Chapter3 Minerals and Rocks October 2012
Introduction to rock-forming Minerals • The Earth is composed of rocks. • Rocks are aggregates of minerals. • Minerals are composed of atoms. • In order to understand rocks, we must first have an understanding of minerals. • In order to understand minerals we must have some basic understanding of atoms - what they are and how they interact with one another to form minerals.
Definition of a Mineral • Naturally formed; it forms in nature on its own, • Solid (it cannot be a liquid or a gas), • With a definite chemical composition (every time we see the same mineral it has the same chemical composition that can be expressed by a chemical formula), and • Characteristic crystalline structure (atoms are arranged within the mineral in a specific ordered manner).
Cont’d Examples • Glass - can be naturally formed (volcanic glass called obsidian), is a solid, its chemical composition, however, is not always the same, and it does not have a crystalline structure. Thus, glass is not a mineral. • Ice - is naturally formed, is solid, and does have a definite chemical composition that can be expressed by the formula H2O. Thus, ice is a mineral, but liquid water is not (since it is not solid). • Halite (salt) - is naturally formed, is solid, does have a definite chemical composition that can be expressed by the formula NaCl, and does have a definite crystalline structure. Thus halite is a mineral. • Therefore, a mineral is a naturally occurring, inorganic, solid with a definite composition and a regular internal crystal structure.
Atomic Chemistry and Bonding • All matter is made up of atoms, and all atoms are made up of three main particles known as protons, neutrons and electrons. • As summarized in the following table, protons are positively charged, neutrons are uncharged and electrons are negatively charged. The negative charge of one electron balances the positive charge of one proton. Both protons and neutrons have a mass of 1, while electrons have almost no mass.
Cont’d • The simplest atom is that of hydrogen, which has one proton and one electron. • The proton forms the nucleus of hydrogen, while the electron orbits around it. • All other elements have neutrons as well as protons in their nucleus. • The positively-charged protons tend to repel each other, and the neutrons help to hold the nucleus together. • For most of the 16 lightest elements (up to oxygen) the number of neutrons is equal to the number of protons. • For most of the remaining elements there are more neutrons than protons, because with increasing numbers of protons concentrated in a very small space, more and more extra neutrons are needed to overcome the mutual repulsion of the protons in order to keep the nucleus together. • The number of protons is the atomic number; the number of protons plus neutrons is the atomic weight. • For example, silicon has 14 protons, 14 neutrons and 14 electrons. Its atomic number is 14 and its atomic weight is 28.
Structure of Atoms • Electrons orbit around the nucleus in different shells, labeled from the innermost shell as K, L, M, N, etc. Each shell can have a certain number of electrons. The K-shell can have 2 Electrons, the L-shell, 8, the M-shell 18, N-shell 32. #electrons = 2N2, where N=1 for the K shell, N=2 for the L shell, N=3 for the M shell, etc. • A Stable electronic configuration for an atom is one 8 electrons in outer shell (except in the K shell, which is completely filled with only 2 electrons). Thus, atoms often loose electrons or gain electrons to obtain stable configuration. • Noble gases have completely filled outer shells, so they are stable. Examples He, Ne, Ar, Kr, Xe, Rn. • Others like Na, K loose an electron. This causes the charge balance to become unequal. In fact to become + (positive) charged atoms called ions. • Positively charged atoms = cations. Elements like F, Cl, O gain electrons to become (-) charged. (-) charged ions are called anions.
Types of bonding: Ionic bonding- caused by the force of attraction between ions of opposite charge. Example: Na+1 and Cl-1. Bond to form NaCl (halite or salt). Covalent bonding - Electrons are shared between two or more atoms so that each atom has a stable electronic configuration (completely filled outermost shell) part of the time. Example: H has one electron, needs 2 to be stable. O has 6 electrons in its outer shell, needs 2 to be stable. So, 2 H atoms bond to 1 O to form H2O, with all atoms sharing electrons, and each atom having a stable electronic configuration part of the time.
Cont’d • Metallic bonding -- Similar to covalent bonding, except innermost electrons are also shared. In materials that bond this way, electrons move freely from atom to atom and are constantly being shared. Materials bonded with metallic bonds are excellent conductors of electricity because the electrons can move freely through the material. • Van der Waals bonding -- a weak type of bond that does not share or transfer electrons. Usually results in a zone along which the material breaks easily (cleavage). A good example is graphite. Several different bond types can be present in a mineral, and these determine the physical properties of the mineral.
