How weathering shapes landscapes

Question 1

Where does weathering occur?

Answer 1

• Predominantly in situ (Latin phrase that translates literally to “on site” or “in position.”) forming saprolite (weathered bedrock) and then soil.
• (Regolith (the layer of unconsolidated solid material covering the bedrock of a planet.) includes the saprolite & soil, but also includes all other non-parent rock material (incl. physically weathered materials)

Question 2

What does the rate of weathering depend on?

Answer 2

• The rate of weathering will depend on rock composition and structure, climate (precipitation, temperature), subsurface water, slope orientation, bioturbation and vegetation.

Question 3

What is bioturbation?

Answer 3

• Bioturbation is defined as the reworking of soils and sediments by animals (most commonly worms & ants) or plants. These include burrowing, ingestion and defecation of sediment grains.

Question 4

What are the first four weathering features that can be seen in the diagram?

Answer 4

• Vertical cracks in the rock (joints)
• Horizontal cracks which can form from sheeting - expansion from release of overburden pressure
• Spheroidal weathering: effects of mechanical and chemical weathering where a rock
• Water seaping into fractures causes chemical weathering gradually from the surface inward

Question 5

What is Spheroidal weathering?

Answer 5

• Spheroidal weathering is a form of chemical & mechanical weathering that affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock that is known as saprolite

Question 6

Study the diagram of the weathering features

Answer 6

https://docs.google.com/document/d/11SlS_6djE3BGAezT2-FeVRTdxG1P1jVyj-u3X2L5qDo/edit?usp=sharing

Question 7

What happens when a landscape gets weathered?

Answer 7

• The topsoil gets eroded away and the cracks in the rock allow for chemical weathering to shape the rocks and soil until there are only remnant sculptured rock such as Granite tors.

Question 8

What are granite tors?

Answer 8

• Tors usually overlie unaltered bedrock and are thought to be formed either by freeze–thaw weathering or by groundwater weathering before exposure.

Question 9

What is Freeze-thaw weathering

Answer 9

• Freeze-thaw occurs when water continually seeps into cracks, freezes and expands, eventually breaking the rock apart

Question 10

what is ground water weathering?

Answer 10

• As slightly acidic water infiltrates the ground to become part of the groundwater system it causes weathering of the rocks. The abundant silicate minerals are chemically weathered into clay minerals and soluble byproducts.

Question 11

Study the diagram for how tors are formed.

Answer 11

• google doc

Question 12

What is an Inselberg

Answer 12

• An isolated hill or mountain rising abruptly from a plain. e.g. Uluru

Question 13

How is an Inselberg formed?

Answer 13

• Inselbergs arise from rocks which erode at a slower rate than that of the surrounding rocks
• Spacing of joints in the granitic rocks controls weathering and development of inselbergs and tors.

Question 14

What is Case Hardening?

Answer 14

• A weathering phenomenon of rock surface induration (A hardened mass or formation). It is observed commonly in: felsic alkaline rocks

Question 15

What do land forms such as tors tell us about the climate and environment?

Answer 15

• They preserve fossils and layers in stalactites and stalagmites show difference in climate over the years

Question 16

What is Duricrust

Answer 16

• A hard mineral crust formed at or near the surface of soil in semi-arid regions by the evaporation of groundwater.

Question 17

Why are Duricrusts a major part of Australian environments?

Answer 17

• Duricrusts have widespread structural controls upon inland Australian land forms
• Duricrusts are actually paleo-soils formed when Australia was a humid tropical continent. When Australia became an arid continent these soils became oxidised and cemented to form hard duricrusts (products of past active weathering).

Question 18

What is Silcrete?

Answer 18

• Silcrete is an indurated soil duricrust formed when surface sand and gravel are cemented by dissolved silica

Question 19

How does Silcrete become a duricrust?

Answer 19

• This is the accumulation of SiO2 as a precipitate leached from the upper soil and deposited as a silica rich horizon, also in humid climates. The silica often cements sandy or gravelly soil particles together forming a very hard horizon of particles and cement. Especially pure silcretes when fractured form an edge like glass and were used by Aboriginies for tools.

Question 20

Study the diagram of the Silcrete

Answer 20

• Google doc

Question 21

How do Physical and chemical weathering interrelate?

Answer 21

• Often there is a process of physical weathering that follows a process of chemical weathering
• Chemical weathering “softens” it up and then a physical weathering processes takes over
• However, there are physical processes that can work in their own right to break down physical materials.

Question 22

What are four examples of physical weathering?

Answer 22

• Water can freeze and expand to shatter rock
• Pressure release
• Water in the rock can be heated to form vapour (rapid expansion) that stresses rock
• Biological

Question 23

What is exfoliation?

