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Philip Galkin
Philip Galkin

The Field Description Of Igneous Rocks



This handbook explains how to observe igneous rocks in the field, from the scale of hand specimens and outcrops to that of regional field relationships. There are 12 chapters: 1, introduction; 2, field techniques; 3, description of igneous rock outcrops; 4, hand specimens and their interpretation; 5, mode of occurence of igneous bodies; 6, volcanic rocks; 7, minor intrusions; 8, plutonic rocks 1, the calc-alkaline association; 9, plutonic rocks 2, the alkaline association; 10, plutonic rocks 3, mafic-ultramafic associations; 11, plutonic rocks 4, anorthositic associations and charnockitic associations; and 12, metamorphism. (TRRL)




The Field Description of Igneous Rocks



Softcover, England: Open University Press, 1985. First Edition. Very Good-. "Written to complement textbooks on igneous petrology, this handbook equips the student with all the information required to study igneous rocks in the field." Ships same or next business day. Previous owner's name on head edge of front cover and tail edge of pages. Spine is slightly skewed and faded. Normal edge wear. Small bumps on spine ends. Reading crease on front cover and full title page. Notes and underlining in pen and pencil throughout. Some soiling and tanning to edges of pages. ; Geological Society of London Handbook Series; B&W Illustrations; 12mo 7" - 7" tall; 154 pages. [Item #75175]


The second edition is well laid out, with an introduction and overview of magmatic rocks followed by two chapters on field skills and the identification and description of igneous rocks in the field. Following...


Igneous rocks are different than minerals in that they are identified by the mineral composition, their texture (size of crystals within) and their color. Igneous rocks are divided into two large groups; intrusive and extrusive igneous rocks. Intrusive igneous rocks are ones that cooled within the Earth's surface. Extrusive igneous rocks are ones that cooled on the Earth's surface. The area where they cool causes them to have very different properties. Because intrusive igneous rocks cooled beneath the Earth's surface, they cooled very slowly, which allowed for the growth of large crystals. In many cases, you can see the crystals contained within the rock. Extrusive rocks cool on the Earth's surface, which causes them to cool very quickly. Because of that, crystals do not have time to form, so the texture of extrusive rocks is generally very fine.


Students will enjoy two week-long residential field camps in Central Otago and Fiordland. The field camps will focus on how to make geological observations, take quantitative and descriptive field notes, and the techniques involved in geological mapping. Students will develop their skills as field geologists by observing a wide range of igneous, metamorphic and sedimentary rocks. Follow-up work in the laboratory will focus on the interpretation of geological maps and the production of short written reports summarising field observations and interpretations. Additionally, students will develop an understanding of the regional geology of Central Otago and Fiordland.


Almost all igneous rocks are silicates meaning that silicon and oxygen are the two most common elements in them. Composition of igneous rocks is expressed as weight percent silica, which ranges from about 45 to 78 (wt % SiO2). Low silica rocks have higher concentrations of iron and magnesium. In turn, high silica rocks have less iron and magnesium, and more sodium and potassium.The two major divisions of igneous rocks based on composition are:


Ultramafic (those with less silica and more iron and magnesium than mafic rocks) plutonic rocks such as dunite, peridotite, and pyroxenite are found in some national parks like Great Smoky Mountains and Kenai Fjords national parks. Ultramafic volcanic rocks are extremely rare, and are not found in any NPS unit.Other terms (such as basic, acidic, felsic, etc.) that generally describe the composition of igneous rocks exist, some of which are older terms and have fallen out of use by many American geologists.


Mafic and silicic rocks have important differences in their characteristics and properties because their composition controls these variables. Two of the most important are melting temperature and viscosity (resistance to flow). Mafic magmas have high melting temperatures and low viscosity. Silicic ones have lower temperatures and high viscosity.Composition also determines what minerals are present in igneous rocks. Mafic rocks contain minerals that are high in magnesium and/or iron such as olivine, pyroxene, amphibole, magnetite, as well as plagioclase feldspar. Quartz (SiO2) cannot form in mafic magmas so this mineral is not found in gabbro or basalt. Feldspars and quartz are the most common minerals in silicic rocks, although these rocks may contain minor amounts of mica (biotite or muscovite), amphibole, magnetite, and other minerals.


Groundmass - The finer grained and/or glassy material between the large crystals (phenocrysts) in an igneous rock. Most often used when describing porphyritic volcanic rocks but may be applicable for some plutonic rocks. The groundmass of volcanic rocks may consist of tiny microscopic crystals and/or volcanic glass.Phenocryst - A large crystal in a porphyritic igneous rock. Phenocrysts may be present in some volcanic rocks as well as in intrusive rocks. In volcanic rocks, phenocrysts usually formed in the magma chamber prior to eruption.


