees Celsius. It also occurs where rocks splits along joints/cracks/lines of weaknesses due to/resulting in block disintegration, where rock composition/minerals are of diff colours, rocks will disintegrate by granular disintegration. It is found most commonly in well-jointed rocks such as basalt and granite. Outer sheets are also easy to erode which helps perpetuate the process.

This exfoliation process creates arch-shaped and dome-shaped features on the exposed landscape

Example: Rock bursts in mines is evidence of rapid expansion of rock by pressure release

Wind, rain and waves

The wind can blow tiny grains of sand against a rock. These wear the rock away and weather it. Rain and waves are pretty self explanatory.

Freeze-thaw/Ice wedging

The Process:As the day becomes warmer, water enters the joints of rocks, which then freezes at night. Water expands by 9% when it freezes into ice. This is why water pipes sometimes burst in the winter. If water from rain or snow gets into a cracks/bedding planes/other openings in a rock and then freezes, the expanding wedge forces the crack further apart exerting great pressure on the rock walls, similar to the pressure produced by driving a wedge into a crack. This occurs quite often in areas where temperatures fluctuate around 0 degrees Celsius. Alternating freeze thaw or frost shattering slowly widens the joints and in time, cause piece of rock to shatter away from the main body.

This block disintegration occurs on steep slopes. The large angular rock fragments gather, forming talus cones and screes at the foot of the slope. The process usually occurs in crevices and joints of rocks where there is limited vegetation cover and is the most widespread form of mechanical weathering. It is common in upland regions of temperate regions like Britain where temperatures fluctuate around the freezing point for several months in winter. However, this does not occur in Polar Regions as temperatures rarely rise above 0 degrees Celsius.

Example:The effects of ice wedging in the Teton Range in Wyoming are seen in both the rugged surface of the mountain peaks and the accumulation of fragmented debris at the base of the cliff. The rock that forms the mountain range is massive granite cut by numerous fractures. Ice wedging, controlled in part by the fractures, produces the sharp, angular texture of the mountain peaks. The debris derived from ice wedging has accumulated in conical slopes near the base of the cliff.

Conditions that allows ice wedging:

  1. Adequate supply of moisture
  2. Pre-existing fractures, cracks or voids
  3. Located where temperatures fluctuates beyond and before the freezing point
  4. Impt as stress applied with each freeze.

Ice wedging is most effective where water is permanently frozen. It occurs more frequently above the timerline and is especially active on the steep slopes above valley glaciers where melt water produced during the warm summer days seeps into cracks and joints before freezing in the night.

Salt Crystallisation Tafoni

The Process:In arid regions, dry weather draws moisture to the surface of the rocks, encouraging the growth of salt crystals in pores and cracks can also pry apart rock. When water that enters the rock begins to evaporate, dissolved minerals in the water grow crystals. Can occur next to salt lakes or just the big ole sea itself. As the crystals grow larger, they exert a force great enough to disintegrate the rock. Rocks like sandstone are broken down by granular disintegration. This process is vividly expressed in the shattering of fence posts near the shore of the Great Salt Lake.

Tafoni: Tafonis are honeycomb weathering pits that are commonly associated with salt weathering. Basically its holey.

Common locations:

  1. Deserts
  2. Semiarid regions
  3. Near coasts where sals can precipitate easily

Pressure Release/Unloading

The Process:Intrusive igneous rock formed when magma cools and solidifies deep within the ground. (The slow cooling of the magma produces coarse-grained crystalline granitic rocks.) They are under great confining pressure from the weight of thousands of meters of overlying rocks. As the landscape is subjected to uplift, the regolith overburden is weathered, eroded and transported away, eventually exposing the granite. As the tremendous weight of overburden is removed from the granite, the pressure of deep burial is relieved. This release in pressure weakens the rock, allowing other agents to enter it and other processes to develop. Where cracks develop parallel to the surface, layer after layer of rock peels off in curved slabs (sheeting ). This exfoliation process creates arch-shaped/dome shaped features on the exposed surface exfoliation domes.

