Lecture 2: Earth Systems Searching for Life on Other Planets The Gaia Hypothesis (from last week) slides The water cycle plus/minus carbon cycle box Models The Tectonic Cycle To understand complicated systems, like the carbon cycle, we try to simplify them down to their most basic elements and study their rates of transfer and abudnances in various parts of the system. This is called a "box model". To understand this method, we'll investigate one that is familiar to us: the Hydrologic cycle. Hydrologic System Draw standard diagram powered by sunlight powered by gravity Energy of water flowing from mountains to sea is very important geological force This is a model. Models are very important in geology and the sciences Used to predict and understand Can be thought of as a box model The Box Model -used to study inputs, outputs, sizes, and residence times of interconnected systems M= the stock -- how much of the thing is in the reservoir, measured in mass or volume T = the residence time -- the average time spent in the system by the material of interest, measured in units of time Fin or Fout= the flows in or out of the reservoir of interest measured in mass or volume/unit time Basic equation -- F=M/T The School example The school (UCSC) contains 10,000 students. Each graduating class has 2,500 students/year. Assumption of steady state - Fin=Fout draw box with arrows Q: what is the average residence time of the students? i.e., how long does the average student spend at UCSC. Note the importance of averages -- it is the average student we are concerned with, not one specific student. If a student has one foot in a bucket of ice and one foot in a fire, on average, heÕs comfortable. T=M/F=10,000 students/2,500 students/year = 4 years What if we add 500 transfers -- both in and out - the school canÕt grow add lines now, T=M/F=10,000/3000=3.333 years The trick in box models is to be able to add up all of the various inputs and outputs to each reservior. Back to the hydrologic system What is the residence time of H2O in the atmosphere? What are inputs and outputs? inputs = evap from sea 337,000km3/yr + evpotrans from land 63,000 km3/yr = 400,000 km3/yr outputs = precip at sea 299,000 km3/yr + precip on land 101,000 km3/yr = 400,000 km3/yr Steady State!! M= 0.0125EE6 km3 T=M/F=12,500/400,000 = 0.03125 years (0.03125 years)(365days/year)=11.4 days use of the model to investigate renewable vs non-renewable reseources Is surface water a renewable resource? F=101,000 km3/yr M=0.125EE6 km3 T=M/F=1.24 years -- renewable resource! Now you should be able to do homework #1. Plate Tectonics Outer portion of the lithosphere is broken into pieces, called plates, that move independently of each other. -Earthquake belts are plate boundaries. The interior of the Earth is still hot and convection moves this heat through the mantle towards the surface. Convection is related to the movements of the plates, but the exact relationships are not clear. --Mantle convection gets the mantle heat out. -- heat is from decay of radioactive elements K, U, Th --Same as large volcano on Mars -- serves same purpose --Lack of meteor impacts on Earth due to constant recycling of Earth Surface, also erosion and biologic activity tend to obscure them. Ocean Basin Topography Mid-Ocean Ridges = spreading centers where new lithosphere and oceanic crust is formed and plates move away from each other. Trenches = long, narrow, deep basins that form where two plates converge and one plate subducts beneath the other. Plate Motions Wegener story. -- example of scientific method Continental drift first proposed based on obvious convergence of coastlines and rocktypes on either side of Atlantic. Regected because of lack of a mechanism 1960's observations included weak asthenosphere, mid-ocean ridges Deduced that oceanic lithosphere and continental lithosphere could be conected and put them together to formulate plate-tectonics. Motions deduced by 1)magnetic properties of rocks 2) satelite and laser measurements velocity = 1-10 cm/y = 10-100 km/my = about as fast as our fingernails grow. Plate Margins 1) Divergent Margins Spreading ridges where two plates move away from each other. --Basaltic magmatism forms new oceanic crust. This is a passive process! Weak asthenosphere moves up to fill space formed by diverging plates. --Asthenosphere cools to form new mantle lithosphere 2) Transform Faults form where spreading centers are offset. draw diagram and point out plate motions. --San Andreas Fault may be example, but term usually reserved for features in the ocean basins. 3)Subduction Zones form where plates converge and one plate subducts beneath the other. --Subduction Zones a) trenches b) chain of volcanoes, called volcanic arcs, on the over-riding plate. Three Driving mechanisms of the Earth 1) Sun 2)Gravity 3) Heat from Earth -- Tectonics The Rock Cycle The Three Rock Families 1) Igneous -- rocks fromed from magmas. All volcanic rock and rock that formed from magma that cooled slowly beneath the ground. 2)Sedimentary -- rocks fromed from particles transported by water, wind, or ice. Deposited sediments eventually become sedimentary rocks. Rocks formed through chemical precipitation from waters are also sedimentary. 3)Metamorphic -- rocks that have changed as a result of high T and/or P. The Rock Cycle [Draw diagram] Start at spreading center; go to subduction zone. Combine Rock cycle and water cycle [Draw diagram] Sedimentary part is driven by the water cycle (solar-driven) igneous and metamorphism are driven by tectonics, or heat energy from the mantle