Crystal Structure • Solids having a regular, orderly arrangement of their internal atoms are said to have crystalline structure and are known as crystals. Most solid substances, including rocks and minerals, are made up of aggregates of many small crystals. • Crystals are characteristically bounded by flat surfaces, which are large-scale reflection of the internal arrangement of the atoms in the crystal. • Study of the arrangement of faces in natural crystals showed that there are six basic groups, each with a characteristic symmetry of the faces. • Packing of atoms in a crystal structure requires an orderly and repeated atomic arrangement. Such an orderly arrangement needs to fill space efficiently and keep a charge balance. • Since the size of atoms depends largely on the number of electrons, atoms of different elements have different sizes. • Crystal structure depends on the conditions under which the mineral forms.
The Six basic Systems of Crystal symmetry • Isometric: Three equal-length axes at right angles to each other. Example; Garnet Magnetite, Halite, Pyrite • Tetragonal: Two equal axes and a third either longer or shorter, all at right angles. Example; Zircon • Orthorhombic: Three unequal axes, all at right angles. Example; Olivine, Aragonite, Anhydrite, Goethite • Monoclinic: Three unequal axes, two at right angles and a third perpendicular to one but oblique to the other. Example; Pyroxene, Amphibole, Micas, Orthoclase, Gypsum • Triclinic: Three unequal axes meeting at oblique angles. Example; Plagioclase, Aluminosilcate • Hexagonal: Three equal axes in the same plane intersecting at 600 and a fourth perpendicular to the plane of the other three. Example; Quartz, Calcite, Hematite
Composition of Minerals The variety of minerals we see depend on the chemical elements available to form them. In the Earth's crust the most abundant elements are as follows: • 1. O, Oxygen 45.2% by weight • 2. Si, Silicon 27.2% • 3. Al, Aluminum 8.0% • 4. Fe, Iron 5.8% • 5. Ca, Calcium 5.1% • 6. Mg, Magnesium 2.8% • 7. Na, Sodium 2.3% • 8. K, Potassium 1.7% • 9. Ti, Titanium 0.9% • 10. H, Hydrogen 0.14% • 11. Mn, Manganese 0.1% • 12. P, Phosphorous 0.1% The most common minerals are those based on Si and O: the Silicates. Silicates are based on SiO4 tetrahedron. 4 Oxygens covalently bonded to one silicon atom.
ionic radii Ionic radii of ions commonly found in rock-forming minerals.
Polymorphs are minerals with the same chemical composition but different crystal structures. The conditions are such things as temperature (T) and pressure (P), because these affect ionic radii. At high T atoms vibrate more, and thus distances between them get larger. Crystal structure changes to accommodate the larger atoms. At even higher T substances changes to liquid and eventually to gas. Liquids and gases do not have an ordered crystal structure and are not minerals. Increase in P pushes atoms closer together. This makes for a more densely packed crystal structure. Examples: • Carbon (C) has two different polymorphs. At low T and P pure carbon is the mineral graphite, (pencil lead), a very soft mineral. • At higher T and P the stable form is diamond, the hardest natural substance known.
Ionic Substitution (Solid Solution) • Ionic substitution - (also called solid solution), occurs because some elements (ions) have the same size and charge, and can thus substitute for one another in a crystal structure. Examples: • Olivines Fe2SiO4 and Mg2SiO4. Fe+2 and Mg+2 are about the same size, thus they can substitute for one another in the crystal structure and olivine thus can have a range of compositions expressed as the formula (Mg, Fe)2SiO4. • Alkali Feldspars: KAlSi3O8 (orthoclase) and NaAlSi3O8, (albite) K+1 can substitute for Na+1 • Plagioclase Feldspars: NaAlSi3O8 (albite) and CaAl2Si2O8 (anorthite) NaSi+5 can substitutes for CaAl+5 (a complex solid solution).
