Chapter 6 - Weathering, erosion, and soil

Question 1

Define Weathering

Answer 1

• Mechanical breakdown and chemical alteration of rocks and sediment when exposes to air, moisture, and organic matter.
• Weathering mechanical or chemical

Question 2

What is Mechanical (physical) Weathering. Where does mechanical weathering start and how are they formed?

Answer 2

Mechanical weathering is the disintegration of rocks

• begins at joints (fractures without movement/cracks)
• formed by cooling or unloading or deformation

Question 3

Define chemical weathering

Answer 3

Decomposition of rocks and sediments via chemical (and microbial) reaction that mainly dissolve minerals

Question 4

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

Answer 4

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

Question 5

Main products of weathering;

Answer 5

Silicates - regolith and soil

Carbonates - karst + caves

Question 6

Pressure release; granite. What is it? What kind of weathering is it associated to?

Answer 6

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

Question 7

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

Answer 7

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

Question 8

Explain the result of weathering at the intergranular level, and where it occurs

Answer 8

crystal dissolution and growth - intergranular

Question 9

What is spalling? What is it a result of?

Answer 9

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

Question 10

What is frost wedging? Why does it happen?

Answer 10

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

Question 11

What is root wedging?

Answer 11

Type of mechanical weathering

• is When the roots of trees break apart the rock, the regolith (unconsolidated rocky material covering bedrock) specifically

Question 12

What is desiccation? How does it produce a mud flat?

Answer 12

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

Question 13

What are playas? What are they composed of?

Answer 13

lake that carries water for a quick period of time, then dries out.
-they have an alkaline, salty crust

Question 14

Whats the lowest place on earth?

Answer 14

Death valley

Question 15

How does chemical weathering work?

Answer 15

Chemical weathering happens through chemical reactions that dissolve minerals

Question 16

What is the cause of chemical weathering?

Answer 16

Thermodynamic instability

Question 17

Rate of weathering is a function of:

Answer 17

• degree of thermodynamic instability
• climate
• surface area

Question 18

Define soil

Answer 18

naturally occuring, unconsolidated or loose material on the surface of the earth, capable of supporting life. Soil has three components: solid, liquid, gas

Question 19

Define Karst

Answer 19

terrain with distinctive characteristics of relief and drainage, including caves, arising primarily from the solution of soluble bedrock by natural waters

Question 20

Define Regolith

Answer 20

greek for blanket rock

-it is the layer of loose heterogeneous material covering solid rock. It includes dust, soil, broken rock

Question 21

How do you know what will dissolve first from chemical weathering?

Answer 21

(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

Question 22

Rank the minerals in the bowman reaction series from Highest temp to lowest temp that they were formed

Answer 22

Highest to lowest:

from olivine to pyroxene to amphibole(hornblende) to biotite to potassium feldspar to muscovite/mica to quartz)

Question 23

How does climate/moisture change the type of weathering that occurs?

Answer 23

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

Question 24

How does surface area effect weathering?

Answer 24

Once the water gets in the joints, it attacks the edges and joints. Most susceptible. Edges get weathered… spheroidal weathering weathering

Question 25

What is spheroidal weathering? What type of weathering is this?

Answer 25

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

Question 26

Soil as a function of climate and vegetation

Answer 26

• 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

Question 27

Layered soil, whats on top? whats in the middle? whats on bottom?

Answer 27

Bedrock at the bottom, partially disintegrated bedrock in middle, then regolith on top, with soil right on it

Question 28

Name 3 types of soil, and properties of them

Answer 28

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