Lecture notes Bill Engstrom: Instructor Earth s Interior GLG 101 Physical Geology

Transcription

1 Lecture notes Bill Engstrom: Instructor Earth s Interior GLG 101 Physical Geology We memorized the layers of the Earth early in the semester. So, how do we know what s beneath the Earth s surface. In the last section, we found out about seismology. Seismic measurements are one of the key ingredients to figuring out what the Earth is made of (not Swiss Cheese that s the moon [just kidding of course????]). There are also a number of other methods we use, including gravity and magnetic measurements, heat flow studies, the study of meteorites, and the study of material behavior. We have observed xenoliths in Basalt of upper Mantle material as evidence for the rock type characteristic of the upper Mantle.but how do we know what s below the upper Mantle? Probing Earth s Interior Most of our knowledge of Earth s interior comes from the study of earthquake waves. And, what we know is that: Travel times of P (compressional) and S (shear) waves through the Earth vary depending on the properties of the materials. Variations in the travel times correspond to changes in the materials encountered. The nature of seismic waves We also know that: Velocity depends on the density and elasticity of the intervening material. Within a given layer, the speed generally increases with depth due to pressure forming a more compact elastic material. Compressional waves (P waves) are able to propagate through liquids as well as solids. Shear waves (S waves) cannot travel through liquids. In all materials, P waves travel faster than do S waves. When seismic waves pass from one material to another, the path of the wave is refracted (bent). The bottom line for all of this is that. Waves travel at different speeds depending on the medium. In general, denser, more rigid material (crystalline rocks) transmit waves faster than lighter, less cohesive rocks.

2 P Waves & S waves We can represent seismic waves traveling through the Earth as wavefronts or ray paths perpendicular to wavefronts. We start with a simple assumption that the Earth is homogeneous (all the same material/rock type) In that case, we would expect the rays to travel at the same rate if the Earth is homogeneous. However, the arrival times of the waves are almost always too soon at farther seismic recording stations which indicates that that the waves must have travelled faster than predicted. And, we know that stations farther away receive waves from deeper within the Earth. So, the rocks that are deeper in the Earth are more rigid/dense than those at or near the surface. What this means The Earth is layered. Seismic Waves and Earth s Structure Conclusions from all of the evidence Abrupt changes in seismic wave velocities that occur at particular depths helped seismologists conclude that Earth must be composed of distinct shells. Layers are defined by composition. Because of density sorting during an early period of partial melting, Earth s interior is not homogeneous. Velocities and Discontinuities Seismic refraction and reflection of waves allow us to construct a picture of the velocities versus depth within the Earth. Velocities increase with depth with a few exceptions and the jumps are called discontinuities. Let s look at some of the major discontinuities (changes in velocity and material) Crust Upper Mantle Boundary = The Moho (aka the Mohorovicic Discontinuity) At this boundary there is a P wave velocity change consistent with a change of rock type from felsic/intermediate/mafic in the crusts to ultramafic in the Mantle. The samples we have of upper Mantle xenoliths of peridotite confirms this change. Boundaries in Upper Mantle LVZ (Low Velocity Zone) for P waves Asthenosphere both S and P waves transmitted so material must behave like solid in short time frames.

3 Large drop suggests material is not all that rigid. 400km and 670Km Discontinuities Sharp P wave velocity increases. Possibly due to phase change or chemical change at these levels. Phase change is simplest explanation under pressure atoms rearrange into denser more stable structure To help us figure out lies beneath surface, we also need to know how seismic waves behave when they encounter different types of rock and liquids. Seismic waves are refracted (Seismic Refraction) and reflected (Seismic Reflection). We ll look at how waves are bent (refracted) by and bounced off (reflected by) rock layers) In Seismic refraction Waves are bent by passage into a layer of different velocity. If the new layer is denser (faster velocity) the waves are bent more shallowly than entrance angle. If new layer is less dense (slower velocity) the waves are bent more steeply. The core mantle boundary (aka the Gutenburg Discontinuity). The outer core is a liquid. P waves are refracted at this boundary and S waves will not pass through because they cannot travel through liquids. Discovered in 1914 by Beno Gutenberg P waves are refracted. The P waves can travel through liquids, but velocities slowed down (waves refracted). Refraction makes the waves enter the core more steeply. Based on observation P waves die out at 105 degrees from the earthquake and reappear at about 140 degrees. This 35 degree wide belt is named the P wave shadow zone. No S waves observed beyond 103 degrees from epicenter (shadow zone) The fact that S waves do not travel through the core provides evidence for the existence of a liquid layer/core beneath the rocky mantle. In Seismic reflection Waves bounce off layers in the Earth Seismic reflection Other experiments with explosions indicated that some waves are not refracted but bounce right back they are reflected. This would be difficult to explain in a homogeneous Earth. The outer inner core boundary (aka the Lehman Discontinuity)