Properties of Minerals Physical properties of minerals allow us to distinguish between minerals and thus identify them, as you will learn in lab. Among the common properties used are: • Habit - shape • Color • Streak (color of fine powder of the mineral) • Luster -- metallic, vitreous, pearly, resinous (reflection of light) • Cleavage (planes along which the mineral breaks easily) • Density (mass/volume) • Hardness: based on Mohs hardness scale as follows: 1. Talc 2. Gypsum (fingernail) 3. Calcite (penny) 4. Fluorite 5. Apatite (knife blade) 6. Feldspar (Orthoclase) 7. Quartz 8. Topaz 9. Corundum 10. Diamond Two German Captains Fought AFrenchQueen To Cause Death
Formation of Minerals Minerals are formed in nature by a variety of processes. Among them are: • Crystallization from melt (igneous rocks). • Precipitation from water (chemical sedimentary rocks, hydrothermal ore deposits). • Biological activity (biochemical sedimentary rocks). • Change to more stable state - (the processes of weathering, metamorphism, and diagenesis). • Precipitation from vapor. (not common, but sometimes does occur around volcanic vents).
Group of Minerals • We group minerals into classes on the basis of their predominant anion or anion group. These include oxides, sulphides, carbonates, silicates, and others. • Silicates are by far the predominant group in terms of their abundance within the crust and mantle.
Silicate Minerals • The vast majority of the minerals that make up the rocks of the earth's crust are silicate minerals. • These include minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and a great variety of clay minerals. • The building block of all of these minerals is the Silica tetrahedron, a combination of four oxygen atoms and one silicon atom. • These are arranged such that planes drawn through the oxygen atoms describe a tetrahedron (a four-faced object)—which is a pyramid with a triangular base. Silicon-oxygen tetrahedron
Cont’d • The bonds in a silica tetrahedron have some of the properties of covalent bonds and some of the properties of ionic bonds. • As a result of the ionic character, silicon becomes a cation (with a charge of +4) and oxygen becomes an anion (with a charge of -2), hence the net charge of a silica tetrahedron Si04 is -4. • Silica tetrahedra are linked together in a variety of ways to form most of the common minerals of the crust. • Most minerals are characterized by ionic or covalent bonds or a combination of the two, but one other type of bond which is geologically important is the metallic bond. • Elements that behave as metals have outer electrons that are relatively loosely held. When bonds between such atoms are formed these electrons can move freely from one atom to another. • A metal can thus be thought of as an array of positively charged nuclei immersed in a sea of mobile electrons. This characteristic accounts for two very important properties of metals: their electrical conductivity and their malleability.
Silicon-oxygen tetrahedron Structure The silica tetrahedron and the structure of silicate minerals. a. The silica tetrahedron consists of a central silicon atom bound to 4 oxygens. b. In orthosilicates such as olivine, the tetrahedra are separate and each oxygen is also bound to other metal ions that occupy interstitial sites between the tetrahedra. c. In pyroxenes, the tetrahedra each share two oxygen and are bound together into chains. Metal ions are located between the chains. d. In sheet silicates, such as talc, mica, and clays, the tetrahedra each share 3 oxygens and are bound together into sheets.
Origin of Igneous rocks • An igneous rock is any crystalline or glassy rock that forms from cooling of magma. • Magmaconsists mostly of liquid rock matter, but may contain crystals of various minerals, and may contain a gas phase that may be dissolved in the liquid or may be present as a separate gas phase.
Cont’d • Magma can cool to form an igneous rock either on the surface of the Earth - in which case it produces a volcanic or extrusive igneous rock, or beneath the surface of the Earth, in which case it produces a plutonic or intrusive igneous rock. • At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface. • This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles. • Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced.
Cont’d The composition of the gases in magma is: • Mostly H2O (water vapor) with some CO2 (carbon dioxide) • Minor amounts of Sulfur, Chlorine, and Fluorine gases • The amount of gas in magma is also related to the chemical composition of the magma. • Rhyolitic magmas usually have higher dissolved gas contents than basaltic magmas. Types of Magma Types of magma are determined by chemical composition of the magma. Three general types are recognized, but we will look at other types later in the course: 1. Basaltic magma (1000-1200oC)-- SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na 2. Andesitic magma (800-1000oC)-- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K 3. Rhyolitic or Granitic magma (650-800oC)-- SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na Viscosity of Magmas Viscosity is the resistance to flow (opposite of fluidity). Depends on composition, temperature, & gas content. Higher SiO2 content magmas have higher viscosity than lower SiO2 content magmas. Lower Temperature magmas have higher viscosity than higher temperature magmas.
