Rocks and weathering

Layers of the earth

Lithosphere

Asthenosphere

Mesosphere

Outer core

Inner core

Lithosphere

Crust and uppermost solid portion of the mantle

Asthenosphere

Plastic upper portion of the mantle

Theory of plate tectonics

The lithosphere is made up of large, moving plates, which float on the semi-molten asthenosphere and interact with each other to cause geographic phenomena.

2 types of crusts

Continental crust and oceanic crust

Differences between oceanic and continental crusts

Age: oceanic is younger, continental older

Thickness: oceanic thinner, continental thicker

Density: oceanic dense, continental lighter

Rock types: oceanic basalt, continental granite and andesite

The 2 theories for continental drift

Convection current theory

Dragging theory

Convection current theory

Tectonic plates in the lithosphere float on the semi-molten asthenosphere

Convection currents are created as heat in the lower mantle melts rock, which rises, cools at the upper mantle, and then condenses.

Seafloor spread theory

Ridge push - new, lighter crust is formed at divergent plate boundaries, which creates oceanic ridges, and pushes the movement of the older plates away. E.g. mid-atlantic ridge

Slab pull (dragging) - older, denser tectonic plates sink into the mantle at subduction zones, pulling the newer and less dense section of the plate along with it as it sinks. E.g. Japan - Pacific plate and Eurasian plate

The earth stays a constant size as the same amount of crust is being formed as it is being destroyed.

Hotspot

Stationry plume of hot mantle material that rises from deep, exceptionally hot regions in the mantle. It breaks through the crust to form a volcano.

As the hostpot source remains stationery whilst the plates above it keep moving, hotspot volcanic island chains are formed.

E.g. Hawaii volcanic islands

Seafloor spreading theory

process by which new oceanic crust is created at mid-ocean ridges and slowly moves outward, pushing continents apart.

Confirmation of seafloor spreading

Paleomagnetism - the magnetic grains in the cooled lava are an indicator of their age. Because they acquire the direction of the Earth’s magnetic field at the time of their cooling.

Magnetic grains across the mid-atlantic ridge were symmetric on either side.

Types of sea ridges and examples

Fast spreading ridges e.g. East Pacific rise

Slow spreading ridges e.g. Mid-Atlantic ridge

types of plate boundaries

Divergent

Convergent (Subduction, collision)

Conservative

Divergent plates, associated landforms and examples

2 plates of the same density move away from each other

Oceanic + oceanic = seafloor spreading, mid ocean ridges - mid-atlantic ridge

Continental + continental = rift valleys - Great Rift Valley, East Africa

Subduction plates, associated landforms and examples

Denser plate subducts under lighter plate. Steeper on continental side. Older and denser the oceanic plate, the larger the dip.

Denser oceanic + lighter oceanic = deep ocean trenches - Mariana Trench

Denser oceanic + lighter continental = volcanic island arcs - Honshu Island Arc

Collision plates, associated landforms and examples

2 continental plates with the same density collide with each other, and move upwards.

Fold mountains - the Himalayas.

Conservative plates, associated landforms, and examples

Two plates move past each other, not necessarily in different directions, but at different speeds.

Continental + continental

Oceanic + oceanic

Continental + Oceanic (rare)

Earthquake e.g. San Andreas Fault

Physical weathering processes

Freeze-thaw

Heating/cooling (Exfoliation)

Salt crystal growth

Pressure release (dilation)

Vegetation root action

Freeze-thaw

E.g. Snowdonia, Wales

High alpine areas with lots of moisture, and diurnal temperature fluctuations

Heating/cooling (exfoliation/onion skin weathering)

During the daytime, rocks expand when heated by the sun, and contract and night when they cool down, causing the layers to crack and peel.

happens in dry places with diurnal temperature fluctuations (deserts) e.g. the half dome, Yosemite, USA.

Salt crystallisation

Saltwater gets into cracks in rocks. Water evaporates, leaving salt deposits. Salt crystals grow and expand, putting pressure on the rocks, which weakens the rock, causing it to break apart.

Coastal regions, desert regione e.g. the Dead Sea, the Twelve Apostles, Australia

Pressure release (dilation)

Rocks buried under heavy layes of other rocks experience pressure. When the overlying material is removed by erosion, the pressure on the rock decreases, causing the rock to expand, and leading to fractures (dilation joints)

• Creates pseudo-bedding planes in inselbergs and tors

E.g. the tors of Dartmoor, England

Vegetation root action

Plant roots grow into cracks and joints in rocks. As the roots expand, they exert pressure on the rock, causing it to split and break apart.

Chemical weathering processes

Hydrolysis

Hydration

Carbonation

Hydrolysis

Formation of new clay mineral from feldspar-containing rocks

1. Rain picks up CO₂ from the atmosphere, forming carbonic acid (acidic water) 

2. Feldspar (mineral found in granite), reacts with the carbonic acid to form Kaolin, silicic acid and potassium hydroxyl.

