study guide for an exam (geotechnical)

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Geotechnical Engineering

3. Rocks

(Reference to notes: www.geology.csupomona.edu

Textbook: A Geology for Engineers by Blyth & Freitas

Photographs Microsoft Encarta)

https://www.youtube.com/watch?v=7m8tevimgco

YouTube video – Rocks overview:

Contents

1. Introduction

2. Igneous rocks

2.1 Formation of extrusive rocks (volcanoes)

2.1.1 Lava flow from volcanoes

2.1.2 Fragments blown from volcanoes – pyroclastic rocks

2.2 Formation of intrusive rocks:

2.3 Classification of igneous Rocks

2.3.1 Texture of igneous rocks

2.3.2 Mineralogy of igneous rocks

2.4 Identification of igneous rocks

3. Metamorphic rocks

3.1 Contact metamorphism (temperature)

3.2 Dislocation (or dynamic) metamorphism (stress/pressure)

3.3 Regional metamorphism (temperature + stress + large area

3.4 Heat and pressure of metamorphism

3.5 Identification of metamorphic rocks

4. Sedimentary rocks

4.1 Formation of sedimentary rocks

4.2 Deposited and composition of sedimentary rocks

4.3 Identification of sedimentary rocks

5. Rock cycle

6. Weathering and erosion

1. Introduction

  • Minerals are compounds of elements (fixed in properties) that combine with each other to form rocks (mixtures of minerals).
  • The term rock is used to describe the many materials which form the majority of the relatively thin outer crust of the surface of the earth.
  • Rock falls into three broad groups based on the origin of the rock:

Basalt

Granite

Obsidian

Marble

Slate

Conglomerate

1. Igneous rocks

2. Metamorphic rocks

3. Sedimentary rocks

Igneous rocks are those that solidify from the magma through either intrusive (Granite) or extrusive (Basalt or Obsidian) processes

Limestone

Metamorphic rocks are those that have been so altered by heat and/or pressure that they have lost their original character and have often been re-crystallized into new types of rocks (Marble or Slate)

Sedimentary rocks result from external forces on the Earth’s crust and are formed of particles deposited by rivers, glaciers, the wind, the sea or by chemical deposition from lakes or the sea (Conglomerates & Sandstone)

2. Igneous rocks

  • Igneous rocks originate from the hot magma which originates from below the surface of the Earth and which cools and solidifies near to or on the surface.
  • The molten rock will have a temperature between 800 and 1200oC and it will solidify by crystallising into a mass of mineral crystals.
  • The main minerals of igneous rocks are:
  • Quartz, Feldspar, Muscovite, Biotite and Mafics
  • Two types of igneous rocks:

1. Extrusive – when the rock flows out onto the crust & cools. Lava flowing from a volcano would produce extrusive igneous rocks

2. Intrusive – when magma is injected into existing rocks in the crust; e.g. Dyke, Sills, Batholiths, Plutons & Stocks

  • The character of igneous rocks is affected by the chemical constitution of the magma and the rate at which the magma cools.
  • Extrusive material which flows out onto the surface of the land or into the sea cools very rapidly and the resulting rock will be glassy or have very fine crystal grains where minerals have crystallised.
  • Intrusive rocks are often very coarse grained as cooling is slower, giving larger mineral crystals time to form.

Basalt (fine grains)

Granite (large grains)

2.1 Formation of extrusive rocks (volcanoes):

2.1.1 Lava flow from volcanoes

  • The lava usually has low viscosity and flows out to cover large areas.
  • Large areas of basalt have been formed in this way and the Giants causeway is an example of one of such lava field.
  • Volcanoes are holes in the Earth’s surface through which magma is ejected.
  • The lava is usually discharged from the volcano with considerable violence as the lava and the contained gases separate.
  • Fissure eruptions are those where the lava flows from cracks in the surface

The giants causeway is a field of roughly hexagonal basalt columns approximately 40 feet high on the shores of County Antrim in Northern Island. As the lava cools it shrinks, so rock in a solidifying layer is pulled in all direction at once forming the 40,000 pillars (www.nature.com)

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Extrusive

  • Pyroclastic rocks are formed when the fragments of material blown out of the volcano are consolidated into a rock.