Origin of Magma • In order for magmas to form, some part of the Earth must get hot enough to melt the rocks present. • Since rocks are mixtures of minerals, they behave somewhat differently. Unlike minerals, rocks do not melt at a single temperature, but instead melt over a range of temperatures. • Thus, it is possible to have partial melts from which the liquid portion might be extracted to form magma. • The two general cases are: • Melting of dry rocks is similar to melting of dry minerals, melting temperatures increase with increasing pressure, except there is a range of temperature over which there exists a partial melt. The degree of partial melting can range from 0 to 100% • Melting of rocks containing water or carbon dioxide is similar to melting of wet minerals; melting temperatures initially decrease with increasing pressure, except there is a range of temperature over which there exists a partial melt.
Origin of Basaltic Magma • Much evidence suggests that Basaltic magmas result from dry partial melting of mantle. • Basalts contain minerals like olivine, pyroxene and plagioclase, none of which contain water. • Basalts erupt non-explosively, indicating a low gas content and therefore low water content. • The Mantle is made of garnet peridotite (a rock made up of olivine, pyroxene, and garnet). • Under normal conditions the temperature in the Earth, shown by the geothermal gradient, is lower than the beginning of melting of the mantle. • Thus in order for the mantle to melt there has to be a mechanism to raise the geothermal gradient. • Once such mechanism is convection, wherein hot mantle material rises to lower pressure or depth, carrying its heat with it. • If the raised geothermal gradient becomes higher than the initial melting temperature at any pressure, then a partial melt will form. • Liquid from this partial melt can be separated from the remaining crystals because, in general, liquids have a lower density than solids. • Basaltic or gabbroic magmas appear to originate in this way.
Origin of Granitic or Rhyolitic Magma Most Granitic or Rhyolitic magma appears to result from wet melting of continental crust. The evidence for this is: • Most granites and rhyolites are found in areas of continental crust. • When granitic magma erupts from volcanoes it does so very explosively, indicating high gas content. • Solidified granite or rhyolite contains quartz, feldspar, hornblende, biotite, and muscovite. The latter minerals contain water, indicating high water content.
Origin of Andesitic Magma • Average composition of continental crust is andesitic, but if andesite magma is produced by melting of continental crust then it requires complete melting of crust. Temperatures in crust unlikely to get high enough. • Andesitic magmas erupt in areas above subduction zones, suggests relation between production of andesite and subduction. • One theory involves wet partial melting of subducted oceanic crust. But, newer theories suggest wet partial melting of mantle.
Magmatic Differentiation • When magma solidifies to form a rock it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change. • Depending on how many minerals are lost in this fashion, a wide range of compositions can be made. The process is called magmatic differentiation. Over the years, various processes have been suggested to explain the variation of magma compositions observed within small regions. Among the processes are: 1. Distinct melting events from distinct sources. 2. Various degrees of partial melting from the same source. 3. Crystal fractionation. 4. Mixing of 2 or more magmas. 5. Assimilation/contamination of magmas by crustal rocks. 6. Liquid Immiscibility. 7. Combined process (a combination of one of these)
1. Distinct Melting Events • One possibility that always exists is that the magmas are not related except by some heating event that caused melting. In such a case each magma might represent melting of a different source rock at different times during the heating event. The possibility of distinct melting events is not easy to prove or disprove. 2. Various Degrees of Partial Melting • When a multicomponent rock system melts, unless it has the composition of the eutectic, it melts over a range of temperatures at any given pressure, and during this melting, the liquid composition changes. Thus, a wide variety of liquid compositions could be made by various degrees of partial melting of the same source rock. 3. Crystal Fractionation Liquid compositions can change as a result of removing crystals from the liquid as they form. In all cases, crystallization results in a change in the composition of the liquid, and if the crystals are removed by some process, then different magma compositions can be generated from the initial parent liquid. If minerals that later react to form a new mineral or solid solution minerals are removed, then crystal fractionation can produce liquid compositions that would not otherwise have been attained by normal crystallization of the parent liquid.
Bowen's Reaction Series • Norman L. Bowen, an experimental Petrologist in the early 1900s, realized this from his determinations of simple 2- and 3-component phase diagrams, and proposed that if an initial basaltic magma had crystals removed before they could react with the liquid, that the common suite of rocks from basalt to rhyolite could be produced. • This is summarized as Bowen's Reaction Series.