The same process occasionally causes rock bursts in mines and tunnels, when the confining pressure is released during the tunnelling operation. It can also be seen in many valley walls and in excavations for roads, where rock slumping, due to sheeting, can cause serious highway problems.

Hydration

The process:A process involving water, but little chemical change. IN this process, water becomes part of the chemical composition of the mineral such as gypsum, which is hydrous calcium sulfate. When some minerals absorb water, they expand, creating a strong mechanical effect that stresses the rock, forcing grains apart.

They also work together in Carbonation andOxidation to convert feldspar, a common mineral in rocks, to clayminerals and silica.

Biological weatheringPhysical and Chemical Weathering

Burrowing animals mix up the soil and loose rock particles which promotes further break down by chemical means. Lichens grow on the surface of bare rocks and extract nutrients from its minerals by ion exchange, causing physical and chemical alterations to the rock.

Humic acid from the decomposition of vegetation is released by a process calledchelation

The action of bacteria and respiration of plant roots increases the CO2 levels which accelerates the solution process, esp carbonation

Note: presence of vegetation cover significantly reduces the extent of mechanical weathering

Human economic activities release more carbon dioxide, sulphur dioxide and nitrogen oxides into the atmosphere. These gases then form acids in solution in rainwater. Acid rain readily attacks limestone and to a lesser extent sandstones, contributing to chemical weathering.

Humans are one of the most important geomorphic agents and causes of mass removal of materials. While critters like termites or prairie dogs excavate below the ground, human’s pollution, waste and mining/quarrying digs deep into the ground and helps speed up weathering. (eg. Bingham Mine)

The waste of humans and animals also adds to the weathering of the area, as most of them are slightly acidic and can undergo solution or carbonation.

Tree and plant roots often considered as wedges. But, roots follow paths of least resistance, so probably not that important. However, it can burrow through decaying rocks to speed up the weathering process  Roots growing through pavements and uplifting them. Trees swaying in the wind might have some prying capability as well.

Chemical weathering

Chemical weathering is the breakdown of minerals by chemical reactions with the atmosphere or hydrosphere. Rocks are decomposed and the internal structure of the minerals destroyed for the coming of new materials. This results in a significant change in the chemical composition and physical appearance of the rock. They attack minerals selectively, and usually occur in places of alternate wetting and drying.

The major types of chemical weathering are dissolution, acid hydrolysis, and oxidation.

Dissolution

Dissolution is a process whereby a mineral passes completely into a solution, like salt dissolving in water. Some minerals dissolve directly in water and the ions leached/flushed away.

Halite (salt) is best known example.

Gypsum is not as soluble, but it easily dissolves by surface water.

Most of large outcrops of minerals occur in non-humid regions as water is the most effective and universal solvents known. The structure of the water molecule requires the 2 hydrogen atoms to be positioned on the same side of the larger oxygen atom, giving it a concentration of positive charges on one side and a negative change on the opposite. Hence, the molecule is a polar and behaves like a tiny magnet, loosening the bonds of ions at the surface of the minerals it comes in contact with. Due to the polarity of the water, pretty much all minerals are soluble to a certain extent in water, though ionic bonds dissolve more easily.

Carbonation and Solution

Carbonation and solution occur when a mineral dissolves into solution, with water being the universal solvent.

Carbonation = Minerals dissolved by carbonic acid. Water readily dissolves carbon dioxide, thereby yielding precipitation containing carbonic acid (H2CO3). Carbonic acid preferentially dissolves certain rocks and minerals like limestone, marble.

All rain is mildly acidic (average pH ~5.6) but the pH decreases significantly with the addition of pollutants generated from the burning of fossil fuels. This more acidic solution is termed acid rain and typically occurs downwind from large industrial cities or from coal-burning power plants. (Sulfur monoxide and Nitrogen dioxide mixing into the water vapour in the rain that condenses into acid rain)

Solution= minerals dissolving into the water

A prominent product of carbonation and solution would be the Karst Landscape.

