Transcript Document
The Geosphere (Solid Earth) Super continent “Pangea” 200 millions years ago (200 MYA) 145 MYA Northern hemisphere and Southern Hemisphere continents, Laurasia and Gondwanaland. Longitudinal waves P-waves. Like sound waves or acoustic wave. S-Waves or shear waves travel slower than P-waves. The motion Of the wave disturbance is perpendicular to the wave’s motion. If one know the speeds of the P-wave and S-wave (Vp and Vs) , then the difference between P-wave arrival (Tp) and S (Ts)wave arrival can be used to figure out the distance to the Earth quakes. D= Vp Tp D= Vs Ts D (TS TP ) 1 1 V V P S 3 stations need to locate the focus of the Earth quake. Know the difference between Epicenter and hypocenter. Where’s the epicenter? Two seismic stations is not enough. Three will do as long as they are not on the same line. Where’s the epicenter? Point A Two seismic stations is not enough. Three will do as long as they are not on the same line. Wave fronts are represented as rays Rays are drawn perpendicular to the wave fronts and show the direction of travel for the wave front. Refraction Typically in the earth as one goes deeper the wave velocity V increases so V1<V2 is A common refraction. However, when Oil, Natural gas, or water is embedded in the rock, especially sandstone formations, the wave Wave speed changes constantly with depth giving a curved wave trajectory. • P waves can travel through liquids and solids • S- waves (transverse waves) can only travel through solids. Seismic Tomography P-wave shadow zone at 4 and 8 o’clock reveal a liquid outer core. Credit: A. Dziewonski/IRIS Consortium http://www.tectonics.caltech.edu/outreach/animations/mase/mase_tomography.html The lithosphere is hard / brittle The asthenosphere can deform like stiff putty. Earth’s Radius 6400 km Depth to inner core 5000 km Depth to outer core 3000 km Depth to mantle 10 to 50 km (oceanic crust 10 km Continental crust 50 km) Oceanic crust is relatively young (80 million years) whereas continental crust can be billions of year old. Earth Radius 6400 km Depth to inner core 5000 km Depth to outer core 3000 km Depth to lower 1000 km Mantle There are approximately 12 lithospheric plates on Earth’s surface. Most scientists believe that convection within Earth’s lower and upper Mantle is the major driving force linked to plate motion. Convection is driven my temperature differences/density differences and gravity. Juan de Fuca plate. Spreading and subduction Mt St Helens is part of the Cascade Arc Volcanoes Hot spot volcanoes Hawaiian Islands Hot Spots and Mantle plumes reaching the surface. Hot spots tend to be relatively stationary. As an oceanic plate moves over a hot spot new islands can be formed. Most think that the hawiian islands are a result of motion over a hot spot. Plate motion at trenches can be up to 10 to 12 cm year. Continental crust is very old (typically over a billion years old) because it does not get recycled frequently. Oceanic crust is typically 100 million years old on average (fairly young). Oceanic crust is continually recycled.