Bowen's Reaction…cont’d Bowen suggested that the common minerals that crystallize from magmas could be divided into a continuous reaction series and a discontinuous reaction series. • The continuous reaction series is composed of the plagioclase feldspar solid solution series. A basaltic magma would initially crystallize a Ca- rich plagioclase and upon cooling continually react with the liquid to produce more Na-rich plagioclase. If the early forming plagioclase were removed, then liquid compositions could eventually evolve to those that would crystallize a Na-rich plagioclase, such as a rhyolite liquid. • The discontinuous reaction series consists of minerals that upon cooling eventually react with the liquid to produce a new phase. Thus, as we have seen, crystallization of olivine from a basaltic liquid would eventually reach a point where olivine would react with the liquid to produce orthopyroxene. Bowen postulated that with further cooling pyroxene would react with the liquid, which by this time had become more enriched in H2O, to produce hornblende. The hornblende would eventually react with the liquid to produce biotite. If the earlier crystallizing phases are removed before the reaction can take place, then increasingly more siliceous liquids would be produced. This generalized idea is consistent with the temperatures observed in magmas and with the mineral assemblages we find in the various rocks. We would expect that with increasing SiO2 oxides like MgO, and CaO should decrease with higher degrees of crystal fractionation because they enter early crystallizing phases, like olivines and pyroxenes. Oxides like H2O, K2O and Na2O should increase with increasing crystal fractionation because they do not enter early crystallizing phases.
Mechanisms of Crystal Fractionation Crystal Settling/Floating - In general, crystals forming from magma will have different densities than the liquid. Inward Crystallization - Because a magma body is hot and the country rock which surrounds it is expected to be much cooler, heat will move outward away from the magma. Thus, the walls of the magma body will be coolest, and crystallization would be expected to take place first in this cooler portion of the magma near the walls. The magma would then be expected to crystallize from the walls inward. Filter pressing - this mechanism has been proposed as a way to separate a liquid from a crystal-liquid mush. In such a situation where there is a high concentration of crystals the liquid could be forced out of the spaces between crystals by some kind of tectonic squeezing that moves the liquid into a fracture or other free space, leaving the crystals behind. It would be kind of like squeezing the water out of a sponge.
4.Magma Mixing • If two or more magmas with different chemical compositions come in contact with one another beneath the surface of the Earth, then it is possible that they could mix with each other to produce compositions intermediate between the end members. If the compositions of the magmas are greatly different (i.e. basalt and rhyolite), there are several factors that would tend to inhibit mixing. • Temperature contrast - basaltic and rhyolitic magmas have very different temperatures. If they come in contact with one another the basaltic magma would tend to cool or even crystallize and the rhyolitic magma would tend to heat up and begin to dissolve any crystals that it had precipitated. • Density Contrast- basaltic magmas have densities on the order of 2600 to 2700 kg/m3, whereas rhyolitic magmas have densities of 2300 to 2500 kg/m3. This contrast in density would mean that the lighter rhyolitic magmas would tend to float on the heavier basaltic magma and inhibit mixing. • Viscosity Contrast- basaltic magmas and rhyolitic magmas would have very different viscosities. Thus, some kind of vigorous stirring would be necessary to get the magmas to mix.
5. Crustal Assimilation/Contamination • Because the composition of the crust is generally different from the composition of magmas which must pass through the crust to reach the surface, there is always the possibility that reactions between the crust and the magma could take place. • If crustal rocks are picked up, incorporated into the magma, and dissolved to become part of the magma, we say that the crustal rocks have been assimilated by the magma. • If the magma absorbs part of the rock through which it passes we say that the magma has become contaminated by the crust. • Either of these processes would produce a change in the chemical composition of the magma unless the material being added has the same chemical composition as the magma. • In a sense, bulk assimilation would produce some of the same effects as mixing, but it is more complicated than mixing because of the heat balance involved. • In order to assimilate the country rock enough heat must be provided to first raise the country rock to its solidus temperature where it will begin to melt and then further heat must be added to change from the solid state to the liquid state. The only source of this heat, of course, is the magma itself.
