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The Solid Earth Unit Two Part Two We will cover • - Soils • - Plate Tectonics Theory • - Earthquakes and Volcanoes Stepping into the Soil What is soil? • Soil is an important product of weathering. • It covers most of land surfaces • Very important resource Characteristics of Soil • Weathering produces a layer of rock and mineral fragments called regolith • Soil is the part of the regolith that supports plant growth. Regolith - all material between fresh bedrock and the ground surface, including weathered bedrock, deposits and soil. Soil • Soil has four major components: • • • • Mineral matter (Broken-down rock) Organic matter (humus) Water Air Soil Texture • Soils contain particles of all sizes. • The texture of a soil refers to the particle sizes in that soil How its formed • How a soil is formed is dependent on: • • • • • Parent material Time Climate Organisms Slope Parent Material • Source of mineral matter in soil • Can be either bedrock or unconsolidated deposits • Soil formed from bedrock is known as residual soil. • Soil formed from unconsolidated deposits is called transported soil. Time • Time is important because the longer a soil has been forming, the thicker it becomes. Climate • Climate has the greatest effect on soil formation. • Climate also determines what can live on and/or in the soil. • The influence of climate is so great that soil scientist have found that similar soils can be produced from different parent materials in the same climate. Organisms • Depending on what is in the soil, will tell us how rich the nutrients of the soil are. • Plants are the main source of organic matter in soil Slope • The amount of slope and direction of slope influence the soil formation greatly. Soil Profile • Soil varies in composition, texture, structure, and color at different depths. • These variations are divided into zones known as soil horizons. • A vertical section through all the soil horizons is known as the Types of Soil • Three common types 1. Pedalfer 2. Pedocal 3. Laterite 1. Pedalfer • Forms in temperate areas • B horizon in this form of soil is rich with iron oxide and aluminum-rich clays. 2. Pedocal • Drier conditioned soil that contains less clay. • Found in Western United State regions 3. Laterite • Form in hot, wet tropical areas. • Deeper soil due to rapid chemical weathering. What Is an Earthquake? ◆ An earthquake is the vibration of Earth produced by the rapid release of energy ◆ Focus and Epicenter • Focus is the point within Earth where the earthquake starts. • Epicenter is the location on the surface directly above the focus. ◆ Faults • Faults are fractures in Earth where movement has occurred. Focus, Epicenter, and Fault Slippage Along a Fault What Is an Earthquake? ◆ Elastic Rebound Hypothesis • Most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces. • When the strength of the rock is exceeded, it suddenly breaks, causing the vibrations of an earthquake. Elastic Rebound Hypothesis What Is an Earthquake? ◆ Aftershocks and Foreshocks • An aftershock is a small earthquake that follows the main earthquake. • A foreshock is a small earthquake that often precedes a major earthquake. Measuring Earthquakes ◆ Seismographs are instruments that record earthquake waves. ◆ Seismograms are traces of amplified, electronically recorded ground motion made by seismographs. ◆ Surface waves are seismic waves that travel along Earth’s outer layer. Seismograph Seismogram Measuring Earthquakes ◆ Body Waves • Identified as P waves or S waves • P waves - Are push-pull waves that push (compress) and pull (expand) in the direction that the waves travel - Travel through solids, liquids, and gases - Have the greatest velocity of all earthquake waves Measuring Earthquakes ◆ Body Waves • S waves • Seismic waves that travel along Earth’s outer layer - Shake particles at right angles to the direction that they travel - Travel only through solids - Slower velocity than P waves ◆ A seismogram shows all three types of seismic waves—surface waves, P waves, and S waves. Seismic Waves Measuring Earthquakes ◆ Earthquake Distance • The epicenter is located using the difference in the arrival times between P and S wave recordings, which are related to distance. ◆ Earthquake Direction • Travel-time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter. ◆ Earthquake Zones • About 95 percent of the major earthquakes occur in a few narrow zones. Locating an Earthquake Measuring Earthquakes ◆ Historically, scientists have used two different types of measurements to describe the size of an earthquake —intensity and magnitude. ◆ Richter Scale • Based on the amplitude of the largest seismic wave • Each unit of Richter magnitude equates to roughly a 32-fold energy increase • Does not estimate adequately the size of very large earthquakes Measuring Earthquakes ◆ Momentum Magnitude • Derived from the amount of displacement that occurs along the fault zone • Moment magnitude is the most widely used measurement for earthquakes because it is the only magnitude scale that estimates the energy released by earthquakes. • Measures very large earthquakes Earthquake Magnitudes Destruction from Earthquakes ◆ The damage to buildings and other structures from earthquake waves depends on several factors. These factors include the intensity and duration of the vibrations, the nature of the material on which the structure is built, and the design of the structure. Earthquake Damage Destruction from Earthquakes ◆ Building Design • Factors that determine structural damage - Intensity of the earthquake - Unreinforced stone or brick buildings are the most serious safety threats - Nature of the material upon which the structure rests - The design of the structure ◆ Liquefaction • Saturated material turns fluid • Underground objects may float to surface Destruction from Earthquakes ◆ Landslides • With many earthquakes, the greatest damage to structures is from landslides and ground subsidence, or the sinking of the ground triggered by vibrations. ◆ Fire • In the San Francisco earthquake of 1906, most of the destruction was caused by fires that started when gas and electrical lines were cut. Destruction from Earthquakes ◆ Short-Range Predictions • So far, methods for short-range predictions of earthquakes have not been successful. ◆ Long-Range Forecasts • Scientists don’t yet understand enough about how and where earthquakes will occur to make accurate long-term predictions. • A seismic gap is an area along a fault where there has not been any earthquake activity for a long period of time. Earth’s Layered Structure ◆ Earth’s interior consists of three major zones defined by their chemical composition—the crust, mantle, and core. ◆ Crust • Thin, rocky outer layer • Varies in thickness - Roughly 7 km in oceanic regions - Continental crust averages 8–40 km - Exceeds 70 km in mountainous regions Earth’s Layered Structure ◆ Crust • Continental crust - Upper crust composed of granitic rocks - Lower crust is more akin to basalt - Average density is about 2.7 g/cm3 - Up to 4 billion years old Earth’s Layered Structure ◆ Crust • Oceanic crust - Basaltic composition - Density about 3.0 g/cm3 - Younger (180 million years or less) than the continental crust Earth’s Layered Structure ◆ Mantle • Below crust to a depth of 2900 kilometers • Composition of the uppermost mantle is the igneous rock peridotite (changes at greater depths). Earth’s Layered Structure ◆ Core • Below mantle • Sphere with a radius of 3486 kilometers • Composed of an iron-nickel alloy • Average density of nearly 11 g/cm3 Earth’s Layered Structure ◆ Lithosphere • Crust and uppermost mantle (about 100 km thick) • Cool, rigid, solid ◆ Asthenosphere • Beneath the lithosphere • Upper mantle • To a depth of about 660 kilometers • Soft, weak layer that is easily deformed Earth’s Layered Structure ◆ Lower Mantle • 660–2900 km • More rigid layer • Rocks are very hot and capable of gradual flow. Earth’s Layered Structure ◆ Inner Core • Sphere with a radius of 1216 km • Behaves like a solid ◆ Outer Core • Liquid layer • 2270 km thick • Convective flow of metallic iron within generates Earth’s magnetic field Earth’s Layered Structure Earth’s Interior Showing P and S Wave Paths Earth’s Layered Structure ◆ Crust • Early seismic data and drilling technology indicate that the continental crust is mostly made of lighter, granitic rocks. ◆ Mantle • Composition is more speculative. • Some of the lava that reaches Earth’s surface comes from asthenosphere within. Earth’s Layered Structure ◆ Core • Earth’s core is thought to be mainly dense iron and nickel, similar to metallic meteorites. The surrounding mantle is believed to be composed of rocks similar to stony meteorites. Plate Tectonics Continental Drift ◆ Wegener’s continental drift hypothesis stated that the continents had once been joined to form a single supercontinent. • Wegener proposed that the supercontinent, Pangaea, began to break apart 200 million years ago and form the present landmasses. Breakup of Pangaea Continental Drift ◆ Evidence • The Continental Puzzle • Matching Fossils - Fossil evidence for continental drift includes several fossil organisms found on different landmasses. • Rock Types and Structures - Rock evidence for continental exists in the form of several mountain belts that end at one coastline, only to reappear on a landmass across