Intrusive (plutonic) igneous rocks are found in either large bodies (plutons or batholiths) or are thin sheets that either cut across (dikes) or are parallel (sills) to layering in the surrounding rocks. Many intrusive bodies result from the solidification of magma chambers that had been underneath volcanoes and then are later exposed by subsequent erosion. The plutonic rocks in Yosemite, Kings Canyon, and Sequoia national parks are all part of the Sierra Nevada Batholith that formed.


ESS 101 Introduction to Geology and Societal Impacts (5) SSc/NScIntroduction to the processes, materials and structures that shape Earth. Emphasizes the dynamic nature of the earth's tectonic system and its relationship to physical features, volcanism, earthquakes, minerals and rocks and geologic structures. The course emphasizes the intrinsic relationship between human societies and geologic processes, hazards and resources. Not open for credit to students who have taken ESS 210. Optional field trips. Prerequisite: No prerequisite classes required. Offered: AWSpS.View course details in MyPlan: ESS 101


ESS 212 Plate Tectonics and Materials of the Earth (5) NScOrigin, composition and structure of the Earth; identification of important rock-forming minerals; identification and description of igneous, metamorphic, and sedimentary rocks; magmatic, metamorphic, and sedimentary processes; formation of continental and oceanic crust; driving mechanisms for plate tectonics; comparison of Earth to other planets. Recommended: high school or college chemistry (e.g., CHEM 110); and high school or college pre-calculus (e.g., MATH 120). Offered: W.View course details in MyPlan: ESS 212


ESS 301 Geology of the Northwest (5) NScGeologic history of Washington, Oregon, and Idaho. Emphasis on use of geologic principles in interpreting evidence found in landscapes and rocks. Weekend field trips optional. Prerequisite: either ESS 101, ESS 105, ESS 210, ESS 211, or ESS 212.View course details in MyPlan: ESS 301


ESS 312 Earth Materials (5) NScCrystallography, crystal chemistry, and characteristics of rock-forming and ore minerals. Description, phase equilibria, origin, and associations of igneous, sedimentary, and metamorphic rocks. Laboratory study of hand specimens. One one-day field excursion. Prerequisite: a minimum grade of 2.0 in either CHEM 142 or CHEM 145; and a minimum grade of 2.0 in ESS 212; recommended: ESS 211 and ESS 213. Offered: Sp.View course details in MyPlan: ESS 312


ESS 439 Petrology of Igneous Rocks (5) NScSystematic study of the major families of volcanic and plutonic igneous rocks with emphasis on tectonic setting, phase relations, geochemistry, and models of their origin and evolution throughout geologic time. Laboratory emphasizes thin-section study of rocks using transmitted and reflected light. Prerequisite: ESS 316.View course details in MyPlan: ESS 439


ESS 456 Sedimentary Geology and Depositional Environments (4) NScPrinciples of sedimentary geology, including survey of modern processes that produce sedimentary rocks and sequences. Recognition of various depositional environments represented in the geologic record, including terrestrial, marine terrigenous, and carbonate environments. Two field trips required. Prerequisite: ESS 213; recommended: ESS 311 and either ESS 326, ESS 425, ESS 426, or ESS 427View course details in MyPlan: ESS 456


Multidomain type alternating-field (AF) demagnetization characteristics like those reported by Lowrie and Fuller for multidomain (MD) magnetite have been discovered in two rock types: (1) coarse-grained dioritic intrusives, and (2) continental basalts of low oxidation state. These are the first reported examples of MD type whole-rock AF characteristics. Mixed single-domain (SD) and MD type characteristics have been measured for two other rocks, a gabbro from the same area as the diorites and a basalt of oxidation index 3. Hysteresis properties of the diorites and gabbro (remanence ratio Jrs/Js 5) are consistent with predominantly MD remanence carriers, and examination of the oxide petrology suggests that these carriers are homogeneous titanomagnetite grains 200-600, μm in size. However, where a stable component of natural remanence can be isolated, the AF characteristics of this component are of SD type. Hysteresis properties of the basalts are less definitive than those of the intrusives but show a clear trend from low (0.127) to high (0.259) Jrs/Js values with increasing oxidation index, in agreement with the corresponding change from MD type to SD type AF characteristics. The MD remanence carriers appear to be 80-100 μm titanomagnetite grains. The Lambertville diabase and Michikamau anorthosite have an anomalous combination of MD hysteresis and SD type AF characteristics. This behavior is not understood but could conceivably prove to be a characteristic signature of rocks containing sub-microscopic magnetite within silicate minerals. Although the results reported here do not constitute a definitive test, the excellent correlation among grain size of the oxide fraction, overall domain state as indicated by hysteresis, and AF demagnetization results for most of the intrusive and volcanic rocks examined favors the view that the Lowrie-Fuller criterion can detect MD carriers of remanence in nature. 041b061a72


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