Petrology Lecture Notes 2024 PDF

PETROLOGY ENGR. HAZEL NUÑEZ-TOLEDO PETROLOGY The study of the origin, occurrence, structure and history of rocks. PETROGENESIS A branch of petrology dealing with the origin and formation of rocks. Involves a combination of mineralogical, chemical and field data. PETROGRAPHY The description and s...

PETROLOGY ENGR. HAZEL NUÑEZ-TOLEDO PETROLOGY The study of the origin, occurrence, structure and history of rocks. PETROGENESIS A branch of petrology dealing with the origin and formation of rocks. Involves a combination of mineralogical, chemical and field data. PETROGRAPHY The description and systematic classification of rocks, aided by the microscopic examination of thin sections. Rocks  Naturally-occurring aggregate of minerals.  Three types:  Igneous  Sedimentary  Metamorphic The Rock Cycle  Classifying Rocks  When classifying a rock sample geologists observe the rock’s color and texture and determine its mineral composition. Color: the apparent color of the rock, on the inside and the outside. Texture: the size, shape, and pattern of the rock’s grain. Mineral composition: The minerals that make up the different parts of a rock.  Texture: Grain Size  Often, the grains in a rock are large and easy to see.  Such rocks are said to be coarse- grained. In other rocks, the grains are so small that they can only be seen with a microscope. These rocks are said to be fine-grained.  Notice the difference in texture between the fine-grained slate and the coarse- grained diorite below. Texture: Grain Shape The grains in a rock vary widely in shape Some grains look like tiny particles of fine sand Others look like small seeds or exploding stars In some rocks, such as granite, the grain results from the shapes of the crystals that form the rock Texture: Grain Shape In other rocks, the grain shape results from fragments of other rock These fragments can be smooth and rounded, like the fragments in conglomerate, or they can be jagged, like the fragments in breccia You can compare conglomerate and breccia on the next slide Texture : Grain Pattern  The grains in a rock often form patterns.  Other grains form wavy, swirling patterns. Some rocks have grains that look like rows of multicolored beads, as in the sample of gneiss shown  Other rocks, in contrast, have grains that occur randomly throughout the rock. COMPOSITION—What kind of substances the rocks are made of Significance of Studying Rocks: The study of rocks is important for several reasons:  Their minerals and global chemistry provide information about the composition of the Earth’s crust and mantle.  Their ages can be calculated by various methods of radiometric dating, and a time sequence of geological events can be put together. Significance of Studying Rocks: The study of rocks is important for several reasons:  Their features are usually characteristic of a specific tectonic environment, allowing scientists to reconstitute tectonic processes.  Many rocks host important ores that provide valuable raw materials that we rely on for our sustenance and technological development.  Igneous Petrology Defined as:  The branch of petrology concerned with the identification, classification, origin, evolution, and processes of formation and crystallization of the igneous rocks.  ---Britannica  That branch of petrology which focuses on the composition and texture of igneous rocks  ---New World Encyclopedia  Scope  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, which range in age from early Archean to Neogene, and they also include the high-level volcanic extrusive rocks and the plutonic rocks that formed deep within the crust.  Since such is the case the scope of this branch of petrology adheres to the definition such as that of the origin, evolution, process of formation and identification of igneous rocks based on their composition and texture. Igneous rock  Igneous rock (derived from the Latin word ignis meaning fire), or magmatic rock, is one of the three main rock types. Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Igneous rock  Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition.  Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rock  A magma consists 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.  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.  Characteristics of Magma Types of Magma  Types of magma are determined by chemical composition of the magma.  Three general types are recognized: 1. Basaltic magma -- SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na 2. Andesitic magma -- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K 3. Rhyolitic magma -- SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na  Gases in Magmas  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.  Gases in Magmas  Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced. The composition of the gases in magma are:  Mostly H2O (water vapor) with some CO2 (carbon dioxide)  Minor amounts of Sulfur, Chlorine, and Fluorine gases  The amount of gas in a magma is also related to the chemical composition of the magma.  Rhyolitic magmas usually have higher dissolved gas contents than basaltic magmas.  