OC103 Lesson #4: Earth’s Structure The Interior of Earth To understand why the oceans are where they are, and even why we have oceans at all, we must start with the fundamental structure of Earth. Earth is like an egg, with concentric layers of the following: • Crust: hard outside layer, fairly thin compared to the rest, like the shell on an egg • Mantle: solid middle layer • Core: deep inside, includes a liquid outer core and a solid inner core So it looks something like this:This layered structure, and the different properties of each layer, explains the origins of some of the important features we see at the surface, such as continents, ocean basins, mountain ranges (both on land and in the oceans), island chains, and volcanoes. The map below shows the surface of Earth with the water removed from the oceans. The different colors in the oceans represent depths, from orange (shallowest, <200 m deep) to yellow to green to blue to purple (deepest, >6000 m deep). Notice that most of the ocean floor is very deep (>4000 m deep, shown in light-blue to purple), and that there is not much orange and yellow compared to how much blue there is. The shallowest (orange) areas are restricted to narrow bands around the continents, and the somewhat shallow (yellow) areas are mostly along some narrow mountain ranges out in the oceans. Why is so much of the oceans so deep? And why is so little of it shallow? And why is it either deep or shallow, with so little in between (notice how little of the map is green)? To answer these fundamental questions about the nature of the ocean floor we need to back up and discuss what Earth is made of, because that determines the shape of its surface, and thus the shape and depth of the oceans. The next few Lessons will describe the interior and exterior of Earth, and discuss how geologic forces shaped the exterior to make the map below look the way it does. The Shape and Depths of the OceansHow do we determine what's inside Earth? • Could we let Hollywood show us and watch "Journey to the Center of the Earth" or "The Core" and pretend they are documentaries? or • Be scientific about it and make some direct and indirect observations Direct Observations • Until we actually can "journey to the center of the Earth", direct observations are only possible for parts of Earth fairly close to the surface • What about drilling down into Earth to take samples from the interior? So far we have only been able to drill to ~15 km depth, not very deep at all compared to the 6400 km to the center of Earth. • There are a few places on Earth's surface where rocks have been uplifted from deep within Earth and are now exposed on the surface, but this rarely happens to rocks that were ever deeper than ~50 km deep, so also not very deep. • Certain types of molten volcanic material are occasionally erupted from unusually deep, as deep as ~200 km, but still not very deep compared to how far it is to the center of Earth.Indirect Observations of Earth’s Interior We can indirectly measure the physical state of Earth's interior using • Earthquake Waves: different types of earthquake or seismic waves travel through solids and liquids differently. • Primary waves ("p" waves): look like pressure or push-pull waves, and can travel through solids and liquids (see below left). Go to this link to see a demo: http://www.jclahr.com/science/earth_science/tabletop/pslnkmv.html • Secondary waves ("s" waves): look like sideways or shear waves, and can travel through solids, but NOT liquids, because if you shear a liquid sideways, energy dissipates as friction and the liquid does not bounce back (see below left, lower). Go to this link to see a demo: http://www.jclahr.com/science/earth_science/tabletop/sslnkmv.html • When a large earthquake (labeled as “Focus” on figure below right) sends seismic waves through Earth (with paths shown by curved lines on figure), instruments distributed around Earth pick up the "p" waves, but not all of the instruments pick up the "s" waves. The instruments on the opposite side of Earth from the earthquake do not receive the "s" waves (labeled as “S-wave shadow zone on figure), leading to the conclusion that earthquake waves that travel through the center of Earth must encounter a body of liquid that absorbs the "s" waves (shown as yellow area on figure). • Magnetic Field: measuring the strength and shape of the magnetic field surrounding Earth shows that there must be a large body of magnetic iron deep within Earth. p-wave: Like pushing on the end of a slinky. s-wave: Like nudging a slinky sideways.Indirect Observations: Gravity Field • Since gravitational attraction is related to the mass of an object (the more an object weighs the stronger its gravitational attraction), we can determine how much Earth "weighs" by measuring the strength of its gravity field. We can also measure the size of Earth, and by dividing its mass by its size we can calculate its average density. When we compare that density to the density we measure for rocks we find at the surface, it is clear that Earth is much "heavier" than it would be if it were made entirely of the rocks we find at the surface; the deep part of Earth must be much denser and heavier than the rocks we find at the surface. • We can calculate the approximate masses and densities of the parts we can't see by subtracting off the masses of parts we can see from the total. These observations and calculations give the following densities: o Crust: 2-3 gm/cm3; This one we can actually measure because we can sample pieces of the crust and measure their densities. These are the rocks that you would typically find at the surface, and would weigh about what you expect for a rock. Remember that density corresponds to how "heavy" an object feels for its size. o Mantle: 3-6 gm/cm3; We can measure this one on samples from the shallowest part of the mantle brought to the surface by some volcanoes, but we have to calculate the density of the deeper mantle. If you were to pick up a piece of rock from the mantle, it would feel like a rock, but be somewhat heavier than you expected because it is about twice as dense as a crustal rock. o Core: 11 gm/cm3; We do not have samples from Earth's core, so we have to calculate this one, although we can measure the density of certain meteorites that we believe are similar to the material in