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1. Origin of Soils and Rocks(Das, Chapter 2)

Sections: 2.1

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Introduction

•The mineral grains that form the solid phase of a soil aggregate are the product of rock weathering.

•The physical properties of soil are dictated by the size, shape, and chemical composition of the grains, and hence the rock from which is derived.

•Rocks are compact, semi-hard to hard mass composed of one or several minerals.

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On the basis of their mode of origin, rocks can be divided into three basic types:

•Igneous rocks

•Sedimentary rocks

•Metamorphic rocks

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Rock Cycle The formation cycle of different types of rock and the processes associated with them. This is called the rock cycle.

SedimentaryRock

MetamorphicRock

IgneousRock

The rock cycle is important in any discussion of soil formation, especially if we know that soils are nothing but disintegrated rocks.

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1 .Igneous RocksFormed by solidification of molten Magma ejected

from deep within earth’s mantle.

Magma has originated well below the surface and ascended towards the surface, and has crystallized as solid rock either on the surface or deep within the earth's crust as temperature fell.

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II. Classification Based on Mode of Occurrence

A. Volcanic or Extrusive RocksThey are formed when magma reaches the surface, along either wide vertical fissures or pipe-like openings in the earth crust.

B. Plutonic or Intrusive RocksThose rocks are formed when the magma rise through the earth’s crust but does not reach the surface.

Intrusions can be divided into two types:1. Major Intrusions

•Stocks•Batholiths•Sheets

2. Minor Intrusions•Dykes•Sills•Laccolith•phacolith

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– Extrusive/VolcanicExtrusive/Volcanic:: Igneous rocks that form due to Igneous rocks that form due to the freezing of melts the freezing of melts aboveabove the surface of the Earth. the surface of the Earth.

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•Intrusive/Plutonic:Intrusive/Plutonic: Form by freezing of melts Form by freezing of melts below below the surface of the the surface of the Earth.Earth.

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Igneous Intrusions

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Types of Igneous Intrusions

DiscordantDiscordant: cut across pre-existing fabric of rock layers

Dikes: are small igneous intrusions that cut across rocks into which the magma intrudes. They are commonly sheet-like, only a few meters wide, but possibly laterally extensive.

Stocks:Stocks: are fairly large (10’s of miles) igneous intrusions that cut across pre-existing rock layers. In size, they are on the order of an individual mountain peak.

Batholiths: are huge igneous intrusions made of many stocks. Their size is on the scale of an entire mountain range (100’s of miles).

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ConcordantConcordant: follow pre-existing fabric of rock layers without interruption of layering.

Sills: are also small igneous intrusions. They are sheets of rock that, unlike dikes, are parallel to pre-existing rocks. Think of magma invading sedimentary rocks by spreading out between rock layers. That magma would cool to form a sill.

Laccoliths:Laccoliths: are small intrusions having a flat floor and doomed roof.

Lopoliths: are also large, inverted mushroom-shaped (spoon-shaped) intrusions that “sag down” in the middle because of dense rocks.

Types of Igneous Intrusions

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Tabular Intrusions: PlutonsTabular Intrusions: Plutons

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Non-Tabular Intrusions: PlutonsNon-Tabular Intrusions: Plutons• Pluton: Irregular blob-shaped

discordant intrusions that range in size from 10’s of m, to 100’s of km

• Batholith: A pluton that is > 100 km2 in surface exposure

• Stock: A pluton that is <100 km2 in surface exposure

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Non-Tabular Intrusions: PlutonsNon-Tabular Intrusions: Plutons• Laccolith: a dome-like sill that bends the layers above it into a

dome shape

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1 .Igneous Rocks

The types of igneous rock formed by the cooling of magma depends on the i) composition of the magma; ii) the rate of cooling associated with it.

Bowen (1922) was able to explain the relation of the rate of magma cooling to the formation of different types of rock.

Bowen’s reaction principle: describes the order of

crystal formation from magma as cooling occurs.

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Bowen’s Reaction Principles

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1200

oC

1000

o

Bowen’s Reaction SeriesTwo series of minerals formed during crystallization of magma

Intrus. Extrus.

Gabbro Basalt

Diorite Andesite

Granite Rhyolite

Low Silica Magma

High Silica Magma

Fra

me-

D

oubl

e

S

ingl

e

Isol

ated

wor

k

S

heet

Cha

in

Cha

in

Temperature ofCrystallization

750

o

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Mineralogy of Igneous RocksMany different types of minerals occur in igneous rocks, but only about eight are normally present as essential constituents of a rock. They are:

•Quartz•Orthoclase •Plagioclase•Muscovite•Biotite•Hornblende•Augite•Olivine

Which of the eight are present is controlled primarily by the composition of the magma.

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Classification of igneous rocks (Summary)

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2. Sedimentary RocksSedimentary rocks are widely spread over the surface of earth. They constitute about 75% of the area of the continents.

Development

Weathering reduces the exposed rock mass to fragmented particles which can be more easily transported more easily by wind, water, and ice.

When dropped by the agents of transportation, they are called SEDIMENTS.

Sediments are typically deposited in layers or beds termed STRATA.

When compacted and cemented together (a process called LITHIFICATION), they formed sedimentary rocks.

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The process through which sediments are converted into sedimentary rocks is called DIAGENSIS. It includes the following phases:

1. Cementation

Water percolating through the voids (or pores) between the particles of sediment carries mineral matter which coats the grain and acts as cement that bind them together.

