sm 02 soil formation

Soil Mechanics ICE‐222CE 222

Chapter 1: Origin of soil and grain size

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Earth’s core 2

The Crust has a thickness of about 10 to 40 km. It is mostly made up of light silicates with an average γ = 3 g/cm3.

The upper and lower mantle is about 3000 km thick and is madeabout 3000 km thick, and is made up of metallic silicates and sulfides with γ = 3.5 to 6 g/cm3.

The liquid core is about 2100 km thick and is made up primarily of liquid Fe and Ni, with a γ = 10 g/cm3.

The solid inner core has a diameter of 1300 km and a γ = 13diameter of 1300 km and a γ 13 g/cm3.

Different rock types found on Earth’s surface 3

Rock cycle 4

• Basic types of rock :– Igneous– Igneous– SedimentaryMetamorphic– Metamorphic

• ROCK CYCLE : is the formation cycle of different types of rock and the processes associated with them

Igneous rocks 7

• Igneous rocks are created when molten material such as magma (within the Earth) or lava (on the(within the Earth) or lava (on the surface) cools and hardens. The hot materials crystallize into different mineralsdifferent minerals.

• The properties and sizes of various crystals depend on the magma’scrystals depend on the magma s composition and its rate of cooling.

• Examples: Granite Obsidian• Examples: Granite, Obsidian, Basalat, Pumice, Andesite, Diorite, Rhyolite.

Igneous rocks 9

Igneous rocks 10

An example of a dikeAn example of a dike crossing through a limestone formation.

Igneous rocks 11

Igneous rocks 12

This rock is an intrusive igneous rock, that was g ,formed by a pool of lava that hardens under the surface and then becomes visible when the surroundings get erodedsurroundings get eroded.

Sedimentary rocks 13

• Sedimentary rocks are made up of sediments eroded from igneous, metamorphic other sedimentarymetamorphic, other sedimentary rocks, and even the remains of dead plants and animals.

• These materials are deposited in layers, or strata, and then squeezed and compressed into rockand compressed into rock.

• Most fossils are found in sedimentary rocks.y

• Examples: sandstone, shale, conglomerate, limestone, chert, coal, gypsum.


A conglomerate is a sedimentary rock that has pieces of other rocks glued.


Layered limestone at Trenton Falls, New York


A sandstone formation shows the bedding planes.


This pink colored sandstone was formed when sand was buried and compressed, usually offshore of river deltas, but can occur in deserts lalso.

Metamorphic rocks 19

• Metamorphic rocks are much less common at the earth’s surface than are sedimentary rocks.y

• They are produced when sedimentary or igneous rocks literally change their texture and structure asrocks literally change their texture and structure as well as mineral and chemical composition, as a result of heat, pressure, and shear.

• Examples: Marble, slate, quartzite, schist, gneiss


Gneiss is formed from granite, gabbros and diorite that minerals in distinct bands


Marble is the melting and cooling of a limestone. The color is a function of the impurities of the parent limestone (pink for iron, etc.)

Weathering 23

• Weathering converts exposed rock to soil in place

• Erosion transports dissolved or fragmented material fromErosion transports dissolved or fragmented material from the source area where weathering is occurring to a depositional environment.

• Most of the earth’s surface is covered by exposure of sediment or sedimentary rock, by area.

• But the sediment layer is thin in most places, with respect to overall crustal thickness, so sedimentary rock is a minor volume fraction of the crust (in part by definition: once buried to the mid‐crust, sediments get cooked to metasediments)cooked to metasediments).

There are two primary types of weathering:Chemical and Physical/MechanicalChemical and Physical/Mechanical

1. Physical Weathering – the breakdown of rocks and minerals into smaller pieces without a change in chemical composition.

Root/Plant Wedging/Action Ice/Frost Wedging/Action

Exfoliation and Abrasion are also types of physical weathering

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Exfoliation and Abrasion are also types of physical weathering.

Mechanical Weathering 28

• Mechanical Unloading. Vertical expansion, erosion reduces load opening fractures

• Mechanical Loading. Impact and abrasion of wind borne particles in deserts and effect of intense rain drops

• Thermal Loading. Expansion of freezing water, high and low temperatures

• Wetting and drawing. Repeated loss & abrasion of water in certain clays

• Crystallization. Formation of crystals in fissures and pores, originally in solution

• Pneumatic Loading. Waves effect on trapped air in cliffs.

2. Chemical Weathering – the breakdown of rocks and minerals into smaller pieces by chemical action. The rocks breaks down at the same time as it changes chemical composition. The end result is different from the original rock. There are 3 types of chemical weathering:

1 Oxidation oxygen combines with the elements in the rock and it1. Oxidation – oxygen combines with the elements in the rock and it reacts. This the scientific name for rust.

