Rocks and weathering

Mapping hazards

Helps planners to know which areas should be avoided to decrease impacts

Earths structure

Crust
Lithosphere
Asthenosphere
Mantle
Outer core
Inner core

Lithosphere

The earths crust and rigid upper part of the mantle

Asthenosphere

Semi-molten and what tectonic plates float on

Mechanisms of plate movements

Ridge push, Convection drag, Slab pull

Ridge push

Intrusion of magma into spreading ocean ridges pushes plates apart

Convection drag

Convection currents in the mantle drags the crust

Slab pull

Cold, denser oceanic lithosphere sinks due to gravity, and drags the rest of the plate with it

Example of Constructive boundary

Eurasion - NA

Example of Destructive (O-C) boundary

Nazca - SA

Example of Destructive (O-O) boundary

Pacific - Philippine

Example of Destructive (C-C) boundary

Eurasian - IndoAus

Example of Conservative boundary

Pacific - NA

Describe sea floor spreading

Molten magma from inside the earth oozes out and solidifies

Explain sea floor spreading

This occurs in constructive plate boundaries between oceanic plates that are moved by plate mechanisms

Describe subduction

When one plate moves under another it sinks

Describe fold mountains

When plates move towards each other they are forced up as they collide

Describe ocean ridges

Magma rises as the plates move away from each other and cool when it reaches the surface

Describe volcanic island arcs

When lithosphere is subducted the slab melts when the edges reach a depth which is sufficiently hot. Hot, remelted material from the subducting slab rises and leaks into the crust, forming a series of volcanoes.

Explain subduction

Oceanic plates are heavier than continental plates. Zones usually dip at angles 30-70 but older, heavier plates dip more steeply.

Explain fold mountain building

The lighter plate buckles up and folds

Explain ocean ridges

Convection currents cause the uplifting of ocean floors

Explain ocean trenches

When plates converge, denser plates are pushed beneath the lighter plate and deep into the mantle

Example of sea floor spreading

mid atlantic ridge

Example of subduction

Nazca - SA

Example of fold mountains

Himalayas

Example of ocean ridges

Mid atlantic ridge

Example of ocean trenches

Mariana trench

Example of volcanic island arcs

Hawaiian islands

What is weathering?

The decay and disintegration of rocks in situ

In situ

Refers to rock that isn't moving or being eroded

Types of rocks

Metamorphic
Igneous
Sedimentary

Metamorphic

Minerals in rocks are changed underground by heat and pressure

Igneous

Magma cools and becomes solid

Sedimentary

Made of particles of sediment such as sand and clay

Physical weathering

Freeze thaw
Heating and cooling
Salt crystal growth
Pressure release
Root action

Freeze thaw

Water trapped in rock joints freezes when temps drop below 0°. When it turns to ice it expands by 9% and exerts enough pressure to shatter the rock.

Granular Disintegration

Granular disintegration occurs in rocks which consist of more than one mineral as different minerals expand and contract at different temps.

Heating and Cooling

Surface temps can fluctuate between 80° and 0°. This means that rock surfaces will expand and contract daily.

Block disintegration

Occurs where a rock consists of only one mineral

Salt crystal growth

When salt in rocks crystallises out of solution . High temps draw saline groundwater to the surface. Evaporation leaves salt crystals in pores and cracks creating stresses and disintegration. Crystals of sodium sulphate can expand by 300% in areas of high isolation.

Pressure release

The majority of igneous rocks were created deep under the earths surface at high temps and pressures. As erosion brings many of these formations to the surface they become subject to less pressure. This unloading causes rocks to fracture horizontally. Spiralling, the vertical fractures occur due to the bending stresses of unloaded sheets across a 3D plane.

Vegetation root action

Some plants and trees grow within the fractures of rock formations. The roots penetrate deep into the soil in search of water and nutrients. As they penetrate the soil they go through cracks, exerting pressure, progressively making cracks deeper and wider.

Chemical weathering

Hydrolysis
Hydration
Carbonation

Hydrolysis

This is the weathering reaction between mineral ions and ions of water when water and a surface meet. This results in the decomposition of the rock by forming new compounds and by increasing the pH of the solution involved through the release of hydroxide ions

Hydration

This is the process where certain minerals absorb water, expand and change. A larger SA also speeds up reactions.
Anhydrite - Gypsum expands by 0.5%

Carbonation

Rainfall combines with dissolved carbon dioxide or organic acid to form weak carbonic acid

Factors affecting weathering

Climate (Rainfall and temp (Peltier))
Geology
Vegetation
Relief

Climate

Peltiers model focuses on the idea that climate is the dominant factor

Geology

Chemical composition
Nature of cements
Joints and bedding planes

Chemical composition

Limestone contains calcium carbonate (carbonation)
Granite contains feldspar (hydrolysis)

Joints and bedding planes

Course grain weather quickly due to large spaces
Joint patterns exert strong control on water mvmt

Vegetation

Presence of veg can increase weathering through veg. root action and organic acids

Relief

Steep slopes remove eroded material quickly exposing fresh rock faces

Mass movement

Is the downward movement of rock and weathered material by gravity alone.

Factors affecting mass movement

Angle of slope
Nature of debris
Geology
Vegetation
Water
Type and structure of rock
Human activity
Climate

Types of mass movement

Soil creep
Mudflow
Landslip
Rockfall

Water and sediment movement

Rain splash
Surface run off
Sheetwash
Rills

Human causes of slope failure

Excavation
Waste heaps
Loading
Deforestation
Vibrations

Strategies to reduce mass movement

Pinning
Netting
Afforestation
Drainage
Mapping hazards

Soil Creep

This is the slow downslope movement of unconsolidated material and soft rock (rarely more than 1-2cm a year.) It is more likely to occur on saturated thick surface deposits on steep slopes.

