The Sea Floor Chapter 2 Continental Drift • Theory proposed by Alfred Wegner (German geophysicist), • Stated that all the continents were joined together at one time in to a supercontinent called “Pangea.” • Pangea began to break apart 180 million years ago. Pangea Theory of Plate Tectonics • Theory that provides the explanation for continental drift. • It is the process involved in the movement of large plates on the earth’s crust. Mid Ocean Ridge • Discovered from sonar images after WWII. • It is a continuous chain of volcanic mountains that encircles the globe like a baseball’s seams. • It is the largest geological feature on Earth!!! • The formation was made when transformation faults were displaced by cracks in the Earth’s crust. The Sea Floor • The sea floor is geologically distinct from continental land masses. • Geological processes sculpt the shoreline, determine water depth, control if the sea bottom is muddy, sandy, or rocky. • Geological processes create new islands and undersea mountains for colonization and determine specific marine habitats. Earth is a water planet. • Oceans cover 71% of the globe. • Oceans regulate Earth’s climate and atmosphere. • The Northern Hemisphere is 61% covered by oceans. • The Southern Hemisphere is 80% covered by oceans. Ocean Classification • Oceans are divided into four basins. 1) 2) 3) 4) Pacific Basin- deepest and largest Atlantic Basin- second largest Indian Basin- third largest Arctic Basin- smallest and shallowest Connected to basins are seas . • Examples of seas are: Mediterranean, Gulf of Mexico, South China, Red, Dead, Baltic, etc. • All oceans are interconnected and compromise a single “world ocean” referred to as Panthalassa. • The Southern Ocean is a continuous body of water that surrounds Antarctica. Internal Earth Structure The Core • The innermost portion of Earth. • The inner core is made of solid iron. • The outer core is made of liquid iron that has a swirling motion. • This motion is responsible for Earth’s magnetic field. The Mantle • Found outside the core. • Most is solid and very hot, near the melting point of most rocks. • Some flows very slowly like a liquid. The Crust • Outermost layer made of thin, rigid rock that floats on top of the mantle. 1) Continental Crust- mostly made of granite and is light in color. It is less dense than oceanic crust so it floats on the mantle. Approx. 3.8 million yr. old. 2) Oceanic Crust- mostly made of basalt and is denser and thinner than continental crust. It does not float as high on the mantle. Aprox. 180 million years old. Mid Ocean Ridge Classification • In the Atlantic- Called Mid Atlantic Ridge • In the Pacific- Called East Pacific Rise Trenches • Deep depressions in the sea floor especially common in the Pacific. Significance of Mid Ocean Ridge • Lead to the discovery that from time to time the Earth’s magnetic field has reversed direction. • This happens about every 700,000 years. • Reversal takes 5000 years. • Reversal occurs from the movements of materials in the Earth’s core. Magnetic Anomalies • Geologists found striped patterns of magnetic bands in the sea floor. • Show alternate periods of normal and reversed periods of earth’s magnetic field. The Sea Floor • Created by oceanic crust separated at the midocean ridge allowing some mantle to rise through the rift. • When the mantle rises through the rift it cools and solidifies to form new oceanic crust. Sea Floor Spreading • Process of making new oceanic crust. • What do you remember about the oceanic crust? Lithosphere • Litho = “Rock” • “Rock sphere” • Composed of plates of crust and upper mantle. • These plates can contain continental/oceanic crusts or both. • The lithosphere floats on the denser, more plastic athenosphere. Plate Movement Subduction • The “downward” plate movement into the mantle at a subduction zone to form a “trench.” • This movement causes earthquakes and when some molten material rises to the surface it may form volcanoes. • Ex. When oceanic crust subducts under continental crust the Andes mountains of South America were formed. Subduction • When two oceanic crusts collide one will subduct below the other and form trenches that lead to the formation of volcanic island chains called “island arcs.” • Ex: The Aleutian and Marianas Islands • When two continental plates collide both tend to float and buckle and neither is subducted. • This forms mountains ranges such as the Himalayas. • What properties of the continental crust make this happen? Shear Boundary • When two plates slide past each other causing extreme friction. • This stress can cause earthquakes. • Ex: A shear boundary formed the San Andreas Fault in California. Shear Boundary Convection • When heat from the mantle swirls the plates. • This used to be the main hypothesis of plate tectonics. • Now it is believed to be an auxiliary form of plate motion. Convection Geologic History Pangea • 200 million years ago Pangea was surrounded by Panthalassa. • What was Panthalassa? • Panthalassa is thought to be the ancestor to the Pacific ocean. • Tethys Sea- separated Eurasia from Africa (It is the modern Mediterranean Sea. • Sirius Borealis- is the modern Arctic Ocean. 