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Brooklyn College Core Curriculum 3.32 Geology The Science of Our World |
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GEOLOGIST
IN THE FIELD
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Igneous Rocks Igneous rocks (etymology from Latin ignis, fire) are rocks formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle or crust. Typically, the melting is caused by one or more of the following processes — an increase in temperature, a decrease in pressure, or a change in composition.
The
field identification of igneous rocks is based on color, density, composition
(mineralogy), grains size and texture. The classification chart above
shows how these characteristics are used to identify the basic types
of igneous rocks. Pegmatite
is an igneous rock distinguished by its abnormally large crystals. The
crystals are normally larger that a few centimeters and can often be
dozens of centimeters long or much longer (meters long). Unlike other
igneous rocks that develop from the molten state, pegmatites grow from
aqueous solutions. The solutions allow for ease of movement of the nutrients
to the site of crystal growth. Thus pegmatites can produce large crystals
in a short (geologically) period of time. |
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Metamorphic Rocks Metamorphic rocks are the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means "change in form ".The protolith is subjected to heat and pressure (temperatures greater than 150 to 200 °C and pressures of 1500 bars) causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and are classified by texture and by chemical and mineral assemblage (metamorphic facies). They may be formed simply by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above. They can be formed by tectonic processes such as continental collisions which cause horizontal pressure, friction and distortion. They are also formed when rock is heated up by the intrusion of hot molten rock called magma from the Earth's interior.
Temperature, pressure and chemically active fluids can alter existing rocks (while still in the solid state) to produce new rocks (and sometimes minerals) that are stable under the new conditions. These are the metamorphic rocks. There are two types of metamorphic rocks. Those that show the effects of pressure by some parallel structure within the rock like layering or parallel alignment of mineral grains or banding. These are the Foliated metamorphic rocks. All of the other metamorphic rocks (those without parallel structures) are call Non Foliated. The two basic types of metamorphic rocks are: foliated and non foliated. Foliation can be observed in the field by parallel bands within the rocks or it can often be observed on a smaller scale in the hand specimen. This foliation can also be seen microscopically by the alignment of platy minerals. Non foliated as the name implies, does not have any parallel orientation of the grains within the metamorphic rock. Non foliated rocks have recrystallized without producing parallel structures. This can be done in the absence of pressure but more commonly by the lack of elongate or tabular grains. For example sandstone is metamorphosed into quartzite by the normal agents of metamorphism (heat and pressure), but because of the equidimensional nature of the quartz grains, no alignment or parallel structure can take place. Low grade – Rocks that are metamorphosed under temperature and pressure conditions up to 400oC and 400 Mpa.
High grade – Rocks that are metamorphosed under temperature and pressure conditions higher than about 400oC and 400 Mpa.
Migmatite – The result of a composite rock, part igneous and part metamorphic.
Types of metamorphism
Contact metamorphism – Metamorphism that occurs when rocks are heated and chemically changed adjacent to an intruded body of hot magma.
Burial metamorphism – Metamorphism that occurs after diagenesis, as a result of the burial of sediments in deep sedimentary basins.
Regional metamorphism – Metamorphism of an extensive area of the crust , associated with plate convergence, collision, and subduction.
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Sedimentary Rocks Sedimentary
rock is one of the three main rock groups (the others being igneous
and metamorphic rock). Rock formed from sediments covers 75-80% of the
Earth's land area, and includes common types such as chalk, limestone,
dolomite, sandstone, conglomerate and shale. Sedimentary rocks are classified
by the source of their sediments, and are produced by one or more of:
The sediments are then compacted and converted to rock by the process of lithification.
Sedimentary rocks form in two basic ways. One type of sedimentary rock forms at the expense of existing rocks. Existing rocks are broken down by the processes of weathering and erosion. They are then deposited and lithified to form sedimentary rocks. These sedimentary rocks are called Detrital. The other method of sedimentary rock formation is by chemical precipitation. These are call the Chemical sedimentary rocks. For example the evaporation of sea water can produce a sedimentary deposit of salt, gypsum and even limestone. Sedimentary rocks are derived from pre-existing rocks. The processes of weathering and erosion (weathering occurs in situ, or "with no movement", and thus should not to be confused with erosion, which involves the movement and disintegration of rocks and minerals by agents such as water, ice, wind, and gravity), break rocks down through a variety of mechanical and chemical means. The solid remains of this process as well as the dissolved material can go into making the sedimentary rocks. Sedimentary rocks are divided into two big groups: chemical and clastic (fragmental). Chemical rocks are subdivided into crystalline (from precipitates and evaporites) and bioclastic (shells and plant remains). |
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Rocks - Recognizing and Identifying Rocks
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The Rock Cycle
The Rock Cycle is a group of changes. Igneous, metamorphic and sedimentary rocks are subject to tectonic forces that will uplift these rocks to heights where they are subjected to the forces of weathering. After these rocks have been transformed to regolith then erosion will transport these sediments to places of deposition: rivers, lakes, seas, etc. Here these layers will be compacted and lithified. If these sedimentary rocks are subjected to heat and pressure then these sedimentary rocks will change their physical and chemical appearance making them metamorphic rocks. Metamorphic rocks are pressured and heated even further producing new rocks until these rocks become in contact with extreme heat where they will melt and became part of this magma. Eventually this magma will move up to the surface and crystallize either on the surface of the Earth (extrusive) or inside a cool magma chamber (intrusive) creating new igneous rocks, now these igneous rocks could be changed by heat and pressure becoming new metamorphic rocks, then this never ending cycle starts again.
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