Temperature of Magmas  Temperature of magmas is difficult to measure (due to the danger involved), but laboratory measurement and limited field observation indicate that the eruption temperature of various magmas is as follows:  Basaltic magma - 1000 to 1200oC  Andesitic magma - 800 to 1000oC  Rhyolitic magma - 650 to 800oC.  Viscosity of Magmas  Viscosity is the resistance to flow (opposite of fluidity). Viscosity depends on primarily on the composition of the magma, and temperature. Higher SiO2 (silica) content magmas have higher viscosity than lower SiO2 content magmas (viscosity increases with increasing SiO2 concentration in the magma). Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma). Mode of Occurrences: Pahoehoe lava – has a smooth, billowy or ropy surface. Mode of Occurrences: Aa lava – consists of irregular blocks that are commonly covered with small spines. Block lava – is composed of irregular blocks that lack spines. – is typical of siliceous lava. Fissure eruptions – are lava flows reaching the surface of the earth along extensive fissures. Fissure – is an extensive crack in a rock. Terms: Spherulites – are spherical bodies composed of fibers of feldspar with radial arrangement about a common center. Vesicular structure – refers to rock riddled with air bubbles which may be almond-shaped, rounded, ellipsoidal, or even tabular. Scoria – a term applied to basaltic lava in which the gas vesicles are numerous and irregular in shape. Vesicles – are small cavity formed in volcanic rock by entrapment of a gas bubble during solidification. Terms: Pumice – is a frothy siliceous lava, produced in an extreme stage of gas escaping. Amygdules – are infilling of vesicles by secondary minerals such zeolites, carbonates and various forms of silica. Phenocrysts – are large and well-shaped crystals. Groundmass – is a fined-grained or glassy base. Terms: Trachytic structure – refers especially to a mass of sanidine laths in subparallel alignments, like a school of minnows. Porphyritic structure – is used in ancient times as an ornamental and building stone, which contains large, prominent crystals in a deep-reddish fine-grained matrix. Environments of Formation  Specific areas in the upper part of the earth comprise the environments of formation for igneous rocks. Figure 1.Plate Tectonic – Igneous Genesis Relationship Environment of formation are identified as follows and are indicated in Fig.1  1. Mid-ocean Ridges  2. Intracontinental Rifts  3. Island Arcs  4. Active Continental Margins  5. Back-arc Basins  6. Ocean Island Basalts  7. Miscellaneous Intra-Continental Activity  *kimberlites, carbonatites, anorthosites... Classification of Igneous Rocks Igneous rocks are classified by its composition, most likely on the basis of its silica content. Words such as Ultramafic, Mafic, Intermediate and Felsic are used to distinguish one igneous rock from the other. Distinctions between these terms are made in order to specify rocks accordingly.  Ultramafic Igneous Rock – also referred to as ultrabasic rocks; are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). Classification of Igneous Rocks  Mafic Igneous Rock - igneous rock that is rich in magnesium and iron, and is thus a portmanteau of magnesium and ferric. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar.  Chemically, mafic rocks are enriched in iron, magnesium and calcium and typically dark in color. In contrast the felsic rocks are typically light in color and enriched in aluminum and silicon along with potassium and sodium. The mafic rocks also typically have a higher density than felsic rocks. The term roughly corresponds to the older basic rock class.  Intermediate Igneous Rock - Rocks composed mostly of hornblende and intermediate plagioclase feldspars. Intermediate magmas are somewhat more viscous than the mafic magmas. Additionally, they contain somewhat more gas than do the mafic magmas, but not quite as much as the felsic magmas.  Felsic Igneous Rock - Rocks composed mostly of biotite, muscovite, sodium- rich plagioclase feldspars, potassium feldspars, and quartz make up the felsic family of igneous rocks. Felsic magmas are much more viscous than the intermediate magmas. Additionally, felsic magmas may have very high gas contents. Granite is the name given to intrusive felsic rocks, whereas rhyolite is the name given to extrusive felsic rocks. Felsic came from Feldspar and Silica as opposed to Mafic which came from Magnesium and Ferrous/Ferric. Silica Gradation 1. Acidic Igneous Rocks -- SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na e.g. Granites ~ 72 wt% SiO2, granodiorites ~ 68 wt% SiO2 2. Intermediate Igneous Rocks -- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K e.g. Andesite 57 wt% SiO2 3. Basic Igneous Rocks - SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na e.g. Basalts range from 48 to 50 wt% 4. Ultrabasic Igneous Rocks - < 45 wt% SiO2 Figure 3. Difference between the four classification of Igneous Rocks Two Types of Igneous Rock Igneous rock is classified into two major types: Volcanic and Plutonic Rocks  Volcanic Rocks Volcanic rocks or