And if California slides into the ocean, Like the mystics and statistics say it will, I predict this motel will be standing, Until I pay my bill Warren Zevon, Desperadoes Under the Eaves

Assignment: Reading: Explorations, pp. 161-166. plus check out the Union of Concerned Scientists' web pages on ozone depletion in our atmosphere. Sign up for a time slot for Observing Lab #2. Problem Set #5 is due Thursday, 6 October at 1:00 pm

In Class: Question to Ponder Which of the following has the greatest amount of energy? a) a single photon of visible light (wavelength ~ 500nm). b) a single photon of infrared light (wavelength ~ 1000nm). c) a single photon of ultraviolet light (wavelength ~ 200nm). Looking at the Earth as a Planet One we know best. Most interesting feature is that it supports life. What about it makes it conducive to life? What's wrong with the other planets? Are these special traits permanent, or will the Earth become (or can we make it by accident) hostile to life? Understanding the Earth's Interior Lots down there, We can't access it. Need to use remote sensing means Two important ways -- density and seismology. What We Can Learn from the Density of the Earth Density = mass/ volume, so we'll need mass and volume. Get mass from watching the speed and orbit size of a satellite (Moon or artificial). Get volume from V = 4/3 x pi x r3, where r is the radius of the Earth and we assume it's a sphere (close enough). Earth's average density is 5480 kg/m3. Compares with densities of other materials. Gold: 19300 kg/m3 Lead: 11400 kg/m3 Iron: 7900 kg/m3 Rock: 2500 kg/m3 Water: 1000 kg/m3 Ice: 920 kg/m3 Wood: 800 kg/m3 Good bet iron and rock are major constituents of the Earth. Even more, since we know that the Earth's surface is mainly rocky, with a rock-type density, the we can conclude that the deep interior must be even more dense (to make the average as big as 5480 kg/m3). Thus, we know that the core of the Earth must be iron-rich. And we know that the Earth is differentiated, i.e., the heavy stuff is at the center and the lighter stuff is on the surface. That means the Earth was once pretty molten (since stuff had to be able to move around to differentiate). The Earth must've had a hot period in its early history. Using Seismology to Map the Earth's Interior Earthquakes generate waves that travel through the Earth. "P" waves are compression waves, and travel through just about anything. "S" waves are more like "wiggles" on a string; they don't propagate through liquids. Types of waves detected at sites on the other side of the Earth from an earthquake tell us something about the material through which the waves passed. The absence of "S" waves in these kinds of experiments tells us that some part of the Earth's core is liquid. Why is the Earth's Center Hot? The fact that some of the Earth's interior is liquid implies that it's hot. Temperature at center > 5000K. Heat from formation of the Earth; impacts and the proto-Earth coalesced. Heat from radioactive decay. Convection and Plate Tectonics Hot in the center, cool on the outside --> recipe for convection. Hot stuff rises, cool stuff falls. Creates convective "rolls" -- plumes of hot upward-moving stuff surrounded by cooler descending material. Happens very slowly in the gummy sticky mantle material. Crustal material gets dragged along. The crust on top of upwelling mantle is stretched outward and broken. Our Earth's crust is broken into large plates floating on the mantle material. Plate Tectonics Plates move around because of convective flows in mantle. Places where plates separate -- "rift zones" -- new crust is created to fill the gap. Places where plates collide -- "crustal uplift" -- large mountain ranges. Places where one plate overrides another -- "subduction zones" -- plate on bottom is forced downward into the mantle, heated up and melted. Subducted material reappears on the surface in the form of volcanic outflows.