3. The Silicic acid and potassium hydroxyl flow away with the solution, leaving Kaolin (weakened clay material) behind as the end product. The weakened Kaolin crumbles and breaks apart.

E.g. Skull Rock, Joshua Tree National Park

Hydration

1. Water absorbed by certain materials, causing them to increase in volume.

2. The expansion causes internal stress, leading to rock disintegration/cracking.

Carbonation

Erosion of limestone

1. Rain reacts with carbon dioxide, forming carbonic acid.

2. Carbonic acid comes into contact with limestone containing the mineral calcium carbonate, forming calcium bicarbonate

3. Because calcium bicarbonate is soluble, it percolates with the rainwater away, leaving behind weakened rock structure.

4. Gradually forms caves, sinkholes

E.g. the Waitomo caves, New Zealand

Carbonation associated landforms

Karst topography

• Stalagtites and stalagmites

Factors affecting type and rate of weathering (CRRRV)

Climate

• Temperature and rainfall

Rock type e.g. limestone more susceptible to carbonation, feldspare more susceptible to hydrolysis

Rock structure

• Porosity, permeability

Vegetation

• Can both increase and decrease rate

Relief

• Decreased weathering at the top due to decreased buildup of water. Bottom of slope collects water

The Peltier diagram

Predict the type of weatheirng in a place through its rainfall and temperature

The slope as an open system

Contains inputs and outputs of energy

• Input = energy absorbed into the slope - radiation, gravity, water input

• Output = energy released from the slope - re-radiated heat, mass movement, erosion

Exogenic and endogenic factors of a slope

Exogenic = factors created outside of a slope - rain, erosion, chemical and physical weathering

Endogenic = factors created inside the slope - earthquakes

Shear stress and strength of a slope

Shear stress = force that acts parallel to a surface, causing it to deform or slide

Shear strength = slope’s ability to resist shear stress before it breaks or falls

Parts of a slope

Regolith = all loose, unconsolidated material on a surface

• Scree = regolith on the talus slope

Mass movement/slope processes processes

Heaves (soil creep) - very slow movement of soil over time

Flows - fast, fluid movement of material mixed with water e.g. mudflow, lahar

Slides - blocks of consolidated materials moving together along a defined surface

Falls - abrupt movement of unconsolidated material falling through the air from steep slopes/cliffs

Heaves

• very slow movement of soil over time. barely perceptible except for the tilted fences/poles

• Forms terraces

• Expansion and contraction of rock caused by freeze-thaw cycles and moisture changes.

• Happens more in tropical climates with larger diurnal temperature changes

flow and example

• fast, fluid movement of material mixed with water.

• High water content

• Regolith absorbs large amounts of water after a period of significant rainfall, becoming saturated and heavy, and sheer stress exceeds sheer strength.

• Flows along a slip plane

• E.g. the Nevada del Ruiz eruption, Colombia, 1985 - causing mudflow and death of >20,000

Slides and example

• Section of consolidated rocks and soil move as a unit along a well defined surface (slide plane)

• Combination of weak rocks, steep slope, water content, and undercutting (shear stress > sheer strength)

• E.g. Cyclone Gabrielle, North Island, 2023 causing landslides

Rockfall and example

• abrupt movement of unconsolidated material falling through the air from steep slopes/cliffs

• Caused by weathering, undercutting (human and natural) or earthquakes, affecting the talus slope.

• E.g. Kaikoura cliffs, New Zealand

Factors that contribute to mass movement

Increasing shear stress

• Undercutting - natural e.g. waves, human

• Steep slope angle

• Loading of a slope - water, vegetation, human

• Earthquakes

Decreasing shear strength

• Burrowing by animals

• Weathering

• Deforestation

• Rock and soil type

• Deepness of regolith

Effect of water on sediment movement on slopes

Rainsplash

Surface runoff (sheetwash and rills)

Rainsplash

When raindrops fall, depending on its size, its force dislodges soil particles and splashes them short distances. This breaks down soil aggregates, and loosens the regolith.

Surface runoff

• Flowing water erodes unconsolidated regolith (loose soil), transporting them downslope.

  • Sheetwash - gentle, thin layer of surface runoff, removs a thin layer of topsoil

  • Steeper slopes/faster surface runoff creates rills and gullies

Impact of human activity on stability of slopes

• Farming/terracing

• Road cutting

• Deforestation

• Buildings

• Mining/quarrying/excavation

Strategy to reduce mass movement

• Vegetation/afforestation

• Stope stabilising

  • Terracing

  • Grading

  • Retaining walls

  • Geotextiles

  • Pinning

• Rockfall and landslide barriers

  • Rock nets & fences

  • Gabions

Case study of human impact on slopes

Fraser’s hill, Penang, Malaysia

• Landslides, rockfall, mudflow as recurring events at Fraser’s hill → roads undercutting pediment

• Malaysian government used geo-engineering methods to increase slope stability

  • Afforestation

  • Gabions made of bamboo and brush