L06 - Weathering

rock cycle order

outcrop (weathering, erosion), sediment, sedimentary, metamorphic, igneous (magma)

what is weathering

Weathering is the mechanical/chemical breakdown of rock/mineral on earth's surface into smaller particles

weathering caused by?

temp changes

water

chemical reactions

what is erosion?

removal and transportation of rocks/soil from their original locations, by agents (wind, water, ice)

does weathering result in smaller particles having a different composition from the original substance?

it may or may not ( can do both)

which weathering changes the rock composition?

chemical weathering

why should we care about weathering as engineers?

civil: impacts building materials

geological: decreases stability of soil or rock slopes, can cause landslides

why is weathering important?

weathered material is:

- eroded (transported)

- deposited as sediment

- forms sediment leading to sedimentary rocks

- creates clay, sand, fertilizer etc

- creates landforms

what happened in trois rivieres in regards to weathering?

cracking caused by weathering (oxidation of pyrrhotite in concrete aggregate from local quarry) in homes

in trois rivieres, how did the cracking happen?

pyrrhotite in the concrete has iron, was exposed to oxygen and water, which oxidized and caused swelling and cracking, affecting the structural integrity of homes

what are the four types of mechanical weathering?

1. frost wedging

2. salt wedging

3. exfoliation

4. root wedging

what is mechanical weathering?

split of large rocks into smaller ones by exerting forces that exceed the strength of the rock

frost wedging

Process that splits rock when water seeps into cracks, then freezes, expands, and exceeds the tensile strength of the rock, causing rock to split

what does frost wedging produce?

talus aka scree, which are large angular blocks of rock

how often does frost wedging happen?

frequently, it happens in seasonal cycles

what is salt wedging

pressure created by the crystallization of salts in pore spaces and along rock fractures,

when water evaporates in salt wedging, what happens?

minerals are left behind to grow and exert pressure on surrounding rock

where does salt wedging commonly happen? why polluted air?

coastal areas, arid climate, polluted air

polluted air comes with sulphates and many salts in its air

how did salt wedging happen in egypt?

frequent removal of water for irrigation left salt in the rock of monuments, resulting in the damage of historical monuments via salt wedging

how does exfoliation happen as mechanical weathering?

at depth, rocks are under pressure from above rocks

removal of above rocks = rock expands

results in formation of fractures parallel to ground surface (exfoliation)

what is exfoliation?

formation of fractures parallel to the ground surface (aka sheeting)

what is root wedging?

tree roots growing in existing cracks in rock, and as the root grows, this causes the rock crack to expand

root wedging is a _____ weathering force in rock

minor

why does chemical weathering take place in different conditions compared to mechanical weathering?

these rock-forming minerals aren't as stable at low temp/pressure, or at exposure to atmospheric gases and water at the surface

how does instability in mineral composition affect chemical weathering?

instability results in chemical reactions occurring with atmospheric reactants (oxygen, CO2, water) and polluted air

where are rocks found that are prone to chemical weathering?

deep inside the earth where there is an absence of atmospheric gases

what are the 3 types of chemical weathering?

1. solutions (dissolution)

2. hydrolysis

3. oxidation

what is dissolution?

rocks dissolve (break up into ions in solution) when reacting to water (especially acidic water)

what rocks experience dissolution?

evaporites, carbonates (including gypsum, calcite)

what is hydrolysis?

a chemical reaction involving water that produces a NEW mineral

does dissolution result in new minerals?

no

what is the impact of acidity in dissolution/hydrolysis?

acids help both processes

combination of water and CO2 in atmosphere forms carbonic acid

what makes hydrolysis different compared to dissolution?

different: chemical reactions create new/altered minerals, and some ions may be left over (stay in solution)

same: both involve water and acidity helps process

what is oxidation

reaction of free oxygen with metallic minerals

what rocks are more affected by oxidation?

rocks containing metals

examples of oxidation rocks containing metals

ex: iron sulphides (pyrite, pyrrhotite), iron silicates (biotite, amphibole)

what is the effect of oxidation?

coloration of rock

creation of new mineral

example of effect of oxidation

iron oxidation - high iron content results in reddish colour

which mineral group is the most resistant to chemical weathering?

native elements (ex: gold, zinc, etc)

chemical weathering - in low resistance minerals, what type of chemical weathering most commonly happens?

dissolution

chemical weathering - in medium resistance minerals, what type of chemical weathering most commonly happens?

oxidation and hydrolysis

chemical weathering - in high resistance minerals, what type of chemical weathering most commonly happens?

hydrolysis

what factors helps mechanical/chemical weathering?

faults/fractures

porosity

increase of exposed surface area

climate (temperature, moisture)

composition of minerals

how do faults, fractures, and joints aid weathering?

faults and fractures provide conduits (routes) that water and roots can penetrate the rock on a deeper level

how does porosity aid weathering?

porosity = natural cavities and pore space due to rock formation

spaces allow water and gas to penetrate rock more easily

how does mechanical weathering change the surface area/volume ratio?

once a large boulder breaks, splits into smaller parts and more surface area is exposed, exposing each smaller piece more to weathering

how does temp affect chemical weathering?

chemical reaction rates increase with temp

rule of thumb: chemical reactivity rates increase by a factor of 2 for every 10 degrees C

how does temp affect mechanical weathering?

freeze-thaw effects

what are common weathering products that resist physical weathering?

quartz

what are common weathering products that are weak to chemical weathering?

olivine

pyroxene

amphibole

mica

how does quartz weather?

resists physical weathering

non reactive to most chemical weathering

breaks down to sand-sized particles and stops

how does feldspar weather?

resists physical (for a while)

breaks down with chemical weathering

how does weathering time affect feldspar

short weathering time = results in sand

long time = results in clay

how does olivine, pyroxene, amphibole, and mica weather?

weak to chemical weathering

quickly breaks into clay

end products of physical weathering

sand, clastic sediment

end products of chemical weathering

clay, ions in solution