Download Rocks, Soil AP Env Sci Class 14 Dr. Mike Sowa

Rocks to Soil
AP Env Sci 2011-2012
Earth’s Surface is Constantly Changing
• Internal Processes: elevating parts of Earth’s surface
– Mountain Building
– Volcanic Activity
• External Processes: breaking rock and moving it to lower
elevation
– Weathering
– Mass Wasting
– Erosion
• Weathering Processes Make Soil
Soil Formation
• Soil – several ways to define:
– Unconsolidated material overlying bedrock
– * Material capable of supporting plant
growth *
• Soil is produced by weathering
– Physical, Chemical, Biological Processes
– Climate, topography, source material composition,
and time are factors
Mechanical Weathering
• Large pieces are broken up into smaller pieces
– Frost Wedging
– Biological Activity
• Processes do not work alone!
• Chemical processes attack exposed surface, so:
– smaller pieces -> more surface area exposed
– more exposure -> more chemical attack
Frost wedging of Diabase Boulders at Sourlands
Michael Hambrey
2. Mech. weathering – Biological activity
Biological activity does mechanical
work and introduces chemicals to rock
Chemical Weathering
• What does the Weathering?
– Water, with small quantities of dissolved components
– ex. CO2 (forms carbonic acid) -> H2CO3
– ex. O2 (oxidizer)
• Crust is made up primarily of silicate minerals
• Fate of Most Abundant Elements:
– Sodium, Calcium, Potassium, Magnesium – form ions
– Iron – oxidizes to iron oxides (red/yellow)
– Aluminum, Silicon, Oxygen – form clays
Example: Granite
• Granite: mainly quartz & potassium feldspar (KAlSi3O8)
• Potassium Feldspar reacts with Carbonic Acid:
2(KAlSi3O8) + 2(H2CO3) + H2O ->
Al2Si2O5(OH)4 + 2K+ + 2HCO3- +4SiO2
• Product is clay mineral and ions in solution
• Quartz is resistant to chemical weathering
• Quartz (sand) is transported to sea or elsewhere
3. Chemical weathering
Breakdown by Acids
Natural sources:
Decaying organic matter makes CO2.
CO2 + water  carbonic acid
Anthropogenic source (human impact):
SO2 and NOx create acids when
combined with water  acid rain
Factors Affecting Rate of Weathering
• Exposed Surface Area
• Rock Characteristics
– Mineral composition
– Solubility
– Cracks, etc
• Climate
– Temperature
– Moisture
– Warm temperature/high moisture favors chemical
weathering
Weathering rates of gravestones (Kingston)
Marble
Slate
1837
1817
What is Soil?
• Soil is a combination of mineral and organic matter, water, and
air.
• It is the part of the regolith (rock and mineral fragments
produced by weathering) that supports the growth of plants
5%
25%
water
air
45%
mineral
25%
humus (organic)
Characterizing Soil Texture: The Soil Triangle
Let’s Practice….
Source: EID
Soil: Big to Small
• Sand: 2 to 0.2 mm
• Silt: 0.02 to 0.002 mm
• Clay: 0.002 mm and smaller
You Need to Know the Soil Triangle:
Why?
• Texture impacts soil’s ability to retain and transmit air:
– Larger particles have larger spaces between them
– Packing -> loose = easily aerated (sand)
• Texture impacts soil’s ability to retain and transmit water:
– Surface area -> small particles have more SA/volume
– Nutrients and water retained by surfaces
• Sandy soils drain fast, dry out rapidly
• Clay drains slowly
• Texture affects “workability” (agriculture)
• Loam soils supports plant life best
Control of Soil Formation
• Parent Material: Source of weathered mineral
matter
– Parent = Bedrock – residual soils
– Parent = Sediment – transported soils
•
•
•
•
Time: How Long?
Climate: Temperature and Precipitation
Plants and Animals: Source of organic matter
Topography:
– Steep slopes often poor soil
– Bottomlands often poor drainage, much organic
– Best: flat to undulating uplands
Soil Profile
Soil Profile
• O Horizon: Loose and partly decayed organic matter
• A Horizon: Mineral matter mixed with some humus
• E Horizon: Light colored mineral particles. Zone of
eluviation and leaching
• B Horizon: Accumulation of clay transported from above
• C Horizon: Partially altered parent material
• Solum: O, A, E, and B layers. The “true soil” where soil
formation processes are underway and plant and animal
life exist.
Soil and Plant Growth:
Minerals, Water, Air
• Minerals:
– From Weathering
– Lost by Leaching
– Replaced by Fertilizer
• Organic
• Inorganic
– Can you identify the three most important?
Soil and Plant Growth:
Minerals, Water, Air
• Water:
– Absorbed through roots
– Lost through transpiration
– Soil gains water through Infiltration
– Soils retain water to varying degrees: WaterHolding Capacity
Soil and Plant Growth:
Minerals, Water, Air
• Air:
– Roots need air
– Aeration of soil creates space for air to enter
– Compaction has opposite effect
Soil Gains and Losses
• Gain Organic Matter:
– Ample organic matter (humus)
– Nutrients, infiltration and water-holding capacity,
aeration all increase
• Lose Organic Matter:
– Humus loss (how?)
– Nutrients, infiltration and water-holding capacity,
aeration all decrease
• Mineralization: Near Complete Loss of Organic
Component!