rock cycle, and origin of soil

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CE 240

Soil Mechanics & FoundationsLecture 2

Rock Cycle, and Origin of Soil(Das, Ch. 2)



Outline of this Lecture

1.Rock types and rock cycle2.Basic Mineralogy

3.Weathering of rocks1. mechanical weathering

2. chemical weathering

4.Origin of soil



What is a soil?

Soil is defined as the uncemented

aggregate of mineral grains and decayed

organic matter with liquid and/or gas in

the pores between the grains

(A) gas (mostly air);

(B) solid particles (minerals);

(C) liquid (water, contaminant

liquid, etc.);



Where did soil come from?

Soils are formed by weathering of rocks.

More specifically, the mineral grains that

form the solid phase of a soil aggregate

are the product of rock weathering.

So that, we need discuss

(a) rocks and rock type;

(b) weathering of rocks;



What is a ‘rock’?

In Geology (the science studies rocks), ‘rock’ isdefined as the solid material forming the outer rockyshell or crust of the earth. There are three major

groups of rocks by its origin:

(1) Igneous rocks: cooled from a molten state;

(2) Sedimentary rocks: deposited from fluid medium;e.g., products of weathering of other rocks in water;

(3) Metamorphic rocks: formed from pre-existing rocks

by the action of heat and pressure.

Apparently, the igneous rock is the one far moreessential and intrinsic since the other two types are

relative secondary in origin.



The Identification Chart of the Igneous Rocks

(Bowen reaction)

Hi-resist to

weathering

Lo-resist to

weathering



Rock Cycles



Basic Mineralogy of Rocks

Rocks are formed with minerals. What is a mineral?

1) a naturally occurring chemical element or compound;

2) formed by inorganic processes;

3) with an ordered arrangement or pattern for its atoms –

crystalline structure;4) possesses a definite chemical composition or range of compositions.

The opposite of mineral property is amorphous, i.e., theproperty of non-crystal, order-less property possessed byglass, volcanic glass, etc.; oil or coal can neither be

regarded as minerals by their organic involvement.



Basic Mineralogy of Rocks (cont.)

So we can simply express the mineral asmineral = composition + crystalline structure

There are more than 2000 naturally occurred minerals have

been discovered; only a bit more than 100 are common andused in college mineralogy. However, of the 100 commonminerals only about 25 are abundant rock-forming minerals.

The main types of minerals are:metallic minerals;nonmetallic minerals;

carbonate minerals;sulfate minerals;sulfide minerals;silicate minerals;oxide minerals;clay minerals.



Comparison of surface and

subsurface conditionsSubsurface

• High temperature butconstant at which

minerals reachequilibrium

• high confining

pressure (stress)• less water or no water

• no oxygen

Surface

• low temperature, andhighly variable

• little or no confiningpressure (stress)

• abundant of water

• abundant of oxygen



Conclusion can be drawn from the comparison:

Rock at the surface will undergo changes

---- This change is called Weathering

Weathering is the physical breakdown

(disintegration) and chemical alteration(decomposition) of rocks to form soil or loose

particles at or near Earth's surface. Weathering

causes deterioration of building materials. It also

weakens rocks, a great concern when weathered

rocks are used for foundation.



Two types of weathering

Mechanical weathering:

Physical disintegration or degradation of

rock pieces without a change in composition--size reduction

Chemical weathering:

decomposition whereby one mineral species is

changed into another through various chemicalprocesses. Water plays a major role, through:

1, provide oxygen,

2 provide mobility for moving ions.



Mechanical weathering always involves

fracturing--but that can occur by a whole

host of causes.

Chemical weathering tends to weakenrock, thereby making it easier to break.

Likewise, mechanical weathering creates

additional surface area that is prone to

chemical attack. In this way, the two

processes work together.



mechanical weathering:

With or without water makes distinguisheddifference in rock weathering process, in

arid region, since no water or little water

presence, even the carbonate rocks (e.g.,limestone) are not subject to solution but

persist as resistant rocks. The rock mass

may be sharp angular, and topographydeveloped.



Mechanical Weathering (cont.):

Mechanical weathering processes include:

1) freezing & thawing (frost wedge)

Example:

Pottery container with water in winter time,when water is frozen and become ice it can

have 9% of relative volume change, i.e.,

dv/v = 9% = 0.09This is a very large strain! Similarly if there is

water in rock fractures it will force the fracture

to propagate into further depth.




The stress generated by frozen water is

about 550 psi, check with Table 6.1 you can

find out that this is a value comparable tothe tensile strength of most rock types. Or

only one order of magnitude less. But

remember rocks are experiencing many

freezing-thawing cycles before they are

finally breaking down.

σ




2) differential expansion and contraction

As temperature changes (in deserts or from

forest fires), not all parts of a rock or all its

minerals expand or contract by the same amount.

So when rocks are heated or cooled, the mineralgrains are subjected to differential stresses,

which may be sufficient to make the rock spall, or

break off in sheet-like pieces.



Thermal expansion

* repeated daily heating and cooling of

rock;

* heat causes expansion; cooling causes

contraction.

* different minerals expand and contract

at different rates causing stresses along

mineral boundaries.



The thermal expansion coefficient αL and αV

characters how much a mineral change its

dimension in response to a unit degree

increase in temperature . Using a finiteexpression

∆L=LαL∆T

P L

T

l

L)(

1

∂

∂=α

PV

T

V

V )(

1

0 ∂

∂=α




The linear thermal expansion coefficient αL ison the order of 10-5/ºC, and it could be

anisotropic, i.e.,

αLx ≠ αLy ≠ αLz

For example, αmax is in the elongation axis (C-axis).