Karst Topography

Carboniferous limestone is well-jointed and bedded, which results in the development of Karst topography. It is a landscape that is characterized by numerous caves, sinkholes, fissures, and underground streams. Usually, it forms in regions of plentiful rainfall where bedrock consists of carbonate-rich rock, such as limestone, gypsum, or dolomite, that is easily dissolved. Calcium carbonates dissolves and is removed during solution by running water. Thus, surface streams are usually absent from karst landscapes.

Features of Karst Landscapes

Limestone Pavements: Clints and Grikes (Temperate)

Clints = slabs.

Grikes = lines of weakness vertical depressions

Clints and grikes form under relatively deep cover of soil where water, carrying carbonic acid (from carbon dioxide dissolved In water and organic acids from decaying vegetation) pick at the joints. Carbonic acid reacts with limestone pavement, causing a chemical change. Calcium bicarbonate is removed by solution along the joints. As the process repeats itself overtime, the joints will deepen and widen. It can be as wide as 0.5m and as deep as 2m.

Overtime, the soil on the top of the limesone platform will disappear down the newly eroded grikes and taken away from the tops of the crints. Some of the materials will be washed deep into drainage systems of the pavements through connecting fissures, leaving open grikes of a metre or more in depth.

Process was increased when forest clearance and grazing was introduced.

Limestone Peak Forests and Clusters (Tropical)

Limestone peaks are the giant teeth like protrusions in karst landscapes. When there is a piece of hard compact carbonate rock that experiences strong uplift during the monsoon climate of high moisture, these are formed. The area must not be plagued in glaciers for this to happen.

Peak forest = isolated towers

Peak cluster = linked-base towers

Examples: China, Guilin

Stalagmites and Stalactites

Stalagmites = stuff that grows from the ground up

Stalactites = stuff that grows from the ceiling down

Columns = when it extends from the ground to the ceiling

These are just calcium deposits. As the water carrying the calcium carbonate evaporates on the ceiling (stalactites), or drips to the ground before evaporation(stalagmites), the calcium deposits in it is left behind. Over time, the accumulation of calcium forms these structures.

Sinkholes and fissures

Sinkholes are collapsed chambers. There was once a top soil. However, as the limestone below it experiences weathering, after some time it could no longer support the soil on top, causing it to collapse in on itself. Boomz

Advantages and Disadvantages

Advantages: Construction and Tourism

  1. Limestone can be used as building material cement/mine safety dust/glass/animal feed filler/limestone tile
  2. Paint pigment
  3. Bring in tourists/ Recreational purposes
  4. Umpherston Sinkhole: The Sunken Garden
  5. Water sourcewells and springs aquifers ssupply water
  6. Unique ecosystem
  7. Limestone agriculture (mostly livestock)

Disadvantages

  1. Possibility of popping into a sinkhole
  2. Unstable land and irregular space stuff moving around constantly
  3. Drainage issues
  4. Pipes and underground hazards
  5. Hummocky terrain makes it hard to build things on it
  6. As water travels through the limestone, it makes it very alkaline 7-14 pH tastes gross apparently
  7. Infertile farmland (hence mostly livestock)

Limestone landforms

Tropical / Temperate
Closed depression / Cockpit karst, tower karst / Dolines, uvalas
Karren features / Pinnacles (spitzkarren) / Limestone pavements
(need glacial / lateral erosion)
Caves / Less well-developed
(higher rainfall, but not much seeps in to form caves / enter ground. Most flow off to rivers)
Stalagmite (more concentrated) / More well-developed
(drizzle)
Stalagtite (more spread out)
Drainage / Shallow holes (resurgence stream) + dry valleys + blind valleys
*Cross question between geomo and hydro: drainage density falls because stream length falls but area of drainage still the same

Acid Hydrolysis

Naturally (slightly) acidic water erodes rocks.

