6.1 - Weathering And Soils Flashcards
What is the critical zone?
It is the shell of the earth that we live in.
What are the characteristics of the critical zone?
It is disaggregated = broken up into pieces
It is less dense than solid rock = a handful of soil is less heavy than a handful of rock (it has lost mass)
It is different colour from solid rocks = they contain different minerals and other constituents
There are plants in the top of the critical zone and more water in the critical zone than in solid rocks
Why are there plants in the top of the critical zone?
There is pore space for roots and nutrients
Why is there more water in the critical zone than solid rocks?
- There is more pore space in the critical zone
- The minerals are more hydrated
What are the 3 types of weathering?
- Physical
- Chemical
- Biological
What is physical weathering?
Rocks and minerals break into smaller fragments with generally minor effect on composition
Physical weathering processes include
- Temperature
- Abrasion
- Plants and animals
How does temperature drive physical weathering?
Physical weathering is driven by the diurnal cycle: day (hot) and night (cool)
There is alternating expansion and contraction. The differences in expansion leads to differential stresses which causes cracks.
= breaks off rocks
- During the day, rocks and other geological materials can heat up due to exposure to sunlight, which can cause them to expand. At night, these materials can cool down and contract. Over time, repeated cycles of expansion and contraction can cause the materials to break down and crack, which can lead to physical weathering.
- Winds are often stronger during the daytime when temperature differences between land and sea are greater. These strong winds can cause rocks and other materials to rub against each other, which can lead to abrasion and further physical weathering.
What is temperature-induced exfoliation?
It is the peeling away of rock layers because of stresses induced by outer layers being warmer or colder than inner portions.
- When rocks are heated by the sun during the day, they can expand, causing stress on the surface of the rock. At night, when temperatures drop rapidly, the outer layers of the rock can contract and cool down faster than the interior of the rock. This creates tension within the rock, and if this tension becomes too great, the outer layers can split off, resulting in exfoliation.
- Canada experiences extreme temp differences from day to night
- Exfoliation is influenced by underlying mineralogy
What is frost wedging?
It is the breaking of rock when water expands as ice inside pore spaces
How does abrasion drive physical weathering?
It occurs when rocks and other geological materials are worn down by frictional forces. This process is driven by the movement of water, wind, or ice, which can transport sediment and other materials across the surface of rocks, leading to erosion and abrasion.
Water:
- Freeze/thaw
- Flow can dislodge bits of rock
- Debris in water (rocks, sediments) abrades other rocks it flows over. This causes rounding of rocks and grains in rivers/shores
Wind:
- The debris in wind (eg. Slit,clay and sand) abrades rocks
Ice
- Freeze/thaw
- Debris in ice (rocks and sediments) abrades rocls
- Ice also transports lots of ground up rock material
How can plants and animals drive physical weathering?
- Plant roots enhance physical disintegration by entering cracks in rocks and prying them apart
- Burrowing organisms have a very minor effect (although can significantly affect chemical weathering)
Neither of these physical processes are globally significant compared to the actions of water, ice, wind and temperature
What happens when physical weathering breaks up a rock?
The rock is broken up into smaller pieces without changing the chemical make p of the rock.
- The larger the surface area of the rock, it has increased exposure to chemical weathering
Why do chemical reactions alter rocks and minerals at earth surface (near-surface) conditions?
Because most rock-forming minerals (primary minerals) are not thermodynamically stable at earths surface temperature and pressure
- Rock-forming minerals at Earth’s surface are not thermodynamically stable because they have formed under conditions of high temperature and pressure, which are typically found deep within the Earth’s interior. When these minerals are exposed to the lower temperature and pressure conditions found at the Earth’s surface, they can become unstable and undergo chemical reactions or physical changes that lead to their breakdown and alteration.
What factors could affect chemical weathering?
Temperature = chemical reactions are usually faster at higher temps. There is often more liquid water and biology available at higher temps
Water and water flow = carries reactants to the rock and carries products away
Reactants = acids and/or oxygen
Products = if concentration of dissolved products are too high the reaction will slow and stop
What are net results of chemical weathering
Convert primary minerals (eg. Feldspars) to secondary minerals (eg. Clays)
Deliver nutrients (eg. P) from rocks to biosphere in soluble forms