2.1.2 Fragments blown from Volcanoes - Pyroclastic Rocks

2.2 Formation of Intrusive Rocks:

  • When the magma is injected into other rocks and does not immediately reach the surface, the rock is said to be intrusive.
  • Intrusions are either major where the crystals are usually large as the materials cools slowly, or minor when the intrusion is much thinner and fine to medium crystals are formed.

Examples of minor intrusions are dykes and sills

Dyke swarm

Dykes

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Sills

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  • A batholith is defined as a very deep and very large area of extrusive material.
  • Pluton is a moderately large magma flow intruded at one time and contained within a single boundary.
  • Stocks are smaller, near cylindrical vertical columns of intruded material
  • Laccolith is a sheet intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving a dome or mushroom-like form with a generally planar base.

Examples of major intrusions are Batholiths, Plutons and Stocks

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Both major and minor intrusions may eventually become exposed as the overlying and adjacent material is worn away.

Devils Tower:

1,267 feet high

Wyoming, USA

 

  • Igneous rocks are classified on the basis of texture and mineralogy.

2.3 Classification of igneous rocks

www.nysed.gov

2.3.1 Texture of igneous rocks

  • The texture of igneous rocks relates broadly to the way in which the rock was formed e.g. cooling rates:

Rapid cooling rates

Slow cooling rates

  • Coarse texture crystals >2 mm

  • Medium texture crystals 1–2 mm

  • Fine texture crystals <1 mm

 

 

Plutonic rocks

(extrusive)

Volcanic rocks

(intrusive)

  • Plutonic (phaneritic) texture igneous rocks have cooled slowly under considerable cover and they are coarsely crystalline (>2 mm) – clearly visible to the eye
  • Volcanic (aphanitic) texture igneous rocks consisting of small crystals that cannot be seen by the eye with or hand lens. The entire rock is made up of small crystals <1 mm in size. This texture results from rapid cooling in volcanic or (shallow subsurface) environments.

Examples of the types of texture of igneous rocks:

Two main types:

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Subtypes:

Vesicular texture igneous rocks (volcanic types) have holes, pores, or cavities within the rock. Vesicles are the result of gas expansion (bubbles), which often occurs during volcanic eruptions. Pumice and scoria are common types of vesicular rocks.

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  • As a result of Plutonic (cooled slowly) rocks being coarsely grained, classification is based on mineralogy since their individual mineral components can be distinguish.
  • Volcanic rocks are more difficult to classify because either their mineral composition is not visible or the rock has not fully crystallised owing to fast cooling therefore a chemical classification is used.

2.3.2 Mineralogy of igneous rocks

Examples of plutonic rock minerals:

2.3.3. Classification system for ingenious rocks

The classification utilises texture, mineralogy and colour. The latter is a particularly unreliable property, but the classification realises that certain fine-grained (volcanic) igneous rocks contain no visible mineral grains and in their absence colour is the only other available property - use colour only as a last resort.

  • Texture: plutonic (coarse), aphanitic (fine), vesicular, glassy and fragmental (NB do not bother with the latter because we often term all fragmental igneous rocks as tuffs).
  • If it is glassy, vesicular or fragmental you cannot determine mineralogy and hence the name is simply obsidian for a glass, tuff for a fragmental or pumice/scoria for a vesicular rock (the latter are differentiated on the basis or colour and size of the vesicles or holes).
  • Mineralogy: for the plutonic and some volcanic rocks you must determine the mineralogy. Often it is only necessary to identify one or two key minerals, not all of the minerals in the rock.

Example 1: classification of an ingenious rock

  • Texture: You pick up a specimen and notice that it is distinctly coarse grained (Plutonic).
  • This means that it must be one of the rocks in the row labelled coarse (i.e., granite, diorite, gabbro or peridotite).

www.nysed.gov

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  • Minerals: You next place the rock under a microscope and identify the minerals plagioclase and pyroxene.

