SEDIMENTS & SOIL Flashcards
(25 cards)
how does Sedimentary rock from
cementation of sediment
2 parts of Sedimentary rock
Sedimentary ‘Cover’ / “veneer”
Igneous/Metamorphic ‘Basement’
how much of earth surface is covered by sedimentary rocks
80% but its less than 1% of Earth’s mass
How does physical weathering differ from chemical weathering
- Physical = Mechanical breakage / disintegration
- Chemical = Decomposition by reaction with water
Physical Weathering characteristics
- Doesn’t change mineral makeup
- Creates “detritus”
- Jointing - crustal rocks cool and expand at the surface, causing through-going fractures e.g. Exfoliation joints—onion-like sheets (Common in granites)
- Wedging e.g. frost, root, salt
3 types of detritus that physical weathering makes
= Coarse-grained – Boulders, cobbles, and pebbles.
= Medium-grained – Sand-sized.
= Fine-grained – Silt and clay (mud)
where is Chemical Weathering most prominent
- Maximized under warm and wet conditions - tropical weathering turns rock into “saprolite”
- Virtually absent in deserts
Common chemical weathering reactions
= Dissolution
= Hydrolysis
= Oxidation
= Hydration
explain dissolution as a chemical weathering reaction
- Some minerals (halite, gypsum, calcite) dissolve - acidity enhances this effect
- enlarges joints on the surface of a limestone outcrop + dissolves away sharp edges
explain hydrolysis as a chemical weathering reaction
- Water breaks cation bonds in SILICATE minerals (e.g. feldspar, quartz, biotite) - yields Dissolved ions + Alteration residues
explain oxidation as a chemical weathering reaction
= reaction where metal loses electrons
= Important process in mafic silicate decomposition
= Rusting = familiar example
explain hydration as a chemical weathering reaction
- Absorption of water into a mineral structure -> a volume increase
- Important processes in some clay minerals
what determines weather a mineral is more susceptible to weathering
- High T and P minerals tend to weather quickly at low T and P
- Low T and P minerals are more stable at Earth’s surface
organisms that are important weathering agents
= Plant roots
= Fungi
= Lichens
= Bacteria
*Organic acids attack minerals
what determines Chemical Weathering Rate
Mineral composition
how does block geometry (shape) determine chemical weathering rate
= Corners weather fastest; 3 sides of attack
= Edges weather at a moderate rate (2 sides)
= Flat faces weather slowest (1 side)
*Cube-shaped rocks develop “spheroidal weathering.”
what 3 things influences the rate of weathering attack
= Mineral stability
= Degree of compaction / cementation
= Subtle differences in texture, etc
in what way does soil develop
- vertical sequence of layers (soil horizons) called a profile
- develop from the surface downward
- only top is fertile
name of the Upper soil profile
Zone of leaching
- Ions from chemical weathering
- Fine silts and clays infiltrate
name of the lower soil profile
Zone of accumulation
- Ions form new minerals.
- Silts and clays clog pore spaces
5 different horizons that reflect soil-
forming processes
- O Horizon – Dark organic matter-rich surface layer
- A Horizon – Organic and mineral matter
- E Horizon – Transitional layer leached by organic acids
- B Horizon – Organic-poor mineral rich layer
- C Horizon – Slightly altered bedrock (parent material)
Soil-Forming Factors
= Climate – Amount of water and warmth.
= Substrate composition – Soil parent minerals.
= Slope steepness – Soils develop best on low slopes.
= Drainage – Wet soils are more organic-rich.
= Time – Older soils are more developed.
= Vegetation – Controls type of organic matter added.
how is soil classified
= mineral composition
= Environment
*12 soil orders are commonly recognized
3 ways Soil quality is degraded
= Overuse of pesticides
= Industrial contamination
= Salt build-up
