Introduction to Geology GEOL-101 Midterm 1 Review Based on the textbook: Understanding Earth, 6th Edition, by Grotzinger and Press CH 1: earth system Summary The human creative process, field and lab observations, and experiments help geoscientists formulate testable hypotheses (models) for how the Earth works and its history. A hypothesis is a tentative explanation focusing attention on plausible features and relationships of a working model. If a testable hypothesis is confirmed by a large body of data, it may be elevated to a theory. Theories are abandoned when subsequent investigations show them to be false.
Confidence grows in those theories that withstand repeated tests and successfully predict the results of new experiments. A set of hypothesis and theories may become the basis of a scientific model that represents an entire system too complicated to replicate in the laboratory. Often models are tested and revised in a series of computer simulations. Confidence in such a model grows as it successfully predicts the behavior of the system. The elevations of Earth topography averages 1–2 kilometers above sea level for land features and 4–5 kilometers below sea level for features of the deep ocean.
The principle of uniformitarianism states that geological processes have worked in the same way throughout time. Earth’s interior is divided into concentric layers (crust, mantle, core) of sharply different chemical composition and density. The layered composition of the Earth is driven by gravity. Only eight of the 100 or so elements account for 99 percent of Earth’s mass. The lightest element (oxygen) is most abundant in the surface crust and mantle, while the densest (iron) makes up most of what is found deep in the core. Earth’s major interacting systems are the climate system, the plate tectonic system, and the geodynamic system.
The climate system involves interactions among the atmosphere, hydrosphere, and biosphere. The plate tectonic system involves interactions among the lithosphere, asthenosphere, and deep mantle. The geodynamic system involves interactions within the central core that produce occasional reversals of Earth’s magnetic field. As the Earth cooled, an outer relatively rigid shell, called the lithosphere, formed. Dynamic processes driven by heat transfer, density differences, and gravity broke the outer shell into plates that move around the Earth at rates of centimeters per year.
Major components (atmosphere, hydrosphere, biosphere) of Earth’s surface systems are driven mostly by solar energy. Earth’s internal heat energizes the lithosphere, asthenosphere, deep mantle, and outer and inner core. Terms and Concepts Asthenosphere Convection Core Continental lithosphere Continental crust Earth systems Geology Inner core Lithosphere Mantle Oceanic lithosphere Oceanic crust Plate tectonic system Principle of uniformitarianism Scientific method Topography CH 2: plate tectonics Summary For over the last century some geologists have argued for the concept of continental drift based on: he jigsaw-puzzle fit of the coasts on both sides of the Atlantic the geological similarities in rock ages and trends in geologic structures on opposite sides of the Atlantic fossil evidence suggesting that continents were joined at one time the distribution of glacial deposits as well as other paleoclimatic evidence In the last half of the twentieth century the major elements of the plate tectonic theory were formulated. Starting in the 1940s (WWII), ocean floor mapping began to reveal major geologic features on the ocean floor.
Then, the match between magnetic anomaly patterns on the seafloor with the paleomagnetic time scale revealed that the ocean floor had a young geologic age and was systematically older away from the oceanic ridge systems. The concepts for seafloor spreading, subduction, and transform faulting evolved out of these and other observations. According to the theory of plate tectonics, the Earth’s lithosphere is broken into a dozen moving plates. The plates slide over a partially molten, weak asthenosphere, and the continents, embedded in some of the moving plates, are carried along.
There are three major types of boundaries between lithospheric plates: divergent boundaries, where plates move apart convergent boundaries, where plates move together and one plate often subducts beneath the other transform boundaries, where plates slide past each other Volcanoes, earthquakes, and crustal deformation are concentrated along the active plate boundaries. Mountains typically form along convergent- and transform-plate boundaries. Where divergent-plate boundaries are exposed on land, subsiding basins and mafic volcanism are typical.
Various methods have been used to estimate and measure the rate and direction of plate movements. Today seafloor-spreading rates vary between a few to 24 cm per year. Seafloor isochrons provide the basis for reconstructing plate motions for about the last 200 million years. Distinct assemblages of rocks characterize eachtype of plate boundary. Using diagnostic rock assemblages embedded in continents and paleo-environmental data recorded by fossils and sedimentary rocks, geologists have been able to reconstruct ancient plate tectonic events and plate configurations.
Driven by Earth’s internal heat, convection of hot and cold matter within the mantle, the force of gravity and the existence of an asthenosphere are important factors in any model for the driving mechanism of plate tectonics. Currently studies of the plate-driving forces focus on discovering the exact nature of the mantle convection. Questions being addressed include: Where do the plate driving forces originate? At what depth does recycling occur? What is the nature of rising Convection Currents? The assembly and subsequent break up of Pangaea represent a striking example of the effects of plate tectonics acting over geologic time.
