understanding geology and its engineering properties …

Understanding Geology and Its Engineering Properties…

From A Civil Engineer’s Point of View…

Dr.ÖZGÜR YILMAZERGeotechnical Engineer

MSc. Geological EngineeringMSc. Civil Engineering

PhD. Geological Engineering

INTRODUCTIONROCKS (Definition: Minerals)

A rock is defined as a consolidated mixture of minerals. A mixture of minerals implies the presence of more than one mineral grain, but not necessarily more than one type of mineral. A rock can be composed of only one type of mineral, but most rocks are composed of several different types of minerals.

A mineral is;•a naturally occurring substance•that is solid and stable at room temperature, •representable by a chemical formula, •usually abiogenic, and •has an ordered atomic structure.

It is different from a rock, in this sense, which can be an aggregate of minerals or non-minerals, and does not have a specific chemical composition.

INTRODUCTION

There are over 4,900 known mineral species. The silicate minerals compose over 90% of the Earth’s crust. Silicon and oxygen constitute approximately 75% of the Earth's crust, which translates directly into the predominance of silicate minerals.

ROCKS (Definition: Minerals)

INTRODUCTION

Minerals are distinguished by various chemical and physical properties. Differences in chemical compositon and crystal structure distinguish various species, and these properties in turn are influenced by the mineral's geological environment of formation.

Minerals are made of elements that are bonded together to form stable solid matters. Hydrogen and Oxigen elements bond together to form H2O (water) mineral. Similarly, Na and Cl elements come together to form NaCl (salt).

ROCKS (Definition: Minerals)

INTRODUCTION

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, the number of protons in its nucleus.

ROCKS (Definition: Elements)

INTRODUCTION

When two distinct elements are chemically combined, with the atoms held together by chemical bonds, the result is termed a chemical compound.

Q_1. How do atoms bond together to form minerals? A_1. Elements bond by sharing or transferring electrons

Q_2. Why don't elements prefer to remain alone, unbonded?A_2. Elements like to have their outer electron orbital full of electrons, so elements with full orbitals are very stable (e.g., the noble gases He, Ar, Kr, Xe).

ROCKS (Definition: Elements)

ROCK TYPES

Rocks are grouped into three main categories:

ROCK CYCLE

IGNEOUS ROCKSDEFINITION AND SOURCES

Igneous rocks (from the Greek word for fire) form from molten rock (magma), crystallizes and solidifies. Igneous rocks form in three main places: where lithospheric plates pull apart at mid-ocean ridges, where plates come together at subduction zones and where continental crust is pushed together, making it thicker and allowing it to heat to melting.

INTRODUCTIONROCKS

Igneous rocks are divided into two groups, intrusive and extrusive, depending upon where the molten rock solidifies

Intrusive, or plutonic igneous rock forms when magma is trapped deep inside the Earth. Great globs of molten rock rise toward the surface. Some of the magma may feed volcanoes on the Earth's surface, but most remains trapped below, where it cools very slowly over many thousands or millions of years until it solidifies.

IGNEOUS ROCKSDEFINITION : INTRUSIVE ROCKS

Slow cooling means the individual mineral grains have a very long time to grow, so they grow to a relatively large size. Intrusive rocks have a coarse grained texture. The image at right shows granite, an intrusive igneous rock..

IGNEOUS ROCKSDEFINITION : INTRUSIVE ROCKS

Extrusive, or volcanic, igneous rock is produced when magma exits and cools outside of, or very near the Earth's surface.

IGNEOUS ROCKSDEFINITION : EXTRUSIVE ROCKS

These are the rocks that form at erupting volcanoes. 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.

IGNEOUS ROCKSDEFINITION : VOLCANIC (EXTRUSIVE) ROCKS

Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture. Pumice, obsidian, and basalt are all extrusive igneous rocks.

IGNEOUS ROCKSDEFINITION : EXTRUSIVE ROCKS

Igneous rock textures are used by geologists in determining the mode of origin igneous rocks and are used in rock classification. There are six main types of textures; 1) Phaneritic, 2) Aphanitic, 3) Porphyritic,4) Glassy, 5) Pyroclastic and 6) Pegmatitic.Aphanitic (not visible) rocks in contrast to Phaneritic rocks, typically form from lava which crystallize rapidly on or near the Earth‘s surface. Because they make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals. The individual crystals in an aphanitic igneous rock are not distinguisable to the naked eye. Examples of aphanitic igneous rock include basalt, andesite, and rhyolite.

IGNEOUS ROCKSDEFINITION : ROCK TEXTURE

Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals. Examples include obsidian and pumice.

Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below the Earth's surface. As a magma cools slowly the minerals have time to grow and form large crystals. The minerals in a phaneritic igneous rock are sufficiently large to see each individual cyrstal with the naked eye. Examples of phaneritic igneous rocks are gabro, diorite and granite.

Pegmatitic texture occurs during magma cooling when some minerals may grow so large that they become massive (the size ranges from a few cm to several metres).


Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained (aphanitic) matrix. The result is an aphanitic rock with some larger crystals (phenocrysts) imbedded within its matrix. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals.


Pyroclastic (pyro=igneous, clastic = fragment) textures occur when explosive eruptions blast the lava into the air resulting in fragmental, typically glassy material which fall as volcanic ash, lapilli, and volcanic bombs.


SUMMARYIGNEOUS ROCKS

Intrusive igneous rocks are the rocks which cool down really slowly under the surface. All igneous rocks doesn’t cool the same way. That’s the reason why they don’t look all the same too. Intrusive igneous rocks have large crystals. The granite stone is the example of intrusive igneous rock.



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understanding geology and its engineering properties …

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