  • You go to the bottom row of the chart (minerals composition) and look for a part with you mineralogy. You find it in the’ third column’ (pyroxene) and read the name (gabbro) from the coarse row on the chart.

www.nysed.gov

Igneous rocks

Reference: Foundation of Engineering Geology by Waltham

  • The term metamorphism means to change (meta = after, morphe = shape),
  • e.g. when a caterpillar becomes a butterfly.
  • While not as dramatic, similar changes can occur in rocks.
  • Rocks will alter their form and appearance to suit new conditions.
  • It is a slow process that occurs deep within the Earth, the existing rock can either be igneous, sedimentary or rocks which have been previously metamorphosed.
  • The change in the existing rock is brought about by either temperature or by pressure.
  • The rock doesn't melt, but it changes state by one or both of these processes:
  • Either one or both these cause changes in the mineral structure which can change the texture of the rock quite dramatically:
  • Mineral changes - growth of new minerals that are more stable under conditions of high temperature/pressure
  • Textural changes – recrystallisation, alignment of platy minerals, usually as a result of unequal application of stress

3. Metamorphic rocks

(1) Contact metamorphism:

Occurs when heat is the main cause of change.

(2) Dislocation (Dynamic) metamorphism: Occurs when pressure is the major cause of change.

(3) Regional metamorphism: Occurs where heat + pressure over a large area.

https://s3.amazonaws.com/gs-geo-images/cd348e48-b526-42ee-8032-0ff128e8815d_l.jpg

  • Occurs when heat is the main cause of change.
  • This occurs in areas adjacent to igneous intrusions and when movement takes materials down into hotter areas of the earth’s crust.

3.1 Contact metamorphism (heat)

Example:

http://www.chim.lu/ech1274.php

http://static1.squarespace.com/static/51bbeba5e4b0510af19f26f7/t/51df6c21e4b08e613e4b9307/1373596708522/contactmetamorphism.gif

3.2 Dislocation metamorphism (stress/pressure)

  • Occurs when heat and stress occurs over a very large area (mountains – due to continental collision on plate boundaries).

3.3 Regional metamorphism (heat + pressure over a large area)

https://classconnection.s3.amazonaws.com/98/flashcards/813098/png/untitled41334267346089.png

The recrystallisation of the minerals is often at right angles to the predominant pressure, causing foliation (alignment) of minerals:

3.4 Heat and pressures of metamorphism

Regional

metamorphism

(heat + pressure + large area)

Contact

metamorphism

(heat)

Dislocation

metamorphism

(pressure)

Melting

(magma formation)

3.5 Identification of metamorphic rocks

www.nysed.gov

Identification of metamorphic rocks flow chart

Step 1 - Texture

http://geology.com/rocks/metamorphic-rocks.shtml

Reference: Foundation of Engineering Geology by Waltham

Metamorphic rocks

  • Sedimentary rocks are formed from:
  • Fragments derived from other, older rocks, which may be igneous, metamorphic or sedimentary
  • Precipitates from solutions
  • Organic remains

The sedimentary rocks form a relativity thin covering on the earth's crust (generally but not always) overlying the igneous and metamorphic rocks.

  • Formed from the weathering (break down) of existing rocks.
  • The weathering can be by temperature action, water action, glaciation or wind erosion.
  • The resulting particles are then transported by rivers, the sea, glaciers or wind blown to a place where they are deposited

The deposition can be on land or in water

  • During the transportation and deposition, there is likely to be a sorting of the different particle sizes, note that:
  • The land is normally the erosional environment
  • The sea is normally the depositional environment

4. Sedimentary rocks

http://www.thunderboltkids.co.za/Grade5/04-earth-and-beyond/images/gd-0056.png

  • As the sediments accumulate, the process of hardening into sedimentary rocks starts.
  • The texture of sedimentary rock matches the mode of deposition.
  • Two important characters of sedimentary rocks are the porosity and packing.
  • The two process of formation are:

(2) Cementation

  • The particles are cemented by mineral cement which are carried in by groundwater.
  • The strength of the rock depends largely on the strength of the cement
  • Silica is a strong cement
  • Conversely, calcite is a weak cement.

