GEOLOGIST IN THE FIELD

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.
Color is based on the relative amounts of iron and magnesium contained within the minerals of the rock. The color is considered light if there is little iron or magnesium (felsic minerals, Quartz, the mostly Na-plagioclases near Albite, the K-spars, and Muscovite Mica). The color is intermediate if there is an increased concentration of iron and magnesium. The color is considered dark if the rock appears as a dark gray to black (pyroxenes, the amphibole Hornblende, the mostly Ca-plagioclases near Anorthite, and Biotite mica.
Density is a property that is proportional to the composition of the rock. The higher the amount of silica (felsic) the less dense the rock will be. The less amount of silica in the rock the more dense the rock will be.
Composition (mineralogy) refers to the relative amounts of silica vs. mafic minerals. High silica rocks (silica > 60%) are considered felsic. These rocks, like granite and rhyolite are generally light in color. Rocks rich (>60%) in ferromagnesian minerals (mafic) are generally dark in color.
Texture describes the nature of the grains (crystals) that make up the rock. Rocks are considered coarse grainded if you can distinguish the crystals with the unaided eye. Fine grained igneous rocks have at least a portion of the rock matrix that has no visible crystals to the unaided eye. Porphyritic texture is produced by two distinct cooling stages producing both large and small crystals in the same rock. Slow cooling (generally deep underground) produces large crystals. Rapid cooling (at or near the surface of the Earth) produces smaller crystals. In a porphyry, the crystals are of distinctly different size. The smaller crystals are called the matrix or groundmass. The term pegmatite is reserved for igneous rocks that have unusually large crystals. It is a term that is used for any such igneous rock but is normally associated with granites. Pegmatites are unique in that they do not form directly from the igneous melt but instead are formed from fluids derived from or near the igneous rock body. The fluids (generally aqueous and under high temperatures and pressures) allow for a great deal of freedom for the migration of ions (charged atoms or molecules) to the sites of crystallization. The result is the formation of large crystals.

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). Pegmatites form as late fractionation products from a magma, with very high silica content and high water content.


Porphyritic rock is and igneous rocks that contains two distinct crystal sizes. These distinctly different crystal sizes were produced by different cooling of the liquid rock. Large crystals form slowly beneath the surface of the Earth and small crystals form when rapid cooling takes place (normally at or near the surface). The large crystals in a porphyry are called phenocrysts. The term porphyritic is used as an adjective to describe this distinct texture of igneous rock.
Glassy igneous rocks are formed by very rapid cooling. No crystals were formed during the cooling process. Examples are obsidian and pumice.

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.

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:
* clastic rock formed from fragments broken off from parent rock, by o weathering in situ or o erosion by water, ice or wind, followed by transportation of sediments, often in suspension, to the place of deposition;
* biogenic activity; or
* precipitation from solution.

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).

Rocks - Recognizing and Identifying Rocks

IGNEOUS ROCKS

PUMICE
Environment of formation =
extrusive (volcanic)
Texture =
Glassy, vesicular
Grain size =
non-crystalline
Color = light
Density =
low
Composition =
felsic

VESICULAR BASALT
Environment of formation =
extrusive (volcanic)
Texture =
Glassy, vesicular
Grain size =
non-crystalline
Color = dark
Density =
medium
Composition =
mafic

SCORIA
Environment of formation =
extrusive (volcanic)
Texture =
Glassy, vesicular
Grain size =
non-crystalline
Color = dark
Density =
medium
Composition =
mafic

RHYOLITE
Environment of formation =
extrusive (volcanic)
Texture =
fine
Grain size =
less than 1 mm
Color = light
Density =
low
Composition =
felsic

ANDESITE
Environment of formation =
extrusive (volcanic)
Texture =
fine
Grain size =
less than 1 mm
Color = light
Density =
medium
Composition =
intermediate

BASALT
Environment of formation =
extrusive (volcanic)
Texture =
fine
Grain size =
less than 1 mm
Color = dark
Density =
high
Composition =
mafic

GRANITE
Environment of formation =
intrusive (plutonic)
Texture =
coarse
Grain size =
1 mm to 10mm
Color = light
Density =
low
Composition =
felsic

DIORITE
Environment of formation =
intrusive (plutonic)
Texture =
coarse
Grain size =
1 mm to 10mm
Color = medium
Density =
medium Composition = medium

GABBRO
Environment of formation =
intrusive (plutonic)
Texture =
coarse
Grain size =
1 mm to 10mm
Color = dark
Density =
high
Composition =
mafic

PERIDOTITE
Environment of formation =
intrusive (plutonic)
Texture =
coarse
Grain size =
1 mm to 10mm
Color = dark
Density =
high
Composition =
ultramafic

https://encrypted-tbn0.google.com/images?q=tbn:ANd9GcSJ9EpetVs_kVKB4pY2lG12swMXXleRTzm6vKdjaIqqrT9a0lTtgg