Answer 23

• Exfoliation is a physical weathering, common in granite, an intrusive igneous rock which is formed deep within the crust and under great pressure from the overlying materials. When granitic rocks are moved to the surface by tectonic processes, the pressure is released and the granite expands slightly, resulting in exfoliation or in other word, the rock sheds its skin.

Question 24

Study the diagram for Exfoliation of Granite

Answer 24

• Google doc

Question 25

What is Thermal Expansion?

Answer 25

• Thermal expansion, or insolation, is a very important type of weathering, which occurs particularly in desert type areas. It is essentially very similar to freeze-thaw weathering, without the water. The outer layer of the rock is heated greatly by the sun during the day, causing it to expand. At night, the cooling of the rock causes it to contract. After this expansion and contraction has been repeated many times, the outer layer of the rock peels away

Question 26

What is Heat Spalling?

Answer 26

• Heat spalling can be heat from forest fires and brush fires will cause the outer surface layers of rock to expand quickly and break away in spalls (exfoliation)

Question 27

What is Biological weathering?

Answer 27

• Biological weathering is the weakening and subsequent disintegration of rock by plants, animals and microbes.
• Growing plant roots can exert stress or pressure on rock.

Question 28

What are some examples of Biological weathering?

Answer 28

• Biological processes can also produce chemical weathering, for example where plant roots or microorganisms produce organic acids which help to dissolve minerals.
• Microbial activity breaks down rock minerals by altering the rock’s chemical composition, thus making it more susceptible to weathering.

Question 29

What are the main needs for physical weathering?

Answer 29

• The pull of Gravity

- Water

Question 30

What is mass movement?

Answer 30

• The bulk movements of soil and rock debris down slopes in response to the pull of gravity, or the rapid or gradual sinking of the Earth’s ground surface in a predominantly vertical direction.

Question 31

What controls mass movement?

Answer 31

• Chemical and physical weathering processes produce a weakening of surface rocks, making the susceptible to the pull of gravity
• Because gravity is the main force controlling movement, slope is an important variable

Question 32

What are the two main components of mass movement?

Answer 32

• Mass movements can consist of either rigid mass or a mass of individual particles
• Movement can be rapid or slow

Question 33

What makes mass movement rapid or slow?

Answer 33

• The wetter the soil is the faster the process

Question 34

What are some of the different type of mass movement?

Answer 34

• Soil creep
• Translational slide
• Slump-rotational slide
• Rockfall
• Earth flow
• Debris Avalanche
• Mudflow
• Solifluction

Question 35

What is a Slope creep?

Answer 35

• The slow downslope movement of the soil, underlying regolith and particles that occurs on every slope covered with loose, weathered material. At a rate too slow to be observed over a period of months, it is indicated by slow tilting of trees, poles, gravestones, and other objects set into the ground on hillsides.

Question 36

How is a Slope Creep formed?

Answer 36

• Apart from direct gravitational influences, frost heaving causes slope creeps: as interstitial water freezes, surface particles are forced up and out perpendicular to the slope; when let down by melting, these particles are drawn directly downward by gravity and are thereby gradually moved downslope. Other processes involved are the wedging action of root growth and the wetting and drying of soil layers.

Question 37

Study the diagrams of the slope creep and the different types of mass movements and what level of moisture creates them

Answer 37

• Google doc

Question 38

What is the mass movement type Solifluction?

Answer 38

• Solifluction is a slow flow that occurs over slopes of 6 degrees to 20 degrees. In cold climates where summer melting of ground ice provides a saturated zone 50 - 100 cm deep (the active layer) above a permanently frozen layer, solifluction is termed gelifluction.

Question 39

What types of mass movements are landslides?

Answer 39

• Landslides include the mass movements known as Translational slide, Slump-rotational slide, earth flow and Debris Avalanche.

Question 40

When do Landslides occur?

Answer 40

• These occur on steep slopes where a mass of material slides over a shear plane or a series of shear planes (rotational or planar)

Question 41

What are the conditions required for Earth flows and debris avalanches?

Answer 41

• They occur when the material moves relatively quickly as a chaotic mass of particles well lubricated by water or ice, or when the slope is so steep and the material so fragmented that the mass can flow with minimum lubrication
• These occur on steep slopes (26 – 45°)
• Soil type (capacity to fluidize) import.
• Increase in volume downslope
• Non-Newtonian in behavior

Question 42

What is a Scree slope?

Answer 42

• The loose debris or talus accumulated at the foot of a cliff comprising angular stones and boulders.

Question 43

Where do landslides usually occur and what usually needs to happen for them to happen?

Answer 43

• Usually in mountain ranges

- Usually caused by an earthquake coupled with rainfall