2. Hydration – water can dissolve away many earth materials, including certain rocks.

3. Carbonation – carbon dioxide dissolves in water to form carbonic acid. This makes acid rain which chemically weathers (dissolves) rocks. Other acids also combine with water to make acid rain.

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SinkholesSinkholesChemical Weathering

CavesCaves

Disappearing Disappearing streamsstreams

There are 4 factors that effect the rate of weathering:1. Surface Area (exposure) - Exposing more surface area will increase the rate of . Su face ea (e posu e) pos g o e su ace a ea w c ease e a e oweathering. Promote chemical weathering.

2. Particle Size – Larger particles weather slower and smaller particles weather at a faster rate.

3. Chemical Composition (what a rock is made of) – Certain rocks and minerals are naturally weaker than others, while others are more resistant (stronger).

4. Climate – Warmer, moister climates have the most weathering. Heat & Water speed up all chemical reactions. This is the most important factor in weathering.

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There are 2 types of soil relative to the location of h kthe parent rock:

1. Transported2. Residual

Transported Soils – soils that formed in one place and were transported to theirTransported Soils soils that formed in one place and were transported to their present location either by wind, water or glaciers. You can tell when the soil does not chemically match the bedrock below it.

Residual Soils – soils that are located above the rocks that they formed from. In other words, the soil chemically matches the bedrock below it, because it is a product of that rock’s weathering over time.

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Soil deposits 33

• Heterogeneous and anisotropic material• Heterogeneous and anisotropic material

• Layered

What type of weathering occurs in an area that has an average of 165 cm of rainWhat type of weathering occurs in an area that has an average of 165 cm of rain each year and an average temperature of 18°C? ________________________

What type of weathering occurs in an area that has about 140 cm of precipitation

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yp g p peach year and an average temperature of 5°C? _________________________

Transportation of weathering products 36

ICE

WATERGRAVITY TRANSPORTATION AGENTSAGENTS

WINDWIND

There are 4 basic products of weathering, that can be eroded:

1. Soils2. Solid Sediments (boulders, cobbles, pebbles, sand, silt)3. Colloids/Clay Particles (not visible to your eye)3. Colloids/Clay Particles (not visible to your eye)4. Ions (very small electrically charged particles)

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Mount Rushmore There is a pile of weathered material at the bottom. It is slowly being eroded down hill by gravity. It will not be there forever!!

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Transported soils 39

Transported soils: depends on their mode of transportation

• Glacial soils are transported and deposited by glaciers• Glacial soils are transported and deposited by glaciers

• Alluvial soils are transported and deposited by streams d iand rivers

• Lacustrine soils are the deposits in quiet lakes

• Marine soils are deposited under seas

• Aeolian soils are transported and deposited by wind• Aeolian soils are transported and deposited by wind

• Colluvial soils are rapidly deposited soils via gravity in the form of mudslides or landslidesthe form of mudslides or landslides.

As soon as a rock particle (loosened by one of theAs soon as a rock particle (loosened by one of the two weathering processes) moves, we call it

erosion or mass wasting.

Mass wasting is simply movement down slope due to gravitygravity.

Rock falls, slumps, and debris flows are all examples , p , pof mass wasting. We call it erosion if the rock

particle is moved by some flowing agent such as air, water or ice.water or ice.

You can identify which agent of erosion transported each sediment by looking at a few characteristics:

R i W di h h b d h hRunning Water – sediments that have been transported through running water appear rounded and smooth and are deposited insorted piles.pGlaciers – sediments that have been transported by glaciers appear scratched, grooved, and are deposited in completely unsortedil b h d d d i l i Al b ldpiles, because they were dropped during melting. Also, boulders

can only be transported by glaciers.Wind - sediments that have been transported by wind are appearWind sediments that have been transported by wind are appear pitted (random holes) and frosted (glazed look) and are deposited in sorted piles. Only very small particles can be transported by windwind.

Gravity – sediments that are transported by gravity are found in piles at the bottom of cliffs or steep slopes. They appear angular

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piles at the bottom of cliffs or steep slopes. They appear angular and unsorted.

Glacial soils 42

The general term drift is applied to all deposits that are laid downthat are laid down directly by glaciers.

if ll d ill hDrift is called till when un‐stratified (i.e. composed of boulderscomposed of boulders weighing several tons to gravels, sands, silts, and clay particles are all mixed up).

Glacial soils 43

The deposits of till produce topographical features calledmorainesfeatures called moraines.

A terminal moraine is a ridge of till that marks the limit of glacier’s advance whereas a greatadvance, whereas a great amount of till spread out across a valley floor to yform a gentle rolling plain is called a ground

imoraine.

Glacier drift 44

Alluvial soils 45

Water is the most important erosion agent and erodes most often as running water in streams

or rivers.

