Petrology

Petrology

The branch of geology that deals with rocks, including their origin , structure, changes, etc.

Engineering geology

Is concerned with the engineering properties of geologic materials, including their strength, permeability, and compactability, and with the influence of these properties on the selection of locations for buildings, roads and railroads, bridges, dams, and other major civil features.

Urban geology

Involves the application of engineering geology and other fields of geology to environmental problems in urban areas.

Environmental geology

Is generally concerned with those aspects of geology that touch on the human environment.

Environmental and urban geology

Deal in large measure with those aspects of geology that directly influence land use.

Igneous Petrology

Is concerned with the identification, classification, origin, evolution, and processes of formation and crystallization of the igneous rocks.

Igneous Rock

Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene, and biotite. The specimen shown is about two inches (five centimeters) across.

Igneous Petrology

Most of the rocks available for study come from the Earth’s crust, but a few, such as eclogites, derive from the mantle.

The scope of igneous petrology is very large because igneous rocks make up the bulk of the continental and oceanic crusts and of the mountain belts of the world, and they also include the high-level volcanic extrusive rocks and the plutonic rocks that formed deep within the crust.

Igneous Petrology Problems

• Ormand structure of igneous bodies
• Crystallization history of the minerals
• Classification of rocks
• Fractionation of parent magmas
• Mechanism of generation of magmas
• History of formation and the composition of the present oceanic crust
• Evolution of igneous rocks
• Composition of the mantle
• Conditions of pressure and temperature

Sedimentary petrology

This field is concerned with the description and classification of sedimentary rocks, interpretation of the processes of transportation and deposition of the sedimentary materials forming the rocks, the environment that prevailed at the time the sediments were deposited, and the alteration (compaction, cementation, and chemical and mineralogical modification) of the sediments after deposition.

Carbonate petrology

One branch deals with carbonate rocks, namely limestones and dolomites, composed principally of calcium carbonate (calcite) and calcium magnesium carbonate (dolomite).

Clastic petrology

The other principal branch of sedimentary petrology is concerned with the sediments and sedimentary rocks that are essentially non-calcareous.

Metamorphic petrology

Focuses on the composition and texture of metamorphic rocks (rocks such as slate, marble, gneiss, or schist which started out as sedimentary or igneous rocks but which have undergone chemical, mineralogical or textural changes due to extremes of pressure, temperature or both).

Metamorphology

• Means change in form
• In geology, the term is used to refer to a solid-state recrystallization of earlier igneous, sedimentary, or metamorphic rocks.

Contact metamorphism

In which changes induced largely by increase in temperature are localized at the contacts of igneous intrusions.

Regional metamorphism

In which increased pressure and temperature have caused recrystallization over extensive regions in mountain belts.

Metasomatism

A supplement to metamorphism: the introduction and expulsion of fluids and elements through rocks during recrystallization.
When new crust is formed and metamorphosed at a mid-oceanic ridge, seawater penetrates the crust for a few kilometers and carries much sodium with it.
During formation of a contact metamorphic aureole around a granitic intrusion, hydrothermal fluids carrying elements such as iron, boron, and fluorine pass from the granite into the wall rocks.
When the continental crust is thickened, its lower part may suffer dehydration and form granulites.
The expelled fluids, carrying such heat-producing elements as rubidium, uranium, and thorium migrate upward into the upper crust.
Much petrologic research is concerned with determining the amount and composition of fluids that have passed through rocks during these metamorphic processes.