* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Lecture 11 Review
Survey
Document related concepts
Earth's magnetic field wikipedia , lookup
Physical oceanography wikipedia , lookup
Geochemistry wikipedia , lookup
Global Energy and Water Cycle Experiment wikipedia , lookup
Geomorphology wikipedia , lookup
Geomagnetic reversal wikipedia , lookup
Spherical Earth wikipedia , lookup
History of Earth wikipedia , lookup
Magnetotellurics wikipedia , lookup
Age of the Earth wikipedia , lookup
History of geology wikipedia , lookup
Schiehallion experiment wikipedia , lookup
Surface wave inversion wikipedia , lookup
Large igneous province wikipedia , lookup
Plate tectonics wikipedia , lookup
History of geomagnetism wikipedia , lookup
Transcript
Lecture 11 Review 1) The Earth’s internal structure is inferred from seismic studies. There are two types of waves: P-waves are longitudinal, compression waves like sound waves. S-waves are transverse, shear wave like water waves. The wave velocity is proportional to the material properties through which it travels, elastic property β bulk mod ulus = = inertial property ρ density where density is a measure of how many atoms are packed in a given volume and bulk modulus measures the stiffness of the material lattice, i.e. ∆p change in pressure stress applied β= = = ∆ V V fractional compression observed strain wave velocity = Earth’s core was discovered in 1906 when observers found that they received modified seismic waves from earthquakes on the far side of the Earth. Waves from are altered by passing through the core, and certain types of waves from B do not even reach the observer. The conclusion is that rock layers at different depths have different properties. Seismic Data 2) Changes in velocity are an indicator of changes in density and phase. From these data one can infer the following features of the Earth. Core r = 0 to 3500 km, 16% of volume average density = 10 gm/cc average temperature = 2700 K Mantle r = 3500 to 6400 km, 82% of volume average density = 5 gm/cc average temperature = 2000 K Crust r = 10 to 100 km thick average density = 3 gm/cc where water is 1 gm/cc average temperature = 0 to 1500 K Chemical View - differentiation by density A) core of metals B) mantle of rocks C) crust of low density rocks (slag) - same as Moon Mechanical View - layering by rigidity A) core of metals and mantle rock B) plastic, molten slush, asthenosphere C) lithosphere - light crust + upper mantle rock floating on top of asthenosphere 3) Thermal History The Earth is a differentiated planet, as mentioned in the last lecture. Early heating allowed molten iron and nickel to fall to the center of the Earth to form a solid inner core surrounded by a semi-molten outer core. The thermal history of Earth is similar to the other terrestrial planets. For instance, Mars formed by accretion, then became hot enough for differentiation to occur. Cooling followed moderated by radiogenic cooling. Mars is smaller than the Earth and has lost more of its internal heat and its primary atmosphere. Earth is bigger than Mars and still maintains a hot interior, its lithosphere is thin, and volcanism is common. Plates of the lithosphere carry the continents with them. Plate tectonics may be unique to Earth 4) Surface The present surface configuration is relatively young. Two frames in a time-lapse movie of Earth’s surface, showing the dramatic movement of continental land masses due to plate tectonic motions in the last 4 percent of Earth’s history (170 million years). Geologists are not certain of the motions in earlier times, though there is evidence of previous collisions and splitting. A. 170 million years ago, Pangaea, composed of two major regions called Laurasia and Gondwanaland, was just beginning split apart. B. 70 million years ago, the Atlantic was widening and present land masses were beginning to take shape. India had not yet collided with Asia to make the Himalayas. Pangaea, composed of Laurasia and Gandwanaland, splits into major continents 170 million years ago The current surface is composed of seven major “plates” riding on a semi-plastic asthenosphere. Evidence for changes in the magnetic field of the Earth are found at the bottom of ocean floor rifts where outflowing magma solidifies with the current magnetic alignment frozen in place. The origin of a banded magnetic anomaly pattern across a ridge crest is suggested in this schematic drawing. Sea floor at the ridge crest is polarized in the same direction as the present (normal) magnetic field. The flanking negative anomalies are caused by reversed polarity of the remanent magnetization of sea floor, formerly solidified at the ridge crest when the field was reversed. 5) The surface changes with time due to weather. Convection currents flow when hot air flows to meet cold air. Differential velocities result from rotation of the Earth. This is called Coriolis drift and produces clockwise rotation for highpressure areas and counterclockwise rotation for low-pressure areas. 6) Few features of earlier meteor impact are left. Most have either been subsumed by plate movements or eroded by weather and water.