Alluvial soils 47

Deposition more likely at lower velocitiesE i lik l hi hErosion more likely at higher velocities

Alluvial soils 48

Velocity variations in two planes in a stream

Alluvial soils 49

Zones of maximum turbulence in a stream

Alluvial soils 50

River PatternsRiver Patterns

Meandering Stream Processes and Formation of Oxbow Lake

Oxbow LakesOxbow LakesOxbow LakesOxbow Lakes

Lacustrine soils 53

• Lake deposition takes place as the river enters the large body of quiet water, which forces a drop of river’s flow velocity to practically zero.

• Shortly before arriving to the river‐lake interface, the velocity d i ffi i d l i h i b ddrop is sufficient to drop gravels in the river bed.

• In the depths of the lake are found the silts and clays. As lake fill th t i t h bfill up, they turn into a marsh or bog.

• In cold climates they choke with un‐decayed vegetable matter th t ill lti t l d tthat will ultimately produce peat.

• Eventually surface dries out and the lake disappears.

Lacustrine soils 54

Marine soils 55

• There are two distinct marine deposits: offshore and shore.

• Offshore deposits are very similar to lacustrine deposits except thatOffshore deposits are very similar to lacustrine deposits, except that they are much larger in size and flocculation is insensitive due to high concentration of salts in the oceans.

• Shore deposits are the product of waves that break while arriving towards a shore, and drop suspended sand and silts to form a offshore bar or spit.

• Littoral transport is the term used for the transport of non‐cohesive sediments, i.e. mainly sand, along the foreshore and the shorefacedue to the action of the breaking waves and the longshore currentdue to the action of the breaking waves and the longshore current. The littoral transport is also called the longshore transport or the littoral transport.

Marine soils 56

The “Mar Menor” lake in Spain was cut‐off from the Mediterranean Sea by littoral drift

Wind deposits 57

• Also known as aeolian deposits.

Wi d d il ti l b lli th• Winds can move sandy soil particles by rolling them along the ground and lifting them in air.

• Wind deposited sands are known as dunes.

• Fine‐grained soils can be airborne over long g gdistances by winds.

• A wind‐deposited silt is known as loessA wind deposited silt is known as loess.

Wind deposits ‐ dunes 58

Stages of a sand dune’s formation

The Work of Wind

SuspensionSuspensionSuspensionSuspension

SlidingSliding

RollingRollingRollingRolling

SaltationSaltation

l ll i l ( ) i i

The Work of Wind• Only small particles (<20 μm) can remain in

suspension for long‐time periods.

Colluvial soils 61

• Colluvial soils are produced by movement of soil from its original place by gravity, such as mudflows and landslides.

ll h f il i• Usually, the movement of soil is slow, and is called creep.

I t i k f ll d• In mountains, rocks fall and accumulate at the bottom forming a talus.

• Mudflows and landslides are very common in tropical areas due to high rainfall.

Earth / land flowEarth / land flow

humid areashumid areas

hillsideshillsides

rich in clay/siltrich in clay/silt

slow ratesslow rates

Deposition – the process where sediments are released/dropped by their agent of erosion.released/dropped by their agent of erosion.

Most deposition happens in standing/still bodies of water (oceans/lakes).(oceans/lakes).Deposition is caused by the slowing down (loss of kinetic energy) of the agent of erosion.

Th 3 f h i fl h f diThere are 3 factors that influence the rate of sediment deposition:

1. Sediment size –1. Sediment size 2. Sediment shape –3. Sediment density -

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Graded Bedding/Vertical Sorting – a situation where larger particles settle on the bottom and smaller particles settle towards theparticles settle on the bottom and smaller particles settle towards the top. This happens naturally when a fast moving river/stream meets a large standing body of water. This happens because the velocity of th t d i kl (A t f ll t i i t l k )the water decreases very quickly. (A waterfall emptying into a lake)

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Horizontal Sorting – a situation where moving water enters a l ill b d f l l d h l i llarger, still body of water slowly, and causes the larger particles to be deposited closer to the shoreline. Particle size decreases as you move away from the shore. y

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Delta – a fan shaped deposit that forms at the mouth of a i / h i l b d f Thi iriver/stream when it enters a larger body of water. This is seen

under the water. The particles are horizontally sorted.

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Alluvial Fan - a fan shaped deposit of sediments that forms h t / i fl t f t i t fl t d l iwhen a stream/river flows out of a mountain on to flat, dry plains.

These are not under water and are very visible. This only happens on the land. You can call it a “land delta”.

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Cross-Bedding – a situation where layers of sediments are depositedlayers of sediments are deposited at angles to one another as a result of a change of direction of the

i lerosional agent.

These are usually found in sand dunes deltas and alluvial fansdunes, deltas, and alluvial fans.

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Any QuestionAny Question

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sm 02 soil formation

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