6. Liquid Immiscibility • Liquid immiscibility is where liquids do not mix with each other. We are all familiar with this phenomenon in the case of oil and water/vinegar in salad dressing. We have also discussed immiscibility in solids, for example in the alkali feldspar system. Just like in the alkali feldspar system, immiscibility is temperature dependent. Two important properties of immiscible liquids. • 1. If immiscible liquids are in equilibrium with solids, both liquids must be in equilibrium with the same solid compositions. • 2. Extreme compositions of the two the liquids will exist at the same temperature. • Liquid immiscibility was once thought to be a mechanism to explain all magmatic differentiation. If so, requirement 2, above, would require that siliceous liquids and mafic liquids should form at the same temperature. Since basaltic magmas are generally much hotter than rhyolitic magmas, liquid immiscibility is not looked upon favorably as an explanation for wide diversity of magmatic compositions. Still, liquid immiscibility is observed in experiments conducted on simple rock systems. There are however, three exceptions where liquid immiscibility may play a role. • 1. Sulfide liquids may separate from mafic silicate magmas. • 2. Highly alkaline magmas rich in CO2 may separate into two liquids, one rich in carbonate, and the other rich in silica and alkalies. This process may be responsible for forming the rare carbonatite magmas. • 3. Very Fe-rich basaltic magmas may form two separate liquids - one felsic and rich in SiO2, and the other mafic and rich in FeO.
7. Combined Processes • As pointed out previously, if any of these processes are possible, then a combination of the process could act to produce chemical change in magmas. • Thus, although crystal fractionation seems to be the dominant process affecting magmatic differentiation, it may not be the only processes. • As we have seen, assimilation is likely to accompany by crystallization of magmas in order to provide the heat necessary for assimilation. If this occurs then a combination of crystal fraction and assimilation could occur. • Similarly, magmas could mix and crystallize at the same time resulting in a combination of magma mixing and crystal fractionation. In nature, things could be quite complicated.
Mode of occurrence of Igneous bodies Eruption of Magma • When magmas reach the surface of the Earth they erupt from a vent. • They may erupt explosively or non-explosively. • Non-explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to andesitic magmas). Usually begin with fire fountains due to release of dissolved gases Produce lava flows on surface and produce Pillow lavas if erupted beneath water.
Type of volcanic flows AA flow, the left side on the photo is a pahoehoe flow Ropy surface of a Pahoehoe flow If eruption column collapses a pyroclastic flow may occur, wherein gas and tephra rush down the flanks of the volcano at high speed. This is the most dangerous type of volcanic eruption. The deposits that are produced are called ignimbrites. Tephra that falls from the eruption column produces a tephra fall deposit
Structures and Field relationships Volcanoes • Shield volcano – volcanoes that erupt low viscosity magma (usually basaltic) that flows long distances from the vent. • Pyroclastic cone or cinder cone – a volcano built mainly of tephra fall deposits located immediately around the vent.
Volcanoes…cont’d • Stratovolcano (composite volcano) – a volcano built of interbedded lava flows and pyroclastic material. • Crater - a depression caused by explosive ejection of magma or gas.
Volcanoes…cont’d • Caldera - a depression caused by collapse of a volcano into the cavity once occupied by magma. • Lava Dome - a steep sided volcanic structure resulting from the eruption of high viscosity, low gas content magma. • Fissure Eruptions - An eruption that occurs along a narrow crack or fissure in the Earth's surface. • Pillow Lava - Lavas formed by eruption beneath the surface of the ocean or a lake.
Plutons • Dikes are small (<20 m wide) shallow intrusions that show a discordant relationship to the rocks in which they intrude. Discordant means that they cut across preexisting structures. They may occur as isolated bodies or may occur as swarms of dikes emanating from a large intrusive body at depth. • Igneous rocks cooled at depth. • Name comes from Greek god of the underworld - Pluto. • Sills are also small (<50 m thick) shallow intrusions that show a concordant relationship with the rocks that they intrude. Sills usually are fed by dikes, but these may not be exposed in the field.
Plutons…cont’d • Laccoliths are somewhat large intrusions that result in uplift and folding of the preexisting rocks above the intrusion. They are also concordant types of intrusions. • Batholiths are very large intrusive bodies, usually so large that there bottoms are rarely exposed. Sometimes they are composed of several smaller intrusions. • Stocks are smaller bodies that are likely fed from deeper level batholiths. Stocks may have been feeders for volcanic eruptions, but because large amounts of erosion are required to expose a stock or batholith, the associated volcanic rocks are rarely exposed.