Describe weathering through:
Frost cracking
2 points.
- Frost cracking occurs in a narrow temperature range of -3℃ to -10
- Water penetrates rocks and when this water freezes the liquid water migrates to the freezing front and freeze in existing spaces like existing cracks and expands them.
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Describe weathering through:
Salt and Plants
2 points.
- Salt cracking is a type of cracking is seen in arid environments, where crystals grow in existing cracks and expand them (like frost).
- Plant roots also grow in cracks and can widen them.
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Describe weathering through:
Tectonics
2 points.
- Rocks can crack below the surface (deep in the crust) ‘rock crusher tectonics’.
- Rocks arrive at the surface pre-cracked.
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What is regolith?
Rocks arriving at the surface of the Earth arrive in a reactive zone and start becoming regolith (weathered rock) through a variety of process.
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What are the layers of regolith?
3 points.
Regolith is divided into three layers
- Weathered Rock: Fractures appear and some chemical weathering starts
- Saprolite: Retains the original rocks structure but is fragmented enough to dig through
- Mobile Regolith: Detached from the layers below it. In motion vertically and laterally
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What are the processes of erosion?
3 points.
- Weathering: disintegration of rocks on the surface
- Transport: material transported (by rivers) to the ocean
- Corrasion: material in the river further corrades the channel creating more material.
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What is transport-limited erosion?
The fact that the bedrock is blanketed by mobile material indicates transport-limited erosion: what is determining is not the weathering of the bedrock, it is the transport process.
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What is weathering limited erosion?
In weathering limited erosion, erosion is limited by the rate the rock debris is loosened from the bedrock. In these areas, a blanket of regolith (e.g. soil) will not form
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Mobile regolith has a net downslope motion through what processes?
3 points.
- Gravity
- Biology
- Weather (meteorology)
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What is Rainsplash?
Direct impact of raindrops blasts grains from the soil surface. On a slopped surface, the ejected grains travel longer downhill than uphill resulting in a net flux of material downslope which is proportional to the slope
Where vegetation covers the surface, rainsplash and sheet wash are ineffective in moving particles from place to place
NB: Kinetic energy of the drop ∝ # and velocity of grains
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How is soil creep measured?
4 points.
Use of cosmogenic nucleotides like ¹⁰Be
- The creep of regolith down a hill slope is generally very slow (rates like 2 mm per year).
- Can’t be measured in real time
- ¹⁰Be found in the atmosphere. Attaches to clay particles.
- The longer the exposure of the clay particles to air the more ¹⁰Be on the them.
Note: Soil creep is a very slow, but deterministic process, can be modelled (we can imagine what this hill will look like over time)
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What are Stochastic events?
3 points.
- Occur alongside deterministic processes
- e.g. Landslides
- A threshold process: a FAILURE
Interesting but difficult to predict. Faster event
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Why do hillslopes fail?
4 points.
- Tectonic tilting
- Toe cut out by the river
- Cohesion forces have changed
- Water saturation
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What is internal friction?
The friction angle (angle of internal friction)
Property of material
- Dry sand 𝚽 = 31 degrees
- Most regolith = 20-40 degrees
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What is the ‘critical zone’?
3 points.
Top of vegetation to the bottom of the groundwater
Most landscapes on Earth are blanketed by a mobile layer of material derived from bedrock through the process of weathering. This blanket is called the ‘critical zone’ (top of vegetation to the bottom of the groundwater)
- Hillslopes cover the majority of Earth’s surface
- Can be soil mantled (rounded and smooth) or bare (steeper and jagged)
- Lead to channels
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What are the types of rock weathering?
4 points.
- Tectonics
- Salt and Plants
- Frost cracking
- Thermal stress and strain
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Describe weathering through:
Thermal stress and strain
3 points.
- Rocks expand when heated. if this is uneven strain forms and if this exceeds the strength of the material it fails. For example.
- Fires can spall (onion weathering) large proportions of the rocks in the fire zone to a depth of centimeters).
- Day/Night cycles probably result in microfractures. Cracks in boulders tend to have a N/S orientation.
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What impacts internal friction?
Affected by three elements of reality:
- Water
- Cohesion by minerals
- Cohesion by roots
Plants improve cohesion both by cohesion and by removing water (transpiration)
Cohesion can generate slopes that exceed the internal friction of the system
