Chapter 6 - Weathering, erosion, and soil Flashcards
Define Weathering
- Mechanical breakdown and chemical alteration of rocks and sediment when exposes to air, moisture, and organic matter.
- Weathering mechanical or chemical
What is Mechanical (physical) Weathering. Where does mechanical weathering start and how are they formed?
Mechanical weathering is the disintegration of rocks
- begins at joints (fractures without movement/cracks)
- formed by cooling or unloading or deformation
Define chemical weathering
Decomposition of rocks and sediments via chemical (and microbial) reaction that mainly dissolve minerals
For the results of physical weathering;
crystal dissolution and growth
wedging/exfolation from unloading + daily heating and cooling
spalling due to fire
frost wedging
root wedging
dessication
Give examples where this happens
crystal dissolution and growth - intergranular
wedging/exfoliation from unloading + daily heating and cooling - larger scale deserts, savannas (constant expansion and contraction of the rock, fatigue)
frost wedging - tundra, roads
root wedging - regolith
desiccation - mud flats, playas
Main products of weathering;
Silicates - regolith and soil
Carbonates - karst + caves
Pressure release; granite. What is it? What kind of weathering is it associated to?
When granite is uplifted and eroded, its contained energy (from crystallizing with lots of pressure below the surface) is released by outward expansion,
- this outward expansion is called pressure release
- results in origin of fractures called sheet joints, which spread out, causing trouble for nearby roads
- associated to physical weathering; wedging/exfoliation from unloading
For the following scenarios, what weathering is the causation? Give a type of that weathering for each of the scenarios/locations
mud flats, playas
forests
tundra, roads
regolith
intergranular
larger scale deserts, savannas
Physical/Mechanical weathering
crystal dissolution and growth - intergranular
wedging/exfoliation from unloading + daily heating and cooling - larger scale deserts, savannas (constant expansion and contraction of the rock, fatigue)
frost wedging - tundra, roads
root wedging - regolith
desiccation - mud flats, playas
Explain the result of weathering at the intergranular level, and where it occurs
crystal dissolution and growth - intergranular
What is spalling? What is it a result of?
Spalling - breaking of rocks/ores/etc into smaller pieces
-bolders of rocks with chips falling off
Result of forest fires, over a long period of time, they do their toll on a rock
What is frost wedging? Why does it happen?
Type of mechanical weathering
Frost wedging takes place when water seeps into cracks and expands as it freezes. Angular pieces of rock are pried loose by repeated freezing and thawing.
- frost wedging happens becaues water water expands when it becomes a solid
- also the reason potholes exist
What is root wedging?
Type of mechanical weathering
- is When the roots of trees break apart the rock, the regolith (unconsolidated rocky material covering bedrock) specifically
What is desiccation? How does it produce a mud flat?
Type of mechanical weathering
water in playas quickly dries, creating mud flats
- water in playas are frozen, but as they slowly melt, ice breaks up and the ice and water spreads
- ice then hits the rocks and the rock moves
- moves about 2/5m an hour
- creates shallow pond
What are playas? What are they composed of?
lake that carries water for a quick period of time, then dries out.
-they have an alkaline, salty crust
Whats the lowest place on earth?
Death valley
How does chemical weathering work?
Chemical weathering happens through chemical reactions that dissolve minerals
What is the cause of chemical weathering?
Thermodynamic instability
Rate of weathering is a function of:
- degree of thermodynamic instability
- climate
- surface area
Define soil
naturally occuring, unconsolidated or loose material on the surface of the earth, capable of supporting life. Soil has three components: solid, liquid, gas
Define Karst
terrain with distinctive characteristics of relief and drainage, including caves, arising primarily from the solution of soluble bedrock by natural waters
Define Regolith
greek for blanket rock
-it is the layer of loose heterogeneous material covering solid rock. It includes dust, soil, broken rock
How do you know what will dissolve first from chemical weathering?
(remember bowman reaction series, where the mineral at the top forms at the highest temperature)
Think of it as being people born in equator vs north pole
-person who was born in a country in the equator will have a harder time surviving in the north pole than someone who was born there
-essentially, the further you are away from your natural habitat, the more likely you will die
-Olivine will be the first mineral that will dissolve if you put it in a solution at a low temperature
-these minerals will disintegrate in the same way they will dissolve
-relative instability
Rank the minerals in the bowman reaction series from Highest temp to lowest temp that they were formed
Highest to lowest:
from olivine to pyroxene to amphibole(hornblende) to biotite to potassium feldspar to muscovite/mica to quartz)
How does climate/moisture change the type of weathering that occurs?
Chemical: High temp(higher rate of rx)
wet = more weathering (think carbonates + acidic rain)
-if a rock is prone to chemical weathering, more moisture means more weathering
Physical: Low temp = more physical weathering, thing fatigue, and frost wedging
-dryer more physical weathering
How does surface area effect weathering?
Once the water gets in the joints, it attacks the edges and joints. Most susceptible. Edges get weathered… spheroidal weathering weathering
What is spheroidal weathering? What type of weathering is this?
Chemical weathering.
-in spheroidal weathering, a stone, even one that is rectangular to begin with, weathers to form a more spherical shape because that is what is most stable shape it can assume
Soil as a function of climate and vegetation
- climate is the most important factor influencing soil type and thickness
- as temperature goes down, the types of vegetation that you can grow goes down
Layered soil, whats on top? whats in the middle? whats on bottom?
Bedrock at the bottom, partially disintegrated bedrock in middle, then regolith on top, with soil right on it
Name 3 types of soil, and properties of them
Regolith: lots of chemical dissolution, thiock soil (tropical soil), thick brown soil, lots of organic matter
intermediate soil - not as thick
dry soil - very thin soil if any at all, and lots of regolith underneath
