* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download AIM: Introduce you to scientific study of the world`s oceans and seas
Survey
Document related concepts
Post-glacial rebound wikipedia , lookup
Abyssal plain wikipedia , lookup
Spherical Earth wikipedia , lookup
History of geomagnetism wikipedia , lookup
Geochemistry wikipedia , lookup
Algoman orogeny wikipedia , lookup
Tectonic–climatic interaction wikipedia , lookup
Large igneous province wikipedia , lookup
Schiehallion experiment wikipedia , lookup
History of Earth wikipedia , lookup
Age of the Earth wikipedia , lookup
Plate tectonics wikipedia , lookup
History of geodesy wikipedia , lookup
Transcript
Mid-ocean ridges (MORs) •Stand 2.5-4 km above abyssal plains, and may be ~1000 km across •Axial valleys are 600 m – 2 km lower than ridge mountains •Axial valley/ridge mountains abut oceanic fracture zones •Numerous volcanoes have quiescent eruptions •Axial valleys & ridge mountains composed of basalts in distinctive pillow forms MORs continued •Near axial trough, sediments and sedimentary rocks are thin or absent •Sediment thickness increases with distance from axial trough, but never exceeds 1.3 km •Age of oldest sediments increases with distance from axial trough •Sedimentary rocks are cut by faults, but MORs are not like continental mountains Oceanic fracture zones (FZ’s) •Rectilinear (straight), often for 1000’s of km across ocean basin •Narrow, usually <100 km and may be <10 km wide •Vertical relief across a FZ may be several km •Axial valleys offest as much as 1000’s of km across FZ’s •When FZ’s extend into abyssal plains, they are usually covered sedimentary rock Earthquakes in MOR-FZ systems •Earthquakes occur in axial valleys, especially on faults between valleys and adjacent ridge mountains •Earthquakes occur along fracture zones between offset axial valley segments MOR FZ Deep-ocean trenches (DOT’s) •Arcuate •>6 km and often ~10 km deep •Narrow, typically ~5 km wide at base •Have a asymmetric V shape, with a gentle seaward slope, narrow base, and steeper slope rising to volcanic islands or continental volcanoes •Volcanoes may explosive, i.e. Mt. St. Helens or Mt. Pinatubo Earthquakes near DOTs Earthquakes occur along an inclined, planar zone that extends into the earth beneath the landward side of the DOT Dipping zone of seismic activity = Benioff-Wadati Zone Why do continents and oceans differ so much? • To find the answer, need to consider the character of earth’s interior • In a first pass at examining Earth’s interior, we note: –Earth’s interior is warmer than its exterior –Rock at earth’s interior of earth is denser than rock at surface Interior of earth is warmer than the exterior •In boreholes and tunnels, temperature rises with depth •Heat flows steadily from earth’s interior •Molten rock originates inside earth - how? Rocks inside earth are denser than rock at surface •Density of typical crustal rock is low, 2.7 g/cm3 •Average density of earth is moderate to high, 5.5 g/cm3 •Interior of earth must contain high density material - estimated to be 10-11 g/cm3 •Measurements of Earth’s moment of inertia confirm that earth’s mass is concentrated near its center These two observations led earth scientists to the hypothesis of isostacy • Isostacy = floating balance, in which rigid crust floats on an underlying mantle • Mantle is more deformable and more dense than crust • Crust floats on mantle by – Airy mechanism - all crust has similar density. Where elevation is higher, crust is thicker – Pratt mechanism - crustal density differs. Where elevation is higher, crust is less dense What causes differences in average elevation of continents and oceans? • Crustal thickness? – Thickness of continental crust = 25-40 km – Thickness of oceanic crust = 5-7 km • Crustal density? – At surface, continental crustal rocks have low density (often about 2.6-2.7 g/cm3), but lower continental crust is more dense – Aggregate density of continental crust is 2.8 g/cm3 – Aggregate density of oceaic crust is 2.9 g/cm3 • Variation in crustal thickness is more significant factor in explaining two elevations Earth is opaque. How can I state confidently what is the character of rock at depth? How can one determine more information about earth’s crust and its relationship to earth’s interior? Earthquakes and seismology