(1) Compaction

  • Pressure due to overlying material forces repacks the grains
  • Grain to grain contact increases strength and smaller pores are filled with cement.

4.1 Formation of sedimentary rocks

Sedimentary rocks are deposited in the one of three major environments:

(1) Continental environments these are on land areas such as desert or lakes.

(2) Shelf sea environments these are at the edge of a sea – pebbles, sands and muds are deposited in water and these are then further affected by the wave and tide action.

(3) Deep sea environments where the materials characteristically contain no large fragments.

4.2 Deposited of sedimentary rocks

https://www.youtube.com/watch?v=U3eR19ZfBqE&index=2&list=PLrP7jh5S3OYo0kJYw_KgLtujuTSyiZ8Nv

http://geology.com/rocks/sedimentary-rocks.shtml

www.nysed.gov

4.3 Identification of sedimentary rocks

Ref: J.C. Harvey Geology for Geotechnical Engineers

Reference: Foundation of Engineering Geology by Waltham

Sedimentary rocks

5. The rocks cycle

(A) Rocks on the surface are eroded

(A) The rubble gathers on the sea-bed and become sedimentary rock

(A) This returns to the surface or sinks more deeply into the mantle

(B) Igneous rocks are continuously forming & returning to the surface

(C) Rocks which are changed by heat or pressure are metamorphic rocks

it is a never-ending cycle

6. Weathering and erosion

  • Weathering is the breaking up of rocks by water, ice, chemicals, plants and changing temperature
  • The end product of weathering rocks is called soil.
  • Physical (or mechanical) weathering and chemical weathering are the two main types
  • Physical weathering breaks rocks into pieces.
  • Ice is the major force in this type of weathering.
  • Water fills the cracks of rocks naturally during rainstorms but if the temperature falls and causes the water to freeze it expands in the cracks and may push hard enough to split the rock.
  • In a similar way, plants, especially trees,  may grow in the cracks of rocks sending there roots down deep into the cracks looking for water. As the roots grow the pressure can cause the rock to split
  • Chemical Weathering caused by action of water.
  • This type of weathering affects the minerals within the rocks.
  • Rain, streams, and ocean water dissolves minerals from rocks, causing the rocks to crumble.
  • Weathering starts the process of erosion by breaking down rock.
  • Erosion is the process by which weathered rock and soil are moved  from one place to another.
  • Erosion carves the Earth's surface creating canyons, gorges, and even beaches. There are five agents of erosion:
  • Gravity, Running water, Wind, Glaciers & Waves
  • Deposition is the process by which sediments (small particles of rock) are laid down in new locations. Deposition builds new landforms. Usually water is responsible for deposition but landslides can be caused by earthquakes and volcanoes.

Ref: www.powayusd.sdcoe.k12.ca.us/pusdmvms/Faculty/Downs/EarthScience/erosion/erosion.htm

Ref: www.powayusd.sdcoe.k12.ca.us/pusdmvms/Faculty/Downs/EarthScience/erosion/erosion.htm

The rock is peeling away from the mountain in layers - exfoliation

The following slides are taken form:

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Wind and sand action – causing the rocks to become rounded

Frost action slowly breaking up sedimentary rock

1908

1969

Acid Rain

Running water has eroded the slides of this canyon and created V-shaped valleys

Landslide

Land slump

Avalanche

Glacier forms u-shaped valley where it flows dragging rocks and boulders along the way

Pyroxene: the equi- dimensional, green grains are pyroxene.

Plagioclase: the white or chalky looking grain is the common feldspar, plagioclase.

Olivine: the green, glassy grains are olivine

Olivine: the green, glassy grains are olivine

Pyroxene: the equi-

dimensional, green grains

are pyroxene.

Plagioclase: the white or

chalky looking grain is the

common feldspar,

plagioclase.

Olivine: the

green, glassy

grains are

olivine

Olivine: the green,

glassy grains are

olivine