The story begins with the breakup of the ancient supercontinent of Rodinia 750 million years ago. Plate tectonic processes dispersed the fragments of Rodinia forming a system of ancient continents that existed from the late Proterozoic through much of the Paleozoic. Continued tectonic movement eventually resulted in a set of continental collisions and reformation of the ancient continents into Pangaea. Assembly was completed during the early Triasic, about 240 million years ago. Then, about 200 million years ago the rift that would evolve into the Atlantic Ridge began to open and the separation of Pangaea was underway.
By the beginning of the Cenezoic, India was well on its way to Asia, and the Tethys sea that had separated Africa from Eurasia began to close into the modern inland sea that we know as the Mediterranean. Continued changes during the Cenozoic produced our modern world and its geography. Terms and Concepts Continental drift Continent-continent convergent boundary Convergent boundary Divergent boundary Island arc Isochron Lithospheric plates Magnetic anomaly Magnetic time scale Mid-ocean ridge Mountain range Ocean-ocean convergent boundary Ocean-continent convergent boundary Pangaea Plate tectonics
Seafloor spreading Spreading center Subduction Transform boundary Wegener’s hypothesis CH 3: earth materials Summary Minerals are naturally occurring inorganic solids with a specific crystal structure and chemical composition. Minerals form when atoms or ions chemically bond and come together in an orderly, three-dimensional geometric array—a crystal structure. Chemical bonding may occur either as a result of simple electrostatic attraction (ionic bond) or electron sharing (covalent bond). The strength of the chemical bonds and the crystalline structure determine many of the physical properties, e. . , hardness, cleavage of minerals. Silicate minerals are the most abundant class of minerals in the Earth’s crust and mantle. Common silicate minerals are polymorphs of silicon ions arranged in either isolated tetrahedral (olivine), single chains (pyroxene), double chains (amphibole), sheets (mica), or three-dimensional frameworks (feldspar). There are three important groups of silicates: ferromagnesium silicates, e. g. , olivine and pyroxene—common in the mantle feldspar and quartz—common in the crust clay mineral—commonly produced by chemical weathering
Other common mineral classes include carbonates, oxides, sulfates, sulfides, halides, and native metals. A rock is a naturally occurring solid aggregate of minerals. A few rocks consist of only one mineral and a few others consist of non-mineral matter. The properties of rocks and rock names are determined by mineral content (the kinds and proportions of minerals that make up the rock) and texture (the size, shapes, and spatial arrangement of crystals or grains. There are three major rock types: Igneous rocks solidify from molten liquid (magma); crystal size within igneous rocks is largely determined by the cooling rate of the magma body.
Sedimentary rocks are made of sediments formed from the weathering and erosion of any pre-existing rock; deposition, burial and lithification (compaction and cementation) transform loose sediments into sedimentary rocks. Metamorphic rocks are formed by an alteration in the solid state of any preexisting rock by high temperatures and pressure. Terms and Concepts Anion Atomic mass Atomic number Carbonate Cation Cleavage Covalent bond Crystal Crystallization Electron sharing Electron transfer Isotope Magma Mineral Polymorph Precipitate Rock CH 4: igneous rocks Summary
Igneous rocks can be divided into two broad textual classes: coarsely crystalline rocks, which are intrusive (plutonic) and therefore cooled slowly finely crystalline rocks, which are extrusive (volcanic) and cooled rapidly. Within each of these broad textual classes, the rocks are subdivided according to their composition. General compositional classes of igneous rocks are felsic, intermediate, mafic and ultramafic, in decreasing silica and increasing iron and magnesium content. Figures 4. 1, 4. 2, 4. 3 and Table 4. 1 summarize common minerals and composition of igneous rocks.
The lower crust and upper mantle are typical places where physical conditions induce rock to melt. Temperature, pressure, rock composition, and the presenceof water all affect the melting temperature of the rock: Increase temperature: not all minerals melt at the same temperature; refer to Figures 4. 6 and 4. 7, which explain how fractional crystallization results from Bowen’s reaction series. The mineral composition of the rock affects the melting temperature. Felsic rocks with higher silica content melt at lower temperatures than mafic rocks which contain less silica and more iron/magnesium.
Lower the confining pressure: a reduction in pressure can induce a hot rock to melt. A reduction in confining pressure on the hot upper mantle is thought to generate the basaltic magmas which intrude into the oceanic ridge system to form ocean crust; refer to Figure 4. 15. Add water: the presence of water in a rock can lower its melting temperatures up to a few hundred degrees. Water released from rocks subducting into the mantle along convergent plate boundaries is thought to be an important factor in magma generation at convergent plate boundaries.