180 million years ago. . . • A new rift formed between North America and combined the continents of South America and Africa. • This rift formed the early Mid-Atlantic Ridge and the North Atlantic Ocean. • Pangea broke into two continents: 1) Laurasia- North America and Eurasia 2) Gondwana- South America, Antarctica, India, and Australia. A rift split at this time forming the Indian Ocean. Marine Sediments • Lithogenous- sediments derived from physical and chemical weathering of rocks. • These are most common. • Ex: red clay on the open ocean floor. • Biogenous- made of skeletons, shells of marine organisms such as diatoms, radiolarians, foraminiferans, and coccolithophorids. • Composed of calcium carbonate- sometimes referred to as “calcareous ooze.” Diatoms Foraminiferans • Microfossils- microscopic preserved remains of marine organisms that give clues about the type of water the organism lived in such as whether it was cold or warm. Microfossils Radiometric Dating • The use of radioactive isotopes to determine the exact age of a fossil. • Also referred to as carbon dating. Climate and Changes in Sea Level • Earth’s climate fluctuated between warm interglacial periods and cold ice ages. • During ice ages the sea level falls because water is trapped as glaciers. Pleistocene Epoch • Two million years ago- was the last major period of glaciation. • Sea level has risen over the past 3,000 years due to greenhouse gases such as carbon dioxide and methane. • These gases are said to cause global warming therefore melting glaciers and subsequently causing a rise in sea level. Geology of the Sea Floor The sea floor is divided into two main regions: • Continental margin- are boundaries between continental and oceanic crust. • They consist of shallow, gently sloping continental shelf, a steeper continental slope, and a gently sloping continental rise. Continental shelf • Shallowest part of the continental margin. • Makes up only 8% of the ocean’s surface area but are the most biologically rich portion of the ocean. • When the sea level rises- submarine canyons have formed here. • The shelf ends at the “shelf break” where the slope gets steeper 120-400m. Continental Shelf Continental Slope • Begins at the shelf break and descends to deep sea floor. • Submarine canyons cause its depth to reach 3000-5000m. • The continental slope channels sediments to the sea floor. Continental Slope Continental Rise • When sediment from the submarine canyons accumulates and deep sea fans (deposits that are similar to a river delta) piles on the sea floor. Continental Rise Active and Passive Margins Active Margins • Continental margins with intense geological activity. • Earthquakes and volcanoes are characteristic here. • Have steep, rocky shorelines, narrow continental shelves, steep continental slopes, and lack a developed continental rise. Active Margin Passive Margin • Inactive geologically. • Flat, coastal plains with wide continental shelves and gradual slopes, leading to a thick continental rise. Passive Margins Deep Ocean Basins • Most of the deep sea floor lies at a depth of 3000-5000m (10,000-16,500 ft). • Abyssal plain- deep sea floor, rises at gentle slope toward mid-ocean ridge. • Relatively flat but can contain submarine channels, abyssal hills, plateaus, rises and other features. Abyssal Plain Seamounts • Comprised of volcanic islands and submarine volcanoes. • Ex: Guyots- flat-topped seamounts that are common in the Pacific. • These areas have a great diversity of marine life. Trenches • Areas where plates descend into mantle. • Are the deepest parts of the ocean. • The deepest is the Mariana Trench in the Western Pacific at 11, 022m (36,153ft). Mariana Trench Mid-Ocean Ridge and Hydrothermal Vents • The Mid-Ocean Ridge contains a gap called “the central rift valley.” • Formed when fractures and crevices let seawater seep downward where it is then heated to a very high temperature by the mantle. • This heated water is forced back through the crust via hydrothermal vents or deep sea hotsprings. Mid-Ocean Ridge Hydrothermal Vents Deep Sea Hot Springs • Water from hydrothermal vents can be warm, 10-20C (50-68F) or blistering 350C (660F)! • This hot water dissolves minerals such as sulfides. These minerals cool and form deposits around the vents. Black Smokers • One type of deposit. They look like a chimney. • Made of solidified minerals. • The smoke the boils from them is a dense cloud of mineral particles. Black Smokers • Deep Sea hot springs are of interest to geologists and biologists. • They are an abundant source of marine life. • Many marine organisms are found around these vents. Tube worms around a vent.