Carbonic acid is common in natural environments. It can be created when:

  1. Water combine with carbon dioxides in the atmosphere and in the root zones of plants where carbon dioxide is released into the soil
  2. Bacteria in the soil combine oxygen with decaying organic materials. Water seeping through organic remains become more and more acidic, thus increasing its effectiveness as a weathering agent
  3. Human activities
  4. Sulfuric and nitric acid in acid rain
  5. Sulfuric acid from mining coal or sulphide materials

Effects of these acids are seen in the corrosion of buildings and acidification of lakes and rivers and occasionally in the destruction of their biota.

Hydrolysis = chemical reaction where water and another substance decompose into ions in water. It can occur in pure water but in nature, it usually includes reactions with acids. The reaction between a mineral and an acid is called acid hydrolysis.

A good example of the production of secondary minerals is the chemical weathering of feldspar. Feldspar is an abundant mineral in a great many igneous, metamorphic, and sedimentary rocks. It is therefore important to understand how feldspars weather and decompose to make clay minerals. In turn, these clay minerals are transported and deposited to form the most abundant sedimentary rock, shale (or, strictly speaking, mudrocks).

Oxidation

Oxidationis the chemical combination of oxygen, in the atmosphere or dissolvedin water, with certain metallic elements to form oxides Oxygen reacts with iron in minerals to form iron oxide minerals, e.g., hematite (rust), that give rocks a red or yellow coloration. Oxidation causes rocks to crumble more easily

Of the elementsthat have variable charges, iron is the most important in weathering reactionson Earth. In most silicates, iron is present as Fe2+, but in the presence of Earth’s modern oxygen-rich atmosphere,Fe3+ is the favored oxidation state.Therefore,oxidation is especially important in the weathering of minerals that have highiron content, such as Basalt.

Most alkali (e.g., Na and K) and alkaline earth (e.g., Ca and Mg) elements are removed into solution by weathering reactions (Table 10.1) and eventually become enriched in seawater. On the other hand, the solid mineral residue becomes enriched in Al, Si—incorporated in clays—and Fe— incorporated in oxides. These minerals are stable in the surface environment

Comparison: Limestone vs Granite

Limestone / Granite
Class: formation / Sedimentary
Non-clastic
- Calcium from decomposition of marine organisms
- Gaps filled in with mud and clay
- Lithification: cementation and compaction and drying / Igneous (intrusive) (pressure release – exfoliation)
- Crystallisation of magma
- Granitic (viscous) magma
- Rock texture – coarse-grained (phaneritic) because more time for crystals to grow before solidifies into rock
Chemical composition / >50% generally calcite / calcium carbonate (carbonation and solution): homogenous (little regolith left behind) but pure limestone should be >90%
50-90%: impure limestone
Calcite: natural cementing agent so not all rocks with calcite are limestones / Feldspar (hydrolysis – clay), quartz (sand – granular disintegration) (clay, sand and corestones due to spherodial weathering = residual debris): heterogenous(insolation weathering – granular disintegration) (many types of elements)
Mica, biotite
Rock structure / *Only for carboniferous limestone
High secondary permeability
- Bedding planes / joints / faults (selective weathering – block disintegration)
- Shrinkage joints: drying – tensional (folding) / shear joints
- Older: probability of experiencing tectonic forces
Low primary permeability
- Small pore spaces: water cannot penetrate through easily
- Older: more compaction and cementation – less pore spaces / High secondary permeability
- Joints / faults (selective weathering – block disintegration): sheet joints, shrinkage joints: during formation stage of rock cooling process during formation of granite
Low primary permeability
- Crystals close together and interlock during crystallization
Physical hardness / Hard (not prone to physical weathering) – compaction / Hard (not prone to physical weathering) – feldspar and quartz – hard minerals and interlock tightly

Spheroidal Weathering

Spheroidal weathering is a form of exfoliation.