Differential weathering

Twin Tower God’s Garden Colorado



Twin Tower, God s Garden, Colorado



Processes of mechanical weathering

unloading: jointing, exfoliation, and sheeting

Upon removal of overburden, the elastic component of rock

deformation is recovered and the rock expands. Theunloading may occur when the overlying rocks are eroded orrocks are removed from a quarry. The expansion caused by

unloading may be sufficient to fracture the rock. Suchnaturally formed cracks are known as joints.

Typically, large plutons (bodies of igneous rock) or

metamorphic bodies split into sheets that are parallel to themountain face, a process known as exfoliation. It is alsoknown as sheeting if the expansion from unloading occurs in

granite to form rock slabs.



joints are parallel cracks in which rocks on eitherside are not offset; SheetingSheeting rock layers peel likerock layers peel likelayers of an onionlayers of an onion

Chapter 5: 2) Mechanical weatheringChapter 5: 2) Mechanical weathering ) Unloading



Another “sheeting” example

See also T&L Figure 5.8

g



Vertical columns frommagma cooling & shrinking

“columnar jointing”



Glacially polished basalt columns(end view)

Columnar jointing in basalt



Spheroidal Weathering. Granite illustrates weathering forms quite well.Chemical weathering attacks to granite along joints and makes rounded

boulders (Alabama Hills near Lone Pine) .



Chemical weathering

Chemical weathering=chemical processes thatdissolve and decay earth materials;

Chemical weathering needs the rock exposes toair and water. Mechanical weathering could

enhance chemical weathering by disintegration,

i.e., increase the surface area of rock blocks anddebris and this will greatly accelerates chemical

weathering.



Chemical weathering rate depends on1. Temperature

2. Amount of surface area

3. Availability of water or natural acid

Thus, rocks in tropical environmentexperience most severe chemicalweathering.



Because of its dipolar nature

water is able to dissolve many

chemical compounds. In

addition to the solution effect,

water aids decomposition

through acid action,

oxidation, and hydrolysis.



Acidity of Natural Waters

Water is a good solvent.

Acidic water is better!

pH of most naturalwaters ranges from 4

to 9pH > 9 or < 4 occurs inextreme

environments

Chemical Weathering of silicate



Chemical Weathering of silicate

minerals by carbonic acid

feldspar + water + carbonic acid = clayminerals + dissolved ions

2KAlSi3O8 + H2O + 2H2CO3 =

Al2Si2O5(OH)4 + 2K +

+ 4SiO2(aq) + 2HCO3

-



Why is rainwater naturally acidic?

Rainwater contains dissolved CO2 from

atmosphere.Dissolved CO2 reacts with water to form

carbonic acid (H2CO

3)

CO2 + H2O ⇔ H2CO3

Carbonic acid dissociates to producehydrogen ion (H+) and bicarbonate

H2CO3 ⇔ H++ HCO3-



Chemical weathering (cont.)

Common chemical weathering processes are

solution (dissolution), oxidation, and hydrolysis.

Rock reacts with water, gases and

solutions (may be acidic); will add or remove elements from minerals.



Solution (or dissolution)

* Several common minerals dissolve in

water

i), halite; ii), calcite

* Limestone and marble contain calcite

and are soluble in acidic water.

Marble tombstones



and carvings are

particularlysusceptible tochemicalweathering bydissolution. Notethat the urn andtops of ledges are

heavily weathered,but the inscriptionsare somewhat

sheltered andremain legible.Photo taken in oneNew Orleansgraveyard.



A 16th-century

monastery in Mexicoshows the ravages of

weathering mostly from

wind and wind-driven

rain. The rock is volcanic

tuff.

Karst landscape of Guilin China caused



Karst landscape of Guilin, China, caused

dissolved Carbonate rocks.



The other two kinds of chemical weathering

Oxidation

Oxidation - Oxygen combines with iron-bearingsilicate minerals causing "rusting". Iron oxidesare produced that are red, orange, or brown incolor.

Hydrolysis

Hydration-reaction between mineral and water.

Transport of weathering products



Glacial soils

formed by transportation and deposition of glaciers; Alluvial soils

transported by running water and deposited along

streams;

Lacustrine soils

formed by deposition in quiet lakes;

Marine soils

formed by deposition in the sea;

Aeolian soilstransported and deposited by wind;

Colluvial soils

formed by movement of soil from its original place bygravity (e.g., landslides).



Typical Soil Profile



This photo is an outcrop of a glacial till deposit. Glacial till is a heterogeneous mixtureof clay to boulder size particles deposited within or beneath glacial ice. The till type on

this photo is a dense or basal till with lenses of looser, sandy material (sandy till), thesoil type mapped in this area is the Montauk series [the solum(the A and B horizons)has been removed on this photo. photo location: Fearing Hill, Wareham, MA].

A photo of a glacialfluvial deposit (thetopsoil and subsoil of a



topsoil and subsoil of aHinckley soil has been

removed) from a gravelpit. This photo showsthe horizontal stratifiedlayers of sand andgravel on the top of thephoto called the topsetbeds or delta plain. Theinclined or dippinglayers of fine and

coarse sand (visible onleft part of photo) arecalled the foresetbedsor delta slope. The

foreset beds weredeposited into a glaciallake, the contact of thetopsetand foresetbeds(delta plain/delta slope)

marks the former waterlevel of the lake. town

Summary



Soil come from weathering of rocks.Mechanical weathering is accomplished by

physical forces that break rock into smaller

and smaller pieces without changing the

rock's mineral composition.

Chemical weathering involves breaking downrock components and internal structure and

forming new compounds.

Whereas weathering breaks rocks apart,

erosion removes rock debris by mobile agents

such as water, wind, or ice.



Readings:

Ch. 2

rock cycle, and origin of soil

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