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Transcript
Chapter 2 The Planet Oceanus Composition of the Earth • The Earth consists of a series of concentric layers or spheres which differ in chemistry and physical properties. • There are two different ways to describe those layers: Composition and Physical State • The compositional layers of the Earth are: • the Crust • the Mantle • the Core • The Core is subdivided into a molten outer core and solid inner core Figure 02.01a: The Earth's internal mass. An outer crust, a middle mantle, and a center core (solid inner core surrounded by molten outer core). The Effects of Pressure and Temperature • Increasing pressure raises the melting point of a material by forcing atoms and molecules into tighter configurations, and holding them together more strongly. • Increasing temperature provides additional energy to the atoms and molecules of matter. • This allows them to move farther apart, eventually causing the material to melt. Pressure and Temperature (continued) • Both pressure and temperature increase toward the center of the Earth, but at variable rates. • Divisions of the Earth based upon physical state are: – – – – – the Lithosphere the Asthenosphere the Mesosphere the Outer core the Inner core Figure 02.01b: The Earth's interior Three spheres surround the rocky portion of the Earth • Hydrosphere includes all of the “free” water of the Earth contained in: – – – – – – – Oceans Lakes Rivers Snow Ice Water vapor Groundwater 2-1 The Earth’s Structure The Earth’s Spheres (continued) • Atmosphere is the gaseous envelope that surrounds the Earth . – It is mainly a mixture of nitrogen and oxygen. • Biosphere refers to all living and non-living organic matter. The Sea Floor • Physiography and bathymetry (the topography of the submarine landscape) allow the sea floor to be subdivided into three distinct provinces: – continental margins – deep ocean basins – mid-oceanic ridges Figure 02.02: This map shows the major bathymetric features of the ocean basins of the Pacific Ocean, the Atlantic Ocean, and the Indian Ocean. Courtesy of NOAA • Continental margins are the submerged edges of the continents. • They consist of massive wedges of sediment eroded from the land and deposited along the continental edge. • The continental margin can be divided into three parts: – the Continental shelf – the Continental slope – the Continental rise Figure 2-3a Continental Margin 2-2 The Physiography of the Ocean Floor The Sea Floor (Continued) • The Deep Ocean Province lies between the continental margins and the mid-oceanic ridges. • It includes a variety of features from mountainous to flat plains: – Abyssal plains – Abyssal hills – Seamounts – Deep sea trenches Figure 02.03b: The floor of the deep-ocean basin. • Midoceanic Ridge Province consists of a continuous submarine mountain range. • It covers about one third of the ocean floor. • It extends for about 60,000 km around the Earth. Figure 2-3c Midocean Ridge 2-2 The Physiography of the Ocean Floor Continents and Ocean Basins Differ in Composition, Elevation, and Physiographic Features • Elevation of Earth’s surface displays a bimodal (two peaks) distribution. – About 29% above sea level – Much of the remainder 45 kilometers below sea level Figure 02.04b: Here is a frequency plot showing the relative distribution of the Earth's land elevations and ocean depths. Composition, elevation, and physiographic (Continued) • Continental crust is mainly composed of granite. – light colored – lower density (2.7 gm/cm3) – igneous rock – rich in aluminum, silicon and oxygen • Oceanic crust is composed of basalt – dark colored – higher density (2.9 gm/cm3) – volcanic rock – rich in silicon, oxygen, iron and magnesium • The Moho is the boundary between rocks of the crust and the denser (3.3 gm/cm3) rocks of the mantle, which are mostly silicon, oxygen, iron and magnesium. Composition, elevation, and physiographic (Continued) • Isostasy refers to the balance of an object “floating” upon a fluid medium. • Height (elevation) of the mass above the surface of the medium is controlled by: – the thickness of the mass – the density of the mass (similar to ice floating in water) – The density of the medium in (or on) which the mass is floating Figure 02.05a: The longer the block, the higher its top rises above the water surface and the deeper its base extends into the water. Composition elevation, and physiographic (Continued) • The greater the density of the mass relative to the density of the medium, the lower it will sink in the medium. • The greater the thickness of the mass, the higher a portion of it will rise above the medium. Figure 02.05b: The higher an iceberg rises above the sea surface, the deeper the ice must penetrate into the water in order to maintain isostatic balance. Isostasy • Continents are thick (30 to 40 km), have low density and rise high above the supporting mantle rocks. • Sea floor is thin (4 to 10 km), has greater density and does not rise as high above the mantle. Figure 02.05c: The oceanic crust is denser and thinner than the continental crust. Composition, elevation, and physiographic (Continued) • Altimetry uses satellites