EESA06 textbook notes.doc

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Environmental Science
Lisa Tutty

Chapter 3 Earthquakes usually on mid ocean rides and continental edges. Divergent boundary: -two plates move apart and magma fills the space between the plates. Usually causes earthquakes. -mid ocean ridges are divergent plate boundaries where new oceanic lithosphere forms. -a narrow rift forms along the mid ocean ridge because of crust that slip down as spreading occurs. This movement causes small to moderate size earthquakes. -As the plates move apart, solid mantle in the asthenosphere rises toward the surface, which becomes magma and the magma eventually becomes part of oceanic lithosphere -stretching of the crust may split continents apart and cause lakes(red sea) to form, and with continual seafloor spreading, the lake becomes wider and eventually becomes an ocean, just like Atlantic ocean Convergent boundary: two plates move toward one another. One plate slides under the other. Usually causes volcanoes. Called subduction. Many major earthquakes occur in subduction zones. -can include two oceanic plates, oceanic/continental or 2 continental Ocean-ocean convergent -when plate subducts, temperature rises and magma erupts and forms island arcs which are volcanos. Ocean-continental convergent -oceanic plate subducts underneath continental. -volcanos form on the continental and uplifts the region producing high mountain range. Continental-continental convergent -since both continents are thick and have same density, they cannot be easily subducted. They stack up to each other and form enormous mountains. Transform boundary: two plates move past each other. Usually causes earthquakes. -transform faults combine with spreading centers to form zigzag pattern on seafloor. Magnetite: -at MOR, lava erupts to the surface and cools down, as rocks cool, the magnetic field is recorded by the iron rick mineral magnetite. Magnetic stripes tell us the polarity. Study has shown that earths magnetic poles has switched many times. -young crust near the ridge. Sediment should thicken outward from the ridge. Ships drill holes in seafloor to determine the geologic history. -hot spots form a series of volcanic islands, If the plate is moving beyond the hotspot. The older the volcano, the smaller they get because they subside. Chapter 5  Texture of Igneous Rocks • Texture refers to sizes, shapes and arrangements • underground= intrusive/ plutonic ; cools slowly; forms coarse crystals • at surface= extrinsic/volcanic; cools quickly to form fine crystals • classified by composition and texture. • Light colour minerals are felsic rocks and dark rocks are mafic or ultramafic • Felsic are abundant in quartz and feldspar while mafic is rich in magnesium and iron  Temperature and pressure inside Earth • Radioactive decay of elements • Temperature increases as we move deeper in the Earth • Relates to plate tectonics • Conduction: Heat transfer by direct contact • Radiant Heat transfer: heat radiates through the air. • Convection: Flow of liquid  How do rocks melt • Rise in temperature causes rocks to melt • Rise in pressure causes rocks to become solid • Adding water can lower the temperature at which the rock will melt  How do Igneous Rocks Form • First, the process of melting occurs 40-150km beneath the surface in deep crust or mantle. Magma rises up because it is less dense than rocks and may accumulate to form a magma chamber inside the crust, some magma may solidify at the chamber and never reach the surface. Some magma may crystalize in the magma chambers and get carried to the surface. When magma reaches the surface we call it lava and rocks that form on the surface are called extrusive rock. • Any igneous rock that solidify below surface is called intrusive rock. • Complete melting is rare; most rocks melt by partial melting • Felsic minerals melt at lower temp. than mafic, therefore partial melting produces magma more felsic than the source. Felsic is also less dense so it floats upward and mafic sink to bottom. • Since mafic magma is hotter, it can melt felsic walls and mix together called assimilation  How does magma move? • When rocks melt, the volume of the melt is greater than original; so it is less dense and will rise upward if overlying rocks let it through. • There are 4 constraints on how magma ascends: Magma Pressure, density, gas pressure and stress. Pressure pushes the magma into any available direction. Pressure decreases as magma rises. • Mafic magma may be denser than crustal rock so it stops to form a magma chamber. • If magma has low content of dissolved gas, it will not assist in magma upward. • Tectonic stress may trap the magma or help the magma. • Viscosity measures resistance to flow. Controlled by temperature, composition and crystal content. Low temperature is very viscous. Abundant silicate chains and crystals make magma very viscous since more things are in the way.  How does Magma Solidify • It must lose enough thermal heat to turn to solid. • Magma loses thermal energy when it reaches surface due to conduction and release of gases. Water in rocks near magma receives heat by conduction through walls then convection occurs in the water. • Mafic minerals crystalize before felsic. When it crystalizes, its components are extracted so it becomes more intermediate or felsic.  How does Magma Form Along Divergent Boundaries • Most magma form at mid ocean ridges. • As the plates separate, solid asthenosphere rises to fill the gap; pressure decreases and rocks partially melt. The melted magma is less dense so it rises up and accumulates in magma chambers. Magma rises and erupts within the rift to form new crust as it is cooled down. • Melting in the crust: mafic magma rises to melt the crust. The crust melts to produce felsic magma. The mafic magma then loses heat to the crust and solidifies. • Pillows, dikes are all mafic rocks formed at MOR. • Pillows are a series of overlapping mounds formed when lava erupts into water • Dikes are thin fine crystalline basalt cut across pillow basalts from below.  How does magma form along convergent boundaries • As subducted plate descends, pressure + temp. gradually increase -> new minerals formed from subducted plate due to metamorphism -> water bearing minerals break down-> added water lowers melting temp. of mantle above subducting plate-> melting occurs • If magma reaches crust, forms mountain belt or island arc. • At MOR, seawater flows into hot crust and forms water minerals; moves away from the ridge as time passes. When plate subducts, water trapped minerals cause melting. • Continental collisions may produce felsic magma but it usually does not reach surface because it has to pass through solid crust. -> not as many volcanos  Magma generated at Hot spots • Hot spots are site of intense magmatic activity • Rising plume of hot mantle. Magma from lithosphere and plume can reach surface causing large volcano (Hawaii) • Felsic magma tend to be explosive than mafic  How Do Magma Chambers Form and expressed • A reservoir that allows magma to enter or exit. • Magma may evolve, crystalize, mix, and melt walls-> change in composition. • Chambers form above hot spots, within MOR, above subduction zones usually • Solidified magma chamber is called pluton  Small Intrustions • Dike: sheetlike intrusion that cuts across layers; steep • Sill: intrusion parallel to layers in the host rocks • Laccolith: buldge shape Chapter 7  Clastics / characteristics of sed
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