PEGMATITE
Environment of formation =
intrusive (plutonic)
Texture =
very coarse
Grain size =
10mm or larger
Color = ligh
Density =
low
Composition =
felsic

 

METAMORPHIC ROCKS

SLATE
Texture =
foliated (mineral alignment)
Grain size = microscopic
Type of metamorphism =
Regional (low grade)
Composition = mica and clay minerals

PHYLLITE
Texture =
foliated (mineral alignment)
Grain size = microscopic

Type of metamorphism = Regional (foliation surfaces shiny from microscopic mica crystals)
Composition = mica, quartz, feldspar, amphibole, garnet

SCHIST
Texture =
foliated (mineral alignment)
Grain size = fine to medium
Type of metamorphism =
Regional (platy mica crystals visible from metamorphism of clay or feldspar)
Composition = mica, quartz, feldspar, amphibole, garnet

 

Recognition: mica are visible.

GNEISS
Texture =
foliated (banding)
Grain size = medium to coarse
Type of metamorphism =
Regional (high-grade metamorphism, some mica changed to feldspar, segregated by mineral type into bands)
Composition = mica, quartz, feldspar, amphibole, garnet, pyroxene

 

Recognition: felsic (Quartz, Feldspars) and mafic minerals (Hornblende, Biotite) separated into bands..

HORNFELS
Texture =
non-foliated
Grain size = fine
Type of metamorphism =
Contact (heat) (various rocks changed by heat from nearby magma/lava)
Composition = variable

QUARTZITE
Texture =
non-foliated
Grain size = fine to coarse
Type of metamorphism =
Regional or contact (metamorphism of quartz sandstone)
Composition = quartz

MARBLE
Texture =
non-foliated
Grain size = fine to coarse
Type of metamorphism =
Regional or contact (metamorphism of limestone or dolostone)
Composition = calcite and/or dolomite

METACONGLOMERATE
Texture =
non-foliated
Grain size = coarse
Type of metamorphism =
Regional or contact (pebbles may be distorted or stretched)
Composition = various minerals in particles and matrix

AMPHIBOLITE


Grain size = coarse
Type of metamorphism =
Metamorphism of Basalt
Composition = Amphiboles, typically Horneblende

http://www.kristallin.de/Metamorphite/Amphibolit_Pampau_v.jpg

SERPENTINITE


Grain size = coarse
Type of metamorphism =
Hydrothermal Metamorphism of Ophiolites (Basalt, Gabbro, Peridotites e.g. Dunite)

For example at the Mid-ocean Ridges
Composition = Serpentine, Talc

http://www1.newark.ohio-state.edu/Professional/OSU/Faculty/jstjohn/Common%20rocks/Serpentinite3.jpg

 

SEDIMENTARY ROCKS

CONGLOMERATE
Texture =
clastic (fragmental)
Grain size = pebbles, cobbles and/or boulders embedded in sand, silt, and/or clay
Comments =
Rounded fragments
Composition = mostly quartz, feldspar, and clay minerals; may contain fragments of other rocks

Recognition: some rounded grains larger than 2mm.

BRECCIA
Texture =
clastic (fragmental)
Grain size = pebbles, cobbles and/or boulders embedded in sand, silt, and/or clay
Comments =
Angular fragments
Composition = mostly quartz, feldspar, and clay minerals; may contain fragments of other rocks

 

Recognition: some angular grains larger than 2mm.

SANDSTONE
Texture =
clastic (fragmental)
Grain size = sand (0.2 to 0.006 cm)
Comments =
fine to coarse
Composition = mostly quartz, feldspar, and clay minerals; may contain fragments of other rocks

Recognition: when scratched with the knife sand grains fall out.

SILTSTONE
Texture =
clastic (fragmental)
Grain size = silt (0.006 to 0.0004 cm)
Comments =
very fine grain
Composition = mostly quartz, feldspar, and clay minerals; may contain fragments of other rocks

Recognition: grains can be felt with the teeth, the rock seems gritty.

SHALE
Texture =
clastic (fragmental)
Grain size = clay (less than 0.0004 cm)
Comments =
compact may split easily
Composition = mostly quartz, feldspar, and clay minerals; may contain fragments of other rocks

Recognition: no grains can be detected with the teeth.

ROCK SALT
Texture =
crystalline (chemical)
Grain size = varied
Comments =
crystals form from chemical precipitates and evaporites
Composition = halite

ROCK GYPSUM
Texture =
crystalline (chemical)
Grain size = varied
Comments =
crystals form from chemical precipitates and evaporites
Composition = gypsum

DOLOSTONE
Texture =
crystalline (chemical)
Grain size = varied
Comments =
crystals form from chemical precipitates and evaporites
Composition = dolomite

LIMESTONE
Texture =
bioclastic
Grain size = microscopic to coarse
Comments =
cemented shell fragments or precipitates of biologic origin
Composition = calcite

COAL
Texture =
bioclastic
Grain size = varied
Comments =
from plant remains
Composition = carbon

 

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.