4 Sudden increase in seismic velocities. Slight discontinuity in P wave arrivals travelling through inner most part of core Chemical considerations dictate that extra pressure may allow liquid core to go to a solid at depth. Appears that there may be a solid inner core. In addition to the seismic evidence, we have evidence from differences that we see in gravity across the Earth. These differences from normal gravity are called gravity anomalies. Gravity is the name given to the attraction between masses. Gravity is a force. Observations of gravitational attraction of the Earth indicate that the Earth must have more mass than is visible from crustal rocks and those of the upper mantle. Average Earth Density is about 5.5 grams/cc. Average Crust & Upper mantle densities, however, are only about g/cc Calculations indicate that core must have density of about 10 g/cc consistent w/ iron/nickel What are gravity anomalies and how are they useful? Bodies of heavier rock change the usual value of the Earth's gravity field. Anomalies can be used to locate metal ore bodies or depth to crystalline rock in deep sedimentary basins. Positive anomalies produced by heavier rock beneath the surface (field strength is higher than normal) Negative anomalies produced by lighter rock beneath the surface produces negative gravity anomalies (field strength is less than normal). Examples: Negative anomalies typically occur over lighter, less dense material in salt domes, and over less dense material in sedimentary basins. Gravity anomalies and isostacy We would expect to see a positive gravity anomaly over mountain ranges due to the extra mass that we see. However, in most mountain belts there is no gravity anomaly. Less Dense material extends under mountain range. This is effect is called isostacy Lithosphere blocks of extra mass sink into the asthenosphere. Similar to icebergs in water.

5 Displacement of heavy mantle material away from the roots of mountains removes the positive gravity anomaly. Compensation of weight of lithosphere by displacement of asthenosphere is isostasy. As erosion removes mountains, the lithosphere rises isostatic rebound Isostacy and the Himalayan Mountains The stacking of Indian & Asian plates during continental collision sinks the lithosphere into asthenosphere = isostacy Other isostatic adjustments (examples) As mentioned above erosion can cause an isostatic rebound (raising of the boundary between the lithosphere and asthenosphere). During Earth s past, thick sheets of glacial ice have covered large portions of the Earth, depressing the boundary. As this ice melted, a rebound also occurred. Earth s magnetism is also used to provide evidence for what lies at depth. Shape of magnetic field The geometry of magnetic field is the same AS IF a bar magnet was centered in the Earth's core and aligned with the magnetic poles. Generation of the magnetic field It is too hot in the core for a bar magnet to exist However, the field can be generated by an electrical current moving around the Earth's spin axis in the core. Therefore, the Earth is actually more like an electromagnet vs. a bar magnet. Conclusion the Earth's core must be an electrically conducting fluid. Again, iron nickel would work Earth s magnetic field The requirements for the core to produce Earth s magnetic field are met in that it is made of material that conducts electricity and it is mobile. The inner core rotates faster than Earth s surface and the axis of rotation is offset about 10 degrees from Earth s poles. The field is produced by vigorous convection of liquid iron in the outer core

6 The magnetic field is essentially a geodynamo caused by spiraling columns of rising fluid. It is primarily dipolar. Patterns of convection change rapidly enough so that the magnetic field varies noticeably over our lifetimes. Does the magnetic field always stay the same? Not really.. The magnetic field randomly reverses. North and south poles swap direction. Reversal takes only a few thousand years. Dipolar field significantly decreases in strength. Reversals are important as the field creates a magnetosphere around Earth that protects our planet from solar winds. Magnetic field anomalies. Just like with gravity, there are differences in magnetism across the Earth. Bodies of magnetic rock change the usual value of the Earth's magnetic field. Magnetic anomalies can be used to locate metal ore bodies or depth to crystalline rock in deep sedimentary basins AND play an important role in the determinations of plate motions. More magnetic rock beneath the surface magnetized in the direction of the Earth's present magnetic field produces positive magnetic anomalies (field strength is higher than normal). Less magnetic rock beneath the surface OR rock magnetized opposite of the direction of the Earth's present magnetic field produces negative gravity anomalies (field strength is less than normal). More evidence of what lies within the Earth Meteorites Origin of meteorites Trajectories place most of them in asteroid belt between Jupiter & Mars. They are thought to represent either a broken up planet or original material of the solar system that never coalesced into a planet because of Jupiter's enormous gravity field.

7 Conclusions Earth interior is probably made up of similar stuff as found in meteorites. Age of meteorites gives ultimate age of Earth at 4.6 billion years (supported by moon rocks) Composition of meteorites Nickel iron. Stony much like Peridotite in composition Pallasites mixed 8/2011