2. Compaction

The weight of top layers compacts sediments and expels water out.

3. Crystallization

Sometimes grains of sediments are joined together due to crystallization of some of their constituents due to pressure.

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Classification of Sedimentary Rocks

Sedimentary rocks are classified into two major categories:

I.Detrital Sedimentary RocksThey formed from minerals or rock fragments derived from the breakdown of pre-existing rock

SedimentParticle TermSedimentary Rock

Gravel

BoulderCobblePebblegranule

Conglomerate

Sand

Very course sandCoarse sandMedium sandFine sandVery fine sand

Sandstone

MudSiltClay

ShaleMudstone

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ConglomerateThis is a rock consists of cemented boulder, cobbles, pebbles and granules

SandstoneThis rock is composed essentially of pressure- cemented grains of sand. The cement agent determines the degree of indurations, or hardness, of sandstone.

ShaleIt is formed from silts and clays which have hardened into rock, with the principal indurations agent being pressure.

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1. Breccia is a variety of conglomerate, which has angular rather than rounded fragments.

REMARKS

2. When the grains in sandstone are practically all quartz, the rock is referred to as orthoquartzite.

3. For Sandstone, the three chief kinds of cement, in the order of their importance are:

4. Mudstone has a blocky aspect; whereas, in the case of shale, the rock is split into platy slabs.

•Silica•Iron oxides•Carbonates

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6. Shale is the most abundant of the sedimentary rocks. Nearly half of all the sedimentary rocks are shale.

7. Marl is a calcareous mudstone (expansive).

5. In mudstone and shale, size of the particles are generally less than 1/16 mm.

8. From engineering point of view, sedimentary rocks are the most important and relevant because:

• They are the most abundant, and hence encountered.

• They are the most problematic.

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II. Chemical Sedimentary rocks

Sedimentary rock also can be formed by chemical processes. Rocks of this type are classified as chemical sedimentary rock.

Examples of Chemical Sedimentary Rocks:

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Textures of Sedimentary Rocks

There are two main types of textures of Sedimentary Rocks:

1 .Clastic TextureRocks formed by mechanical weathering have clastic texture

2 .Non-Clastic TextureMost chemical sedimentary rocks have a non clastic texture. These rocks have somewhat the same appearance as igneous rocks with crystalline texture.

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Summary of Sedimentary Rocks

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3. Metamorphic RocksMetamorphic rocks are formed if a rock is subjected to

increase in temperature, pressure, or both, to such degree that a new TEXTURE or possibly a new MINERAL composition is produced.

The process of change is referred to as METAMORPHISM. It is derived from the Greek META which means change and MORPH, shape or form.

The original rock may be igneous, sedimentary or metamorphic.

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The origin of metamorphic rocks is the least clear among the three classes of rocks. Because no one has ever seen a metamorphic rock been formed, and for that reason many of our ideas about them are pure conjecture.

During metamorphism the minerals do not melt, but remain largely in the solid state.

The metamorphism process is isothermal process, i.e. the overall chemical composition of the rock is nearly the same before and after recrystallization.

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Classes of MetamorphismThere are three broad classes of metamorphism depending on the control exercised by temperature and pressure.

1. Thermal or Contact Metamorphism

Increased temperature is the dominant agent producing change. (Caused by igneous activity)

2. Dynamic or Dislocation MetamorphismThe dominant control is stress, extra heat is relatively unimportant. (Associated with faults & earthquake zones)

3. Regional Metamorphism

Both temperature and pressure have operated over a large (regional) area. (Associated with tectonic plate activity).

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The degree of metamorphism is related to the conditions of temperature and pressure under which the new metamorphic rock has formed, and may be assessed by the appearance of certain minerals.

The index minerals each of which indicates a particular temperature and pressure at the time it formed are used to define the metamorphic GRADE of the rock in which they occur.

One important characteristic related to the metamorphic grade is that grain size gets coarser with higher grade, i.e. rocks formed at higher temperature have coarse grained particle like GNEISS, and vise versa.

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Original RockMetamorphic

Rock

Sedimentary rocks

Mudstone, ShaleSlate

SandstoneQuartzite

LimestoneMarble

Igneous Rocks

DoleriteHornblende

DiabaseBiotite

Augite&ChloritePlagioclase

GabbroGranulites

Some metamorphic rocks and their original rocks

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CLASSIFICATION OF METAMORPHIC ROCKS

I. Classification Based on Type of Metamorphism

1. Contact Metamorphic Rocks

2. Dynamic Metamorphic Rocks

3. Regional Metamorphic Rocks

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Foliated•Rock has distinct banding or layering•Formed under direct pressure

Non-Foliated•No distinct layering character•Formed under uniform pressure

II. Classification According to Degree of Foliation (Textures)

CLASSIFICATION OF METAMORPHIC ROCKS

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CLASSIFICATION OF METAMORPHIC ROCKSII. Classification According to Degree of Foliation (Textures)

Well-Foliated

Moderately Foliated

Non-Foliated

SlateGneissQuartzite

PhylliteMigmatiteMarble

SchistGranuliteHornfels

MyloniteSerpentine

Amphiblite

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slate phyllite

schist

Foliated Metamorphic Rocks

gneiss

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Classification of metamorphic rocks according to metamorphic grade