to determine bathymetry. • Based upon slight changes in the elevation of the sea surface (averaged over time to account for waves and tides). • These changes result from the greater gravitational attraction of large rock masses on the sea floor, such as volcanoes or the mid-ocean ridges. Satellite Altimetry Figure B02.05: Variations in the height of the sea surface reflect the presence of large bathymetric features that affect the elevation of the sea surface. Echo Sounding and Seismic Reflection • Echo sounding and seismic reflection rely on sound pulses that reflect off the ocean floor and off sedimentary layers. • The fundamental relationship between the speed of sound and the density of the material in which the sound is traveling is the key to using these techniques. Figure B02.03: Seismic reflection profile. The continuous reflection of sound from subbottom layers (see Figure B2–1) produces a geologic cross section. In this profile, layers of sediment have been deformed into a broad fold. Echo Sounding and Seismic Reflection • Seismic refraction examines how sound waves are bent (refracted) as they travel through material. • They reveal: – Densities – Depths – Thicknesses of rock layers Figure B02.04: The refraction (bending) of sound as it travels through different rock types reveals the shape and density structure of the underlying rock masses. Chapter 3 The Origin of Ocean Basins Continental Drift • Alfred Wegner proposed his hypothesis of continental drift based upon – the fit of continental outlines – fossil evidence – geologic evidence • The continents are sections of a past super continent called Pangea, surrounded by a single ocean he called Panthalassa. • Pangea broke apart and the continents drifted to their present locations. Fitting the Continents Together Figure 03.01a: This map, published by Antonio Snider in 1858, shows the arrangement of the continents into a large landmass. Pangaea, 200 to 300 MYBP Figure 03.01b: If the continents are arranged into Pangea, coal beds and glacial deposits that are 200 to 300 million years old fall into latitudinal belts. The Breakup of Pangaea Figure 03.12a: This sequence of maps displays the changing world geography since the time of Pangaea, the megacontinent of the Permian Period. Sea Floor Spreading • Sea floor spreading was proposed and confirmed in the 1960s, long after Wegener’s hypothesis became very unpopular… • It demonstrates that: – the sea floor moves apart at the oceanic ridges – new oceanic crust is added (mainly through volcanic activity) in the rift valleys at the crests of those ridges • Rift valleys along oceanic ridge crests: – indicate tension – are bounded by normal faults – are floored by recently-erupted basaltic lava flows Ridges & Faults Axis of the oceanic ridge (divergent, spreading displacement) is offset by transform (strike-slip) faults which produce lateral displacement. Figure 03.02a: The rift valley at the crest of midocean ridges is formed by tension. The Geomagnetic Field (Continued) • These measurements (called magnetic anomalies) alternate strong (positive) and weak (negative) in response to the influence of the sea floor rocks. • Magnetic anomalies form parallel bands arranged symmetrically about the axis of the oceanic ridge. Figure 03.03: Magnetic anomaly stripes run parallel and are symmetrically arranged on both sides of the midocean ridge axis. The Geomagnetic Field (Continued) • Ocean floor rocks are mostly basaltic, and they contain minerals rich in iron. These minerals aligned themselves with Earth’s magnetic field as it existed at the time when the lava erupted and froze to form volcanic basalt. Figure 03.04a: The lava sequences in areas A, B, and C have alternating directions of rock magnetization. The Geomagnetic Field (Continued) Figure 03.04c: Based on numerous analyses of lava flows on land, the pattern of polarity reversals of the Earth's magnetic field has been accurately established. • This imparts a permanent magnetic field, called paleomagnetism, to the rock. • This is measured by looking at magnetic rocks of known age that are collected from one place to construct a paleomagnetic time scale. The Geomagnetic Field (Continued) Figure 03.04b: During normal polarity, the geomagnetic north pole lies near the geographic north pole. During a reversal, it flips with the geomagnetic south pole. The geomagnetic polarity time scale, a.k.a. the paleomagnetic time scale shows that over geologic time, the Earth’s magnetic field polarity (direction) periodically reverses poles The Sea Floor and the Geomagnetic Field • Because of their paleomagnetism, rocks of the sea floor influence the magnetic field recorded by magnetometers. • Rocks on the sea floor with normal polarity (e.g. the same as today) paleomagnetism locally reinforce Earth’s magnetic field making it stronger and producing a positive anomaly. • Rocks on the sea floor with reverse polarity (e.g. the opposite from today) paleomagnetism locally weaken Earth’s magnetic field, producing a negative anomaly. Magnetic Reversals Magnetic reversals give rise to a magnetic anomaly signature that is the sum total of both the modern field and the influence of the ocean