the ocean. • Ancient Climates Matching Mountain Ranges Glacier Evidence Continental Drift ◆ A New Theory Emerges • Wegener could not provide an explanation of exactly what made the continents move. News technology lead to findings which then lead to a new theory called plate tectonics. Plate Tectonics ◆ According to the plate tectonics theory, the uppermost mantle, along with the overlying crust, behaves as a strong, rigid layer. This layer is known as the lithosphere. • A plate is one of numerous rigid sections of the lithosphere that move as a unit over the material of the asthenosphere. Plate Tectonics ◆ Divergent boundaries (also called spreading centers) are the place where two plates move apart. ◆ Convergent boundaries form where two plates move together. ◆ Transform fault boundaries are margins where two plates grind past each other without the production or destruction of the lithosphere. Three Types of Plate Boundaries Actions at Plate Boundaries ◆ Oceanic Ridges and Seafloor Spreading • Oceanic ridges are continuous elevated zones on the floor of all major ocean basins. The rifts at the crest of ridges represent divergent plate boundaries. • Rift valleys are deep faulted structures found along the axes of divergent plate boundaries. They can develop on the seafloor or on land. • Seafloor spreading produces new oceanic lithosphere. Spreading Center Actions at Plate Boundaries ◆ Continental Rifts • When spreading centers develop within a continent, the landmass may split into two or more smaller segments, forming a rift. East African Rift Valley Actions at Plate Boundaries ◆ A subduction zone occurs when one oceanic plate is forced down into the mantle beneath a second plate. ◆ Oceanic-Continental • Denser oceanic slab sinks into the asthenosphere. • Pockets of magma develop and rise. • Continental volcanic arcs form in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent. • Examples include the Andes, Cascades, and the Sierra Nevadas. Oceanic-Continental Convergent Boundary Actions at Plate Boundaries ◆ Oceanic-Oceanic • Two oceanic slabs converge and one descends beneath the other. • This kind of boundary often forms volcanoes on the ocean floor. • Volcanic island arcs form as volcanoes emerge from the sea. • Examples include the Aleutian, Mariana, and Tonga islands. Oceanic-Oceanic Convergent Boundary Actions at Plate Boundaries ◆ Continental-Continental • When subducting plates contain continental material, two continents collide. • This kind of boundary can produce new such as the Himalayas. mountain ranges, Continental-Continental Convergent Boundary Collision of India and Asia Actions at Plate Boundaries ◆ At a transform fault boundary, plates grind past each other without destroying the lithosphere. ◆ Transform faults • Most join two segments of a mid-ocean ridge. • At the time of formation, they roughly parallel the direction of plate movement. • They aid the movement of oceanic crustal material. Transform Fault Boundary Testing Plate Tectonics ◆ Paleomagnetism is the natural remnant magnetism in rock bodies; this permanent magnetization acquired by rock can be used to determine the location of the magnetic poles at the time the rock became magnetized. • Normal polarity—when rocks show the same magnetism as the present magnetism field • Reverse polarity—when rocks show the opposite magnetism as the present magnetism field Paleomagnetism Preserved in Lava Flows Testing Plate Tectonics ◆ The discovery of strips of alternating polarity, which lie as mirror images across the ocean ridges, is among the strongest evidence of seafloor spreading. Polarity of the Ocean Crust Testing Plate Tectonics ◆ Earthquake Patterns • Scientists found a close link between deep-focus earthquakes and ocean trenches. • The absence of deep-focus earthquakes along the oceanic ridge system was shown to be consistent with the new theory. Testing Plate Tectonics ◆ Ocean Drilling • The data on the ages of seafloor sediment confirmed what the seafloor spreading hypothesis predicted. • The youngest oceanic crust is at the ridge crest, and the oldest oceanic crust is at the continental margins. Testing Plate Tectonics ◆ Hot Spots • A hot spot is a concentration of heat in the mantle capable of producing magma, which rises to Earth’s surface; The Pacific plate moves over a hot spot, producing the Hawaiian Islands. • Hot spot evidence supports that the plates move over the Earth’s surface. Hot Spot Mechanisms of Plate Motion ◆ Scientists generally agree that convection occurring in the mantle is the basic driving force for plate movement. • Convective flow is the motion of matter resulting from changes in temperature. Mechanisms of Plate Motion ◆ Slab-Pull