Topic 2: Igneous Rock

INTRODUCTION.

1. Rock itself is both construction materials (e.g. fill & reclamation materials) or part of engineering structure (e.g. cut slope, foundation & tunnel).

2. According to their manner of formation, rocks are of three 3 types:

• Igneous rock.

• Sedimentary rock.

• Metamorphic rock.

3. Magma : The Parent Material of Igneous Rock.

4. The word "igneous" is derived from the Latin igneus, meaning "fire".

5. Igneous rocks are rocks that have crystallized from molten rock material called magma.

6. Molten rock trapped underground is called magma. Molten rock erupted at Earth's surface is referred to as lava.

How Igneous Rock is Formed?

1. Magma is less dense than the surrounding solid rocks, it rises toward the surface.

2. It settle within the crust or erupt at the surface from a volcano as a lava flow.

3. Rocks formed from the cooling and solidification of magma.

4. Igneous rocks that cool and crystallize beneath the Earth's surface are called intrusive igneous rocks. (plutonic igneous rocks).

5. Igneous rocks that cool and crystallize on the Earth's surface are called extrusive igneous rocks. (volcanic igneous rocks).

6. The slow cooling promotes the growth of minerals large enough to be identified visually without the aid of a microscope (called phaneritic, from the Greek phaneros, meaning "visible").

7. Slow cooling = coarse grains.

8. Intrusive rocks tend to be coarse grained.

9. Magma erupted at the surface is chilled so quickly that the individual minerals have little or no chance to grow.

10. The rock is either composed of minerals that can be seen only with the aid of a microscope (called aphanitic, from the Greek aphanes, meaning "invisible") or contains no minerals at all.

11. Quick cooling = fine grains.
Extrusive igneous rocks tend to be fine grained or glassy.

IGNEOUS ROCK

1. Igneous rock - magma solidifies by crystallizing into a mosaic of minerals.

2. Intrusive igneous rocks - magma solidifies below the surface of the earth, may later be exposed at the surface when the cover rocks are eroded away.

3. Extrusive igneous rocks - form from the magma which is extruded onto the earth surface to create a volcano.

INTRUSIVE IGNEOUS ROCKS

Figure 2.1 : Intrusive Igneous Rocks

1. Intrusive, or plutonic igneous rock forms when magma is trapped deep inside the Earth.

2. Most of this material may cool and solidify before it even reaches the surface.

3. Formed under conditions of high pressure and high temperature.

4. Can also be classified according to the shape and size of the intrusive body.

5. Intrusive rocks have a coarse grained texture.

6. Coarse grained intrusive igneous rocks which form at depth within the earth are termed as abyssal; intrusive igneous rocks which form near the surface are termed hypabyssal.

7. Batholith is the largest of the intrusive bodies.

Figure 2.2 : Batholith

8. Batholith may occupy huge areas of the Earth's surface when exposed by erosion.

9. Dike is a vertical or near vertical intrusive igneous rock body that cuts across rock beds.

10. Laccolith is a dome shaped intrusive body that has intruded between layers of sedimentary rock.

11. Sill is similar to a dike with the exception that sills run parallel to the existing rock bed instead of cutting through it.

Figure 2.3 : Igneous rock

Texture

1. Texture – refer to size, shape & arrangement of mineral grains.

• Interlocking crystalline texture.

2. Aphanitic: fine-grained size : rapid cooling.

3. Phaneritic: course-grained size : slow cooling.

4. Porphyritic: diff size of minerals present in same rock.

5. Glassy: Lava cool rapidly – atoms have no time to become arranged in order.

6. Vesicular: Rock possessing vesicles (water vapor & gasses in cooling lava form holes).

7. Fragmental

8. Texture of an igneous rock -not refer to the roughness or smoothness of the surface but based primarily on crystal size.

9. Have 6 textures;

a. Vesicular Texture

i. Vesicular- vesicles (holes, pores, or cavities) within the igneous rock.

ii. Result of gas expansion (bubbles), which often occurs during volcanic eruptions.

iii. Pumice and scoria are common types of vesicular rocks.

iv. Example;

• Pumice.

• Scoria.

b. Glassy Texture

i. Glassy texture - non-crystalline (the rock contains no mineral grains).

ii. Results from cooling that is so fast that minerals do not have a chance to crystallize.

iii. This may happen when magma or lava comes into quick contact with much cooler materials near the Earth's surface.

iv. Pure volcanic glass is known as obsidian .

v. Example;

• Obsidion

• Basaltic glass

c. Porphyritic Texture

i. Porphyritic texture - a subtype, but usage of the term often confuses the beginner.

ii. Composed by at least two minerals having a conspicuous (large) difference in grain size.

iii. Thought to have undergone two stages of cooling;

• One at depth where the larger phenocrysts formed.