floor rocks. Figure 03.04d: A magnetometer measures simultaneously both the Earth's magnetic field and the fossil magnetization in the rocks. Dating and the Magnetic Field • Rocks forming at the ridge crest record the magnetic field existing at the time (and place) when they solidify. • Sea floor increases in age and is more deeply buried by sediment away from the ridge because sediments have had a longer time to collect. • It also gets deeper as it cools, because colder rocks are denser and “float” lower (isostasy!) Figure 03.05a: Sea-floor speading combined with geomagnetic polarity reversals creates magnetic anomaly stripes Determining Rates of Change • Rates of sea-floor spreading vary from 1 to 10 cm per year for each side of the ridge. • Rates can be determined by dating magnetic anomaly stripes of the sea floor and measuring their distance from the ridge crest. • Continents are moved by the expanding sea floor - they ride on larger blocks we call “Lithospheric Plates”. Figure 03.05b: The rate of seafloor spreading is easily calculated using the age and distance from the ridge crest of any magnetic anomaly stripe. The Changing Ocean Floor • Because Earth’s size has not changed, expansion of the crust in one area requires destruction (throuh convergence) of the crust elsewhere. • Currently, the Pacific Ocean basin is shrinking (because the oceanic lithospheric plate is being forced back into the mantle) as other ocean basins expand. This process is violent, and produces many earthquakes. • Seismicity is the frequency, magnitude and distribution of earthquakes. • Tectonism refers to the deformation of Earth’s crust. 3. Global Plate Tectonics - Because Earth’s size is constant, expansion of the crust in one area requires destruction of the crust elsewhere. - Earthquakes are concentrated along oceanic ridges, transform faults, trenches, and island arcs. - Subduction is the process at a trench whereby one part of the sea floor plunges below another and down into the asthenosphere Earthquakes Earthquakes are concentrated along oceanic ridges, transform faults, trenches and island arcs; larger and deeper earthquakes at trenches, smaller and shallower earthquakes at ridges. Figure 03.06a: Most earthquakes (represented by black dots) clearly coincide with midocean ridges, transform faults, and deep-sea trenches. Subduction Zones • Destruction of sea floor occurs in subduction zones. • Subduction is the process at a trench whereby one part of the sea floor plunges below another and down into the asthenosphere. • The final stage of subduction is the collision between continents that ride on the lithospheric plates… these continents are too low-density to allow subduction to continue (isostasy again!), and mountain belts form Figure 03.12b: These diagrams depict the collision and suturing of the India and Asia plates. Subduction Zones • The Wadati/Benioff Zone is an area of increasingly deeper seismic activity that runs parallel to a subduction trench and slopes down into the mantle parallel to the down-going plate. • It is inclined from the trench downward in the direction of the island arc. Figure 03.06b: Sketch map of the south Fiji Basin and cross section showing a plot of earthquakes that clearly defines a Benioff zone. Lithospheric Plates Figure 03.07a: The edges of large lithospheric plates are indicated by bands of seismicity. The arrows indicate relative plate motions. Plate Movement Movement of plates is caused by thermal convection of the “plastic” rocks of the asthenosphere which drag along the overlying lithospheric plates. Figure 03.08: Convection in the asthenosphere drags lithospheric plates away from the crests of ocean ridges. Figure 03.07b: A brittle lithospheric plate, which includes the crust and the upper mantle, overlies and moves relative to the plasticlike asthenosphere. The Wilson Cycle • The Wilson Cycle refers to the sequence of events leading to the formation, expansion, contracting and eventual elimination of ocean basins. This cycle is thought to repeat over 200-500 million year time scales. • Stages in basin history are: – Embryonic - rift valley forms as continent begins to split. – Juvenile - sea floor basalts begin forming as continental fragments diverge. – Mature - broad ocean basin widens, trenches eventually develop and subduction begins. – Declining - subduction eliminates much of sea floor and oceanic ridge. – Terminal - last of the sea floor is eliminated and continents collide forming a continental mountain chain. The Wilson Cycle Figure 03.11: The Wilson cycle depicts ocean-basin development as proceeding through a sequence of distinct stages. The Red Sea • The Red Sea is a juvenile ocean basin that is forming as the African plate diverges from the Arabian plate. • New basaltic ocean crust is just beginning to form in the center of the Red Sea. Figure B03.04c: New oceanic crust is forming in the axial trough of the Red Sea by the process of seafloor spreading. Figure B03.04a: This satellite image shows the terrain of the Red Sea region. Figure B03.04b: The Red Sea is a juvenile ocean basin that has just recently opened up as the Arabian plate separates from the African plate.