and Ridge-Push • Slab-pull is a mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along. It is thought to be the primary downward arm of convective flow in the mantle. • Ridge-push causes oceanic lithosphere to slide down the sides of the oceanic ridge under the pull of gravity. It may contribute to plate motion. Mechanisms of Plate Motion ◆ Mantle Convection • Mantle plumes are masses of hotter-than-normal mantle material that ascend toward the surface, where they may lead to igneous activity. • The unequal distribution of heat within Earth causes the thermal convection in the mantle that ultimately drives plate motion. Mantle Convection Models The Nature of Volcanic Eruptions ◆ Factors that determine the violence of an eruption • Composition of the magma • Temperature of the magma • Dissolved gases in the magma ◆ Viscosity • Viscosity is the measure of a material's resistance to flow. The Nature of Volcanic Eruptions ◆ Viscosity • Factors affecting viscosity - Temperature (hotter magmas are less viscous) - Composition (silica content) 1. High silica—high viscosity (e.g., rhyolitic lava) 2. Low silica—more fluid (e.g., basaltic lava) The Nature of Volcanic Eruptions • Pyroclastic Materials • Pyroclastic materials is the name given to particles produced in volcanic eruptions. • The fragments ejected during eruptions range in size from very fine duct and volcanic ash (less than 2 millimeters) to pieces that weigh several tons. The Nature of Volcanic Eruptions • Pyroclastic Materials • Types of pyroclastic material - Ash and dust—fine, glassy fragments - Pumice—frothy, air-filled lava - Lapilli—walnut-sized particles - Cinders—pea-sized particles • Particles larger than lapilli - Blocks—hardened lava - Bombs—ejected as hot lava The Nature of Volcanic Eruptions • The three main volcanic types are shield volcanoes, cinder cones, and composite cones. • Anatomy of a Volcano • A volcano is a mountain formed of lava and/or pyroclastic material. • A crater is the depression at the summit of a volcano or that which is produced by a meteorite impact. • A conduit, or pipe, carries gas-rich magma to the surface. Anatomy of a “Typical” Volcano The Nature of Volcanic Eruptions • Shield Volcanoes • Shield volcanoes are broad, gently sloping volcanoes built from fluid basaltic lavas. • Cinder Cones • Cinder cones are small volcanoes built primarily of pyroclastic material ejected from a single vent. - Steep slope angle - Rather small in size - Frequently occur in groups Shield Volcanoes Cinder Cones The Nature of Volcanic Eruptions • Composite Cones • Composite cones are volcanoes composed of both lava flows and pyroclastic material. - Most are adjacent to the Pacific Ocean (e.g., Mt. Rainier). - Large size - Interbedded lavas and pyroclastics - Most violent type of activity Composite Cones Mount St. Helens Before and After the May 18, 1980, Eruption Profiles of Volcanic Landforms Intrusive Igneous Activity • Geologists conclude that magma originates when essentially solid rock, located in the crust and upper mantle, partially melts. • The most obvious way to generate magma from solid rock is to raise the temperature above the level at which the rock begins to melt. Plate Tectonics and Igneous Activity • The basic connection between plate tectonics and volcanism is that plate motions provide the mechanisms by which mantle rocks melt to generate magma. • Ocean-Ocean • Rising magma can form volcanic island arcs in an ocean (Aleutian Islands). • Ocean-Continent • Rising magma can form continental volcanic arcs (Andes Mountains). Convergent Boundary Volcano Plate Tectonics and Igneous Activity • The greatest volume of volcanic rock is produced along the oceanic ridge system. • Lithosphere pulls apart. • Less pressure on underlying rocks • Partial melting occurs • Large quantities of fluid basaltic magma are produced. Plate Tectonics and Igneous Activity • Intraplate volcanism is igneous activity that occurs within a tectonic plate away from plate boundaries. • Most intraplate volcanism occurs where a mass of hotter than normal mantle material called a mantle plume rises toward the surface. • The activity forms localized volcanic regions called hot spots. • Examples include the Hawaiian Islands and the Columbia Plateau. Kilauea, an Intraplate Volcano 1. Describe how the direction and rate of movement for the North American plate has affected the local climate over the last 600 million years 2. Describe the differences between oceanic and continental crust. 3. Using your knowledge of plate tectonics, tell how our community would be different if there were no moving of plates. How would our world look? What kind of affect would it have on our economy?