Mudflow

This involves the rapid movement of rock and weathered debris mixed with water down valleys. it often occurs on the slopes of active volcanoes in the form of lahars.

Landslip

Single dramatic events when a section of a hillside becomes unstable and moves downhill after shearing has occured.

Rockfall

Tectonic stresses and erosion cause granite rock to fracture. Rockfalls later occur along these fractures. Fractures that develop parallel to the surface are called sheeting joints.

Rain splash

Raindrops impact break up clumps of soil. The lighter materials such as sand and silt are then carried off by surface runoff, leaving behind pebbles and gravel. The fine particles can mix with water and clog up pores, making it impermeable and forming puddles on flat land. On a slope, unabsorbed water flows downhill and carries away loosened soil particles. On a 5° slope 60% of mvmt is downslope. This increases to 95% on a 25° slope and is most effective on 33-45°

Surface run off

Surface wash occurs when the soils infiltration capacity is exceeded and water therefore flows downhill as a result of gravity. This is more likely to occur in the UK during winter months as ground freeze and drains become saturated.

Sheet wash

Sheet-wash is the unchanneled flow of water over a soil surface. It is capable of transporting weathered material and often occurs on footpaths or moorlands.

Excavation

Ground is removed eg in a road and railway cuttings to make level ground.
In areas where there is soft/unconsolidated rock this creates a steep slope liable to mvmt.

Waste heaps

Waste heaps from mining are often steep and porous which is unstable

Loading

Building on slopes adds mass that may trigger mass movement

Deforestation

Decreases interception and takes away binding agents

Vibrations

Movement of heavy vehicles can trigger mvmt

Pinning

Drilling a long hole into loose rocks and deep into stable rocks below and bolting them together, essentially binds loose rock down.

Netting

Metal netting is fastened to road cuttings to prevent loose rocks from falling

Drainage

Moving water away from vulnerable slopes reduces mass mass and lubrication

Afforestation

Increasing interception and binding soil together

Benioff zone

A narrow zone of deep earthquake foci at a subduction zone

Continental crust

The part of the Earth's crust that forms the continents

Continental drift

The theory that the continents have drifted across the surface of the globe.

Convergent plate boundaries

Margins where plates are moving towards each other. This can be oceanic-continental, as in the case of the Nazca and South American plates, or it can be continental-continental, as in the case of the Eurasian and Indian plates.

Core

The central part of the Earth below 2900 km from the surface.

Crust

The outermost layer of the Earth. It may be divided into continental (sial) and oceanic (sima)

Divergent plate boundaries

Plate boundaries in which the plates are moving apart, e.g. the North American and European plates. Plates are characterised by a mid-ocean ridge and/or a rift valley. They are also known as constructive plate margins.

Hotspot

A relatively small area where magma rises through a continental or ocean plate. As the plate moves across the hotspot a chain of volcanoes may form.

Island arcs

Chains of volcanic islands on the continental side of an ocean trench. They are associated with subduction zones

Lava

Molten magma that has reached the Earth's surface. It may be liquid or may have solidified.

Lithosphere

The outermost layer of the Earth, comprising the crust and part of the mantle.

Magma

<olten rock within the Earth. When it reaches the surface it is called lava.

Mid-ocean ridges

Lines of mainly mountains formed where two ocean plates are separating

Oceanic crust

That part of the crust underlying the oceans. It is basaltic in composition.

Oceanic trenches

Arc-shaped depressions formed at subduction zones where one tectonic plate (usually an oceanic one) plunges under a less dense continental one. These are the deepest parts of the oceans.

Oceanic ridges

The largest feature of the ocean floor. They are essentially a linear belt of submarine mountains. They occur at divergent (spreading or constructive) plate boundaries. New magma forces it way up between two plates and forces them apart. The ridges are characterised by a wide rift valley in their centre.

Sea-floor spreading

Where ocean floors grow as plates move apart at constructive margins.

Subduction zone

The area where one plate slides beneath another, where earthquakes occur.

Biological weathering

The form of weathering caused by the activities of living organisms. There are two types: chemical and mechanical (physical). Chemical weathering occurs with the release of organic acids as vegetation decays.

Carboniferous limestone (karst)

Formed of the remains of organic matter, mostly plants and shells. Limestone scenery is unique because it:
● is very hard but also permeable, and
● dissolves very slowly in acid water.

Chemical weathering

The form of weathering brought about by chemical attack of rocks, usually in the presence of water. Chemical weathering involves the 'rotting' or breakdown of the original minerals within a rock to produce new minerals. Some chemicals are dissolved and carried away from the weathering source.

Freeze-thaw

The form of physical weathering, common in mountains and glacial environments, caused by the expansion of water as it freezes. Water in a crack freezes and expands in volume by 9-10 per cent as it turns to ice. This expansion exerts great pressure on the rock, causing the crack to enlarge. After many cycles of freeze-thaw, rock fragments may break off to form scree slopes.

Granite

Coarse-grained, intrusive, igneous rock, typically consisting of the minerals quartz, feldspar and mica.

Joint

A vertical crack in a rock, often formed by compression; it is usually several metres in length. The weathering of joints in rocks such as limestone and granite is responsible for the formation of features such as swallow holes and tors.