Structure of the Earth (Why?) Recall Hypsometric Curve? Continental lithosphere is very different from oceanic lithosphere. To understand this, we need to know more about the structure & composition of Earth. Earthquake (seismic) waves are the key to understanding earth s structure. Seismic wave paths traveling through Earth provide important information about Earths structure They allow us to image and locate the boundaries between Earths many layers (e.g., the core-mantle boundary) Changes in wave paths allow indicate the changes in composition and rheology of the Earth (e.g., liquid outer Fe-core) See Chapter 5 and 6 of your textbook. A detailed reading guide is given for you on Compass. 1
From your reading, you will see That the science leading to our understanding of Earth s structure is interesting and complicated. It involves Snell s Law and the physics of wave propagation, wave theory and spherical geometry combined with experimental highpressure mineral physics. We will not cover the geophysics of seismic waves and how we know Earth s internal structure. I ve included it in your assigned readings to satisfy your curiosity. If you have any questions over this material, don t hesitate to see me What you need to know in order to understand the oceans Earth can be divided on the basis of its composition or its rheology Rheology = behavior in response to stress (e.g. rigid, plastic, fluid) Earth s composition layers Crust, Mantle and Core Earth s rheologic layers Lithosphere, Asthenosphere, Mesosphere, Liquid Outer Core, Solid Inner Core The details of these two ways of seeing Earth s structure follow Earth s composition layers Composition layers on left side of figure: Oceanic Crust: 0-10 km thick Si - Mg - Fe-rich (dense) Continental Crust: 35-75 km thick Si - Al - Ca - Mg - Fe (not dense) Mantle: ~2900 km thick Si - Mg - Fe Very rich in Mg-Fe (very dense) Extends from base of crust (Moho) to the top of the core (CMB) Core: ~3500 km thick Fe (Ni-S) - rich (supper dense) 2
Earth s Rheologic layers (on the right side of the figure) Lithosphere: 100-150 km thick Includes crust and rigid upper mantle This is the rigid plate of plate tectonics Base is defined by 1280ºC boundary Asthenosphere: 100/150 to 700 km depth This is a plastic region that the lithospheric plates ride/float on Begins at ~100 km depth in oceans, 150 km depth under the continents Mesosphere extends from 700 to ~2900 km depth Often called the Lower Mantle At 700 km depth the pressure is so great that the minerals are forced to compress their structure (thus change to denser minerals) This is a plastic layer denser than the asthenosphere Liquid Outer Core: 2900 to 5155 km Made of dense, convecting liquid iron Generates Earth s magnetic field Solid Inner Core 5155 to 6370 km Solid, crystalline Fe The lithosphere includes crust and part of the mantle. A. True B. False The Asthenosphere is composed entirely of mantle. A. True B. False 3
Now we can begin to understand the hypsometric curve Why is the ocean floor deep compared to the continental surfaces? In other words, why do continents (e.g., Antarctica), rise above the ocean floor? To answer this question, we need to understand a few things: If the asthenosphere is a plastic, then the rigid lithosphere floats on it. A perfect analogy to this concept is a block floating on putty, or Another analogy: blocks floating in water Archimedes - a Greek philosopher played around with this Buoyancy Model What will happen if the thickness of the block is increased? A. The base of the block will sink deeper. B. The top of the block will rise higher C. Both A and B D. Nothing will happen, the position of the block will not change. Z U Z A Z B model 4
concept of isostacy Continental lithosphere sinks deeper and rides higher than oceanic lithosphere due to it s thickness and lower density. In this picture the thick board represents the continental lithosphere. The thinner boards represent the oceanic lithosphere. Note, if you remove the top of a continent, it bobs up in such a way that the root is shallower, and the surface is still above the oceanic lithosphere. Isostacy: Examples of Isostacy: Thick ice sinks deeper and rides higher than thin ice 5
More Isostacy: animation Now we can understand the hypsometric curve Ocean floor is deeper than continents because Ocean lithosphere is thinner than continental lithosphere Oceanic lithosphere is denser than continental lithosphere Thus, due to relative buoyancy the oceanic lithosphere does not ride as high on the asthenosphere compared to the continental lithosphere. This is the concept of isostacy Continental lithosphere sinks deeper into the asthenosphere and rides higher than oceanic lithosphere due to it s thickness and lower density Now we understand this graph and image! 6
Review Questions What does the hypsometric curve show you? What are the composition layers of the Earth? What is the radius of the Earth? How is the asthenosphere different from the lithosphere? How is the mesosphere different from (a) the asthenosphere and (b) the lithosphere? Why are oceans deep compared to continents? 7