As subduction begins water trapped in the rock is subjected to increasing temperature and pressure. Eventually the water is released into sedimentary layers above where it melts parts of the overlying plate; refer to Figure 4. 16. Terms and Concepts Andesite Basalt Batholith Bomb Concordant intrusion Country rock Decompression melting Dike Discordant intrusion Diorite Extrusive igneous rock Felsic rock Fractional crystallization Gabbro Granite Granodiorite Intermediate rock Intrusive igneous rock Lava Mafic rock Magma chamber Magmatic differentiation Partial melting Pegmatite Peridotite Pluton Rhyolite Porphyry
Pumice Pyroclast Rhyolite Sill Ultramafic rock Volcanic ash xenolith CH5: sedimentary rocks Summary Plate tectonic processes play an important role in producing depressions (basins) in which sediments accumulate. Sedimentary basins result from rifting, thermal sag, and flexure of the lithosphere. The sedimentary stages of the rock cycle involve the overlapping processes of weathering, erosion, transportation, deposition, burial, and diagenesis. Weathering and erosion produce the clastic particles and dissolved ions that compose sediment. Water, wind, and ice transport the sediment downhill to where it is deposited.
Burial and diagenesis harden sediments into sedimentary rocks via pressure, heat, and chemical reactions. The two major types of sediments are clastic and chemical/biochemical. Clastic sediments are formed from rock particles and mineral fragments. Chemical and biochemical sediments originate from the ions dissolved in water. Chemical and biochemical reactions precipitate these dissolved ions from solution. Understanding the characteristics of sediments and modern sedimentary environments provides a basis for reconstructing past environmental conditions using the rock record.
Sedimentary structures like bedding, ripple marks, and mud cracks, provide important clues about the sedimentary environment. Diagenesis transforms sediment into sedimentary rock. Burial promotes this transformation by subjecting sediments to increasing heat and pressure. Cementation is especially important in the lithification of clastic sediments. The classification of clastic sediments and sedimentary rocks is based primarily on the size of the grains within the rock. The name of chemical and biochemical sediments and sedimentary rock is based primarily on their composition. Terms and Concepts Carbonate rock
Carbonate sediment Cementation Chemical weathering Compaction Conglomerate Cross-bedding Crude oil Diagenesis Evaporite rock Flexural basin Foraminifera Graded bedding Gravel Limestone Lithification Physical weathering Porosity Ripple Salinity Sandstone Sedimentary basin Sedimentary structure Shale Siliciclastic sediments Sorting Subsidence Thermal subsidence basin CH 6: Metamorphic rocks Summary Metamorphism is the alteration in the solid state of preexisting rocks, including older metamorphic rocks. Increases in temperature and pressure and reactions with chemicalbearing fluids cause metamorphism.
Metamorphism typically involves a rearrangement (recrystallization) of the chemical components within the parent rock. Rearrangement of components within minerals is facilitated by: higher temperatures, which increase ion mobility within the solid state; higher confining pressure compacts the rock; directed pressure associated with tectonic activity can cause the rock to shear (smear), which orients mineral grains and generates a foliation; and chemical reactions with migrating fluids may remove or add materials and induce the growth of new minerals.
The two major types of metamorphism are regional metamorphism, associated with orogenic processes that build mountains, contact metamorphism, caused by the heat from an intruding body of magma, and seafloor metamorphism, also known as metasomatism. Other less common kinds of metamorphism are: burial metamorphism, associated with subsiding regions on continents, high-pressure metamorphism, occurring deep within subduction zones and upper mantle, and shock metamorphism due to meteor impact; refer to Figure 6. 4.
Metamorphic rocks fall into two major textural classes: the foliated (displaying a preferred orientation of minerals, analogous to the grain within wood) and granoblastic (granular). The composition of the parent rock and the grade of metamorphism are the most important factors controlling the mineralogy of the metamorphic rock. etamorphism usually causes little to no change in the bulk composition of the rock. The kinds of minerals and their orientation do change. Mineral assemblages within metamorphic rocks are used by geoscientists as a guide to the original composition of the parent rock and the conditions during metamorphism.
Metamorphic rocks are characteristically formed in subduction zones, continental collisions, oceanic spreading centers, and deeply subsiding regions on the continents. Terms and Concepts Amphibolite Burial metamorphism Contact metamorphism Eclogite Foliation Gneiss Granoblastic rock marble Metasomatism Migmatite Phyllite Porphroblast Quartzite Regional metamorphism Schist Seafloor metamorphism Shock metamorphism Slate Adapted for the GEOL101 course by Alfonso Benavides (2012)
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