• Second at or near the surface where the matrix grains crystallized.

Figure 2.4 : Porphyritic Texture

iv. Example;

• Rhyolite

• Andesite

• Basalt.

d. Aphanitic Texture

i. Aphanitic texture - small crystals that cannot be seen by the eye with or hand lens.

ii. Made up of small crystals, which are generally less than 1/2 mm in size.

iii. Results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments.

iv. Example;

• Rhyolite

• Andesite

• Basalt

e. Phaneritic Texture

i. Phaneritic textured - comprised of large crystals that are clearly visible to the eye with or without a hand lens or binocular microscope.

ii. Made up of large crystals, which are generally 1/2 mm to several centimeters in size; no fine matrix material.

iii. Forms by slow cooling of magma deep underground in the plutonic environment.

Figure 2.5 : Phaneritic texture

iv. Example;

• Granite

• Diorite

• Gabbro

• Dunite

f. Fragmental Texture

i. Fragmental - reserved for pyroclastic rocks, those blown out into the atmosphere during violent volcanic eruptions.

ii. Terminology for fragmental rocks - voluminous, or "tuff".

iii. It is comprised of numerous grains or fragments that have been welded together by the heat of volcanic eruption.

iv. It will often feel grainy like sandpaper or a sedimentary rock.

v. Spot shards of glass embedded in the rock.

vi. Example;

Figure 2.6 : Aphanatic rocks

Figure 2.7 : Igneous rock textures.

Figure 2.8 : Granite – Intrusive igneous

Extrusive igneous rock

Figure 2.9 : Extrusive igneous rock

1. Extrusive igneous rocks are formed at the Earth's surface as a result of the partial melting of rocks within the mantle and crust.

2. The melt, with or without suspended crystals and gas bubbles, is called magma. Magma rises because it is less dense than the rock from which it was created. The magma, called lava when molten rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere.

3. Magma which erupts from a volcano behaves according to its viscosity, determined by temperature, composition, and crystal content. High-temperature magma, most of which is basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle.

4. Felsic magma such as rhyolite is usually erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma commonly erupt explosively, and rhyolitic lava flows typically are of limited extent and have steep margins, because the magma is so viscous.

5. Because lava cools and crystallizes rapidly, it is fine grained. If the cooling has been so rapid as to prevent the formation of even small crystals after extrusion, the resulting rock may be mostly glass (such as the rock obsidian). If the cooling of the lava happened slowly, the rocks would be coarse-grained.

6. Because the minerals are fine-grained, it is much more difficult to distinguish between the different types of extrusive igneous rocks than between different types of intrusive igneous rocks.

7. Common types of extrusive igneous rocks are lava rocks, cinders, pumice, obsidian, and volcanic ash and dust.

Figure 2.10 : Extrusive Igneous Rocks

Figure 2.11 : Obsidian rocks are igneous rocks that form when lava cools quickly above ground. Obsidian is actually glass and not a mixture of minerals. The edges of this rock are very sharp.

Figure 2.12 : Extrusive igneous

Figure 2.13 : Microscope

Figure 2.14: Phorphyritic texture:
(a)Aphanitic extrusive igneous rock
(b) Porphyritic intrusive igneous rock

Mineral compositions

1. Main composition of igneous rock - silica minerals (olivine, pyroxene, feldspars, quartz).

2. Bowen’s reaction series – arrangement of minerals became.

3. Ultramafic (<45%>65% silica).

MINERALOGICAL CLASSIFICATION

1. For volcanic rocks, mineralogy is important in classifying and naming lavas. The most important criteria is the phenocryst species, followed by the groundmass mineralogy. Often, where the groundmass is aphanitic, chemical classification must be used to properly identify a volcanic rock.

2. Mineralogic contents - felsic versus mafic

i. Felsic rock, with predominance of quartz, alkali feldspar and/or feldspathoids: the felsic minerals; these rocks (e.g., granite) are usually light coloured, and have low density.

ii. Mafic rock, with predominance of mafic minerals pyroxenes, olivines and calcic plagioclase; these rocks (example, basalt) are usually dark coloured, and have a higher density than felsic rocks.

iii. Ultramafic rock, with more than 90% of mafic minerals (e.g., dunite).

3. For intrusive, plutonic and usually phaneritic igneous rocks where all minerals are visible at least via microscope, the mineralogy is used to classify the rock. This usually occurs on ternary diagrams, where the relative proportions of three minerals are used to classify the rock.

