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Forces Within Earth Objectives • Define stress and strain as they apply to rocks. • Distinguish among the three types of faults. • Contrast three types of seismic waves. Vocabulary – stress – secondary wave – strain – surface wave – fault – focus – primary wave – epicenter Forces Within Earth Forces Within Earth • Earthquakes are natural vibrations of the ground caused by movement along fractures in Earth’s crust, or sometimes, by volcanic eruptions. Earthquakes rip buildings, anything, apart… The earth shifted 30+ feet here You can see the fault line Improperly built buildings disappear… Forces Within Earth Stress and Strain • Most earthquakes occur when rocks fracture, or break, deep within Earth. • Fractures form when stress exceeds the strength of the rocks involved. • Stress is the forces per unit area acting on a material. Forces Within Earth Stress and Strain • There are three kinds of stress that act on Earth’s rocks: – Compression is stress that decreases the volume of a material. – Tension is stress that pulls a material apart. – Shear is stress that causes a material to twist. • Strain is the deformation of materials in response to stress. Forces Within Earth Stress and Strain • There is a distinct relationship between stress and strain that can be plotted as a stress-strain curve. – A stress-strain curve usually has two segments: a straight segment and a curved segment. – Low stresses produce the straight segment, which represents the elastic strain of a material. – If the elastic strain is reduced to zero, the deformation disappears. Forces Within Earth Faults Types of Faults – There are three basic types of faults: • Reverse faults are fractures that form as a result of horizontal compression. • Normal faults are fractures caused by horizontal tension. • Strike-slip faults are fractures caused by horizontal shear. Forces Within Earth Earthquake Waves • Most earthquakes are caused by movements along faults. • Irregular surfaces in rocks can snag and lock, causing stress to build in the rocks. • When the rocks reach their elastic limit they break, and this produces an earthquake. Forces Within Earth Earthquake Waves Types of Seismic Waves – The vibrations of the ground during an earthquake are called seismic waves. – Every earthquake generates three types of seismic waves. • Primary waves, or P-waves, squeeze and pull rocks in the same direction along which the waves are traveling. Forces Within Earth Earthquake Waves Types of Seismic Waves • Secondary waves, or S-waves, cause rocks to move at right angles in relation to the direction of the waves. • Surface waves travel along Earth’s surface, moving in two directions as they pass through rock. Forces Within Earth Earthquake Waves Types of Seismic Waves – P-waves and S-waves, also called body waves, pass through Earth’s interior. – The focus of an earthquake is the point of failure of rocks at the depth where an earthquake originates. – The epicenter of an earthquake is the point on Earth’s surface directly above the focus. Seismic Waves and Earth’s Interior Seismometers and Seismograms Seismic Waves and Earth’s Interior Seismometers and Seismograms • A seismogram is the record produced by a seismometer. Seismic Waves and Earth’s Interior Clues to Earth’s Interior Measuring and Locating Earthquakes Seismic Belts Earthquakes and Society Objectives • Discuss factors that affect the amount of damage done by an earthquake. • Explain some of the factors considered in earthquake probability studies. • Define seismic gaps. Vocabulary – tsunami – seismic gap Earthquakes and Society Seismic Risk What is a mineral? Mineral Characteristics Definite Crystalline Structure – At times and fairly rare, a mineral will form in an open space and grow into one large crystal, possibly taking the shape of one of the six major crystal systems. Tetragonal Cubic Orthorhombic Hexagonal Monoclinic Triclinic Identifying Minerals • Classify minerals according to their physical and chemical properties. Vocabulary – luster – fracture – streak – specific gravity – hardness – ore – cleavage – gem Identifying Minerals Mineral Identification Color – One of the most noticeable characteristics of a mineral is its color. – Color is sometimes caused by the presence of trace elements or compounds within a mineral. – In general, color is one of the least reliable clues to a mineral’s identity. Identifying Minerals Mineral Identification Luster – Luster is the way that a mineral reflects light from its surface. – metallic or nonmetallic. Identifying Minerals Mineral Identification Texture – Texture describes how a mineral feels to the touch. – The texture of a mineral might be described as smooth, rough, ragged, greasy, soapy, or glassy. Streak – Streak is the color of a mineral when it is broken up and powdered. Hardness – Hardness is one of the most useful and reliable tests for identifying minerals. – Hardness is a measure of how easily a mineral can be scratched. What is this? Identifying Minerals Mineral Identification Cleavage and Fracture – Minerals break along planes where atomic bonding is weak. Identifying Minerals Mineral Identification Density and Specific Gravity – Density is expressed as a ratio of the mass of a substance divided by its volume, or D = M/V. Identifying Minerals Special Properties • Special properties of minerals also can be used for identification purposes. – light to be bent in two directions, a process known as double refraction, when it passes through the mineral. – Calcite (CaCO3) fizzes when it comes into contact with hydrochloric acid (HCl). – Magnetite, an iron ore, is naturally magnetic. – The mineral sphalerite produces a distinctive rotten-egg odor when it is rubbed vigorously across a streak plate. Identifying Minerals Mineral Uses • Minerals are virtually everywhere. • They are used to make computers, cars, televisions, desks, roads, buildings, jewelry, beds, paints, sports equipment, and medicines, just to name a few uses. Identifying Minerals Mineral Uses Ores – An ore is a mineral that contains a useful substance that can be mined at a profit. – Examples of ores include Coal (shown), Hematite, which contains the element iron and bauxite, which contains the element aluminum. Identifying Tectonic Plate Movement • Plate tectonic theory, 1915 when Alfred Wegener "continental drift" • Coastlines look like they fit together like a puzzle • Fossils of similar species found on continents that are now separated by great distance • Glaciers also separated by great distances • Paleomagnetic studies • Seismometers showed earthquakes, volcanoes, and other active geologic features for the most part aligned along edges of tectonic plates Superposition Superposition • Top Picture— A is the oldest, then C then G then P then B then D then J Lowest layers are older then came volcanic intrusion through and between layers Devil’s Tower Soil used to cover it Erosion wore away softer dirt You can see erosion paused for a while right here Cooled Magma Remains of Organisms in the Rock Record Types of Fossils • original preservation – soft and hard parts, have not undergone any kind of change since the organisms’ deaths – Such fossils are uncommon, need special circumstances Altered Hard Parts – Altered hard parts organic material removed and changed – Permineralization pore spaces in a fossil are filled in with mineral substances Remains of Organisms in the Rock Record Types of Fossils Altered Hard Parts – Altered hard parts are fossils in which all the organic material has been removed and the hard parts of a plant or animal have been changed either by mineral replacement or by recrystallization. – Permineralization is the process by which pore spaces in a fossil are filled in with mineral substances. – During the process of recrystallization, the exterior of the shell or a bone remains the same, but the shell microstructures are destroyed. • APPMC— • Archaic [Archaeozoic, microorganisms] • Professional [Proterozoic, microorganisms] • Paleontologists [Paleozoic, small sea creatures to plants and amphibians] • Mezmorize [Mesozoic, dinosaurs] • Centurions [Cenozoic, mammals] Renewable Resources Non-renewable Energy