extrusive rocks are those that are expelled from volcanoes with faster rate of cooling Characteristics of Volcanic Rocks according to Texture  Aphanitic Texture: An aphanitic texture is the result of lava having been extruded out of a volcano cooling and crystallizing much faster than magma that has cooled and crystallized underground. The resulting crystals are extremely small–too small, in fact, to be seen with the unaided eye. Figure 2.Rhyolite exhibiting aphanitic texture Figure 3.Basalt exhibiting aphanitic texture Characteristics of Volcanic Rocks according to Texture  Glassy Texture: If an igneous rock solidifies so quickly that it does not have time to form crystals, then it is said to have a glassy texture. Obsidian is an example of an igneous rock with this type of texture. Figure 5.Obsidian or volcanic glass Characteristics of Volcanic Rocks according to Texture  Porphyritic Texture: A porphyritic texture, results when some of the minerals have cooled and crystallized while the magma was still underground, thus producing larger crystals, while other minerals cooled and crystallized above ground after having been extruded from a volcano in the form of lava. Characteristics of Volcanic Rocks according to Texture  The rock that crystallizes from lava above ground quickly cools into a surrounding matrix of rock with an aphanitic texture around the larger crystals, thus creating a texture of large crystals called phenocryst embedded in a matrix of tiny crystals, too small to be seen with the unaided eye. Porphyritic andesite or porphyritic diorite are examples of rocks with this kind of a texture. Also rhyolites and granites sometimes have a porphyritic texture. Figure 8.Andesite Porphyry Figure 9.Granite Porphyry Matrix – otherwise known as groundmass is the compact, finer-grained material in which the crystals are embedded in a porphyritic rock. Grains – also known as phenocryst is a large or conspicuous crystal in a porphyritic rock, distinct from the groundmass. Figure 10.Porphyritic texture showing the phenocrysts and the groundmass Two Types of Igneous Rock  Plutonic Rocks  Plutonic rocks or rocks otherwise known as intrusive rocks are those that solidified from a melt at great depth with slow rate of cooling thereby producing larger forms of crystal. Textures present in Plutonic Rocks Phaneritic Texture: Intrusive igneous rocks produced from magma that has cooled and crystallized underground (e.g. granite) have a phaneritic texture, meaning a coarse-grained texture with crystals that are readily seen by the unaided eye. This is because intrusive igneous rocks cool and crystallize slowly underground, allowing them time to form larger crystals. Figure 6. Syenite exhibiting Figure 7.Quartz Diorite phaneritic texture Textures present in Plutonic Rocks  Pegmatitic Texture: A pegmatitic texture is one in which the mineral grains are exceptionally large. The largest ones are, by convention, more than about 3 cm long. This texture is found in intrusive rocks.  The extra large size does not mean that they cooled extra slowly. Instead, the large crystals of a pegmatite formed in a magma that was extra rich in dissolved water. The water allowed the necessary elements to diffuse extra fast to the sites of crystallization. Textures present in Plutonic Rocks  Pegmatites often form in veins that opened up at the end of the crystallization of a large intrusive mass. Most igneous minerals exclude water from their structure, so the last magma to cool often ends up enriched in water and other incompatible elements. Most of the large igneous minerals that are seen in geology labs are from pegmatites. Figure 8.Pegmatite Figure 9.Tourmaline Pegmatite Volcanic and Plutonic Equivalents Types of Intrusion Sills: form when magma intrudes between the rock layers, forming a horizontal or gently-dipping sheet of igneous rock. Types of Intrusion Dykes: form as magma pushes up towards the surface through cracks in the rock. Dykes are vertical or steeply-dipping sheets of igneous rock. Types of Intrusion Batholiths: are large, deep-seated intrusions (sometimes called Plutons) that form as thick, viscous magma slowly makes its way toward the surface, but seldom gets there. 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, associated volcanic rocks are rarely exposed. Types of Intrusion Laccoliths: are mushroom-shaped bodies with a flat floor and a domed roof. Thus, they appear to have begun forming in the same way as sills; however, as magma continued to intrude, it pushed up the overlying layers rather than continuing to spread out laterally. Types of Intrusion Lopolith: igneous intrusion associated with a structural basin, with contacts that are parallel to the bedding of the enclosing rocks. They can be several miles to several hundred miles in diameter, with thicknesses up to several thousand feet, are some of the largest igneous intrusions known. Types of Intrusion Pacholith: a pluton parallel to the bedding plane or foliation of folded country rock. More specifically, it is a typically lens-shaped pluton that occupies either the crest of an anticline or the trough of a syncline.