Table 2.1 : Simple subdivision of igneous rocks according both to their composition and mode of occurrence.

Figure 2.15 : Bowen’s reaction series

Figure 2.16 : Classification of igneous rock

Figure 2.17 : Classification by texture

Figure 2.18 : Relative proportions of chief mineral
component in igneous rocks.

Figure 2.19 : Subduction of an ocenic plate beneath either ocenic plate or continental plate produce magma. Magma form plutons and some erupted onto seafloor or at volcanoes along convergent plate boundries.

Table 2.2 : Rock properties

Table 2.3 : Classification of igneous rock

Figure 2.20 : Mineral Percent Abundance Chart

Figure 2.21 : Composition key Phaneritic Igneous Rock

Felsic rock

1. Granite

• Granite is an igneous rock that is composed of four minerals. These minerals are quartz, feldspar, mica, and usually hornblende. Granite forms as magma cools far under the earth's surface. Because it hardens deep underground it cools very slowly. This allows crystals of the four minerals to grow large enough to be easily seen by the naked eye.

• Granite is an excellent material for building bridges and buildings because it can withstand thousands of pounds of pressure. It is also used for monuments because it weathers slowly. Engravings in the granite can be read for hundreds of years, making the rock more valuable.

2. Obsidian

• Obsidian is a very shiny natural volcanic glass. When obsidian breaks it fractures with a distinct conchoidal fracture.

• Obsidian is produced when lava cools very quickly. The lava cools so quickly that no crystals can form. When people make glass they melt silica rocks like sand and quartz then cool it rapidly by placing it in water.

• Obsidian is produced in nature in a similar way.

• Obsidian is usually black or a very dark green, but it can also be found in an almost clear form. Ancient people throughout the world have used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds.

• Today obsidian is used as a scalpel by doctors in very sensitive eye operations.

3. Pumice

• Pumice rocks are igneous rocks which were formed when lava cooled quickly above ground.This rock is so light, that many pumice rocks will actually float in water.

• Pumice is actually a kind of glass and not a mixture of minerals. Because this rock is so light, it is used quite often as a decorative landscape stone.

• Pumice is ground up and used today in soaps, abrasive cleansers, and also in polishes.

Intermediate rock

Mafic igneous rock

1. Basalts

• Basalts are dark colored, fine-grained extrusive rock. The mineral grains are so fine that they are impossible to distinguish with the naked eye or even a magnifying glass. They are the most widespread of all the igneous rocks.

• Most basalts are volcanic in origin and were formed by the rapid cooling and hardening of the lava flows. Some basalts are intrusive having cooled inside the Earth's interior.

2. Gabbro

• Gabbro is a dark-colored, coarse-grained intrusive igneous rock.

• Gabbro is very similar to basalt in its mineral make up. It is composed mostly of the mineral plagioclase feldspar with smaller amounts of pyroxene and olivine.

Igneous Rocks–Engineering properties

1. Engineering properties – strength, porosity & deformation behaviour.

2. High compressive strength

3. Joints control behavior

• Tectonicjoints formed by stress changes after cooling.

4. Usually three sets, sometimes more

• Exfoliationjoints formed by unloading parallel to the exposed surface (sheet joints).

5. Increase in frequency close to the surface.

6. In Situ Stress.

7. Residual stress combined with gravity induced stress

• Can produce localized overstressing and rock bursts.

8. Residual Stresses.

9. High residual stresses lead to rapid physical and chemical weathering.

10. Unaltered intrusive igneous rock:

• Course-grained size.

• Interlocking texture.

• Massive structure.

• Great strength.

• Very little water can move through.

• Elastic.

11. Extrusive igneous rock:

• Fine-grained size.

• Interlocking texture.

• Massive structure.

• Porosity > than intrusive igneous rock.

• Less uniform of strength & deformation behaviour (rate of magma solidify).

• Plastic-elastic.

Diagram 2.1 : Stress-strain relationship of rock under uniaxial compression

· Type A, elastic: This type of curve is exhibited by massive and very hard rock material.

· Type B, plastic-elastic: The hard rock that undergoes some densification during initial loading.

· Type C, elastic-plastic: The hard rock with mixed composition with weaker component gradually fail.

· Type D, plastic-elastic-plastic: The rock that experiences densification during initial loading and then gradually failure of weaker components. This is the most common stress-strain relationship of rock.

Table 2.4 : Rock Classification

Figure 2.22 : Unaltered intrusive igneous rock

Figure 2.23 : Extrusive igneous rock

Figure 2.24 : Simulated fracture process of a rock specimen subjected to a confining pressure.