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Transcript
6 & 7-1 Geologic Time The Earth is very old. Current estimates place its age at 4.6 billion years. Difficult to fathom. All of human history (~5000 years) is but 0.0001% of this age. Condense all of Earth history into one year and human history would occupy the last 30 seconds. While difficult, it is important to try and keep geologic time in perspective. Why do we think the Earth is 4.6 billion years old? How do we measure such large periods of time? How can we tell the ages of various landforms? This is what we will discuss the next couple of days. Two types of dating are used: Relative dating—here we try to determine which of two objects is younger, which older. However, we do not attempt to determine the actual ages. In the following chapters (and some we have seen) we will see what geologic forces can do over such immense periods of time. Absolute dating—here we try to determine the actual age of a feature. Relative Dating 2. Principle of Original Horizontality We can attempt to determine the relative ages of features by making use of some common sense assumptions and principles. When we see sedimentary layers being layed down today they are always at or nearly horizontal. Thus if a layer is tilted what must have happened and when? 1. Principle of Uniformitarianism 3. Principle of Superposition “The present is the key to the past” In general, new sedimentary layers are deposited on top of preexisting layers. We assume that processes which occur today also occured in the past. Where would the youngest layers be seen? Older? Physical laws of the Universe do not change in time. But what if a layer has been deformed or overturned? 6 & 7-2 4. Principle of Cross-Cutting Relationships 6. Principle of Faunal Succession Sometimes igneous rocks intrude into country rock or a fault cuts through various rock layers. Sometimes rocks contain fossils. Which is older, the igneous rock (or fault) or the country rock? Typically, organisms only appear within a certain time frame on Earth. The presence of a fossil constrains the age of the rock to the period when the organism existed. 5. Principle of Inclusions Sometimes rocks contain inclusions within themselves. Which is older, the inclusion or rock layer? Organisms which make only a brief appearance are particularly useful—if present they can tie the age down well (index fossils). Thus, the types of fossils seen can be used to gauge the age of a rock. (And vice-versa, using our previous rules, paleontologists use rocks containing fossils to determine the relative ages of fossils.) With all these tools we can use to date features it should be easy, right? Now suppose the environment changes to one favoring erosion over deposition— Alas, mother nature throws a few curves our way. Erosion removes some of the upper (younger) layers. Not all (or most) sedimentary rock layers present a nice clean history. Then the environment changes again to one favoring more deposition. What would we see in this case? Consider the following sequence of events: Material is initially deposited in a region. As material is deposited the layers are built up nicely, young on top, older underneath. When this occurs we have an unconformity. Unfortunately, this sequence of events is more the rule than the exception. Several types of unconformities exist. 6 & 7-3 Nonconformity Disconformity When sedimentary layers are observed overlying an unlayered body of plutonic igneous rock or metamorphic rock. In a disconformity, the layers are still horizontal but a definite gap exists in the record. Intrusive igneous rock and metamorphic rocks do not get produced on the Earth’s surface. ==> Must have been some erosion exposing the rock before the new deposition took place. Angular unconformity Sometimes sedimentary layers are layed on top of tilted or deformed sedimentary layers. Perhaps the fossils contained in adjacent layers suggest quite different ages. Sometimes evidence of erosion and weathering can be seen in the underlying layer. Unfortunately, virtually all outcrops contain unconformities. Most contain information on only a small fraction the Earth’s history. What must the sequence of events have been in this case? Correlation Absolute Dating Techniques: If most of the information is missing in a given outcrop, how do we try to get a coherent history of a region? Radioactive dating The main technique used to determine the absolute age of features makes use of radioactive elements. Most atoms are stable. How do we relate one region to another? i.e how can we tell which layers were formed at the same time? However, a few are not stable but decay in time into different atoms by emitting various particles. These atoms are said to be radioactive. The initial atom is known as the parent isotope while the decay product is known as the daughter isotope. How can we use radioactive elements to date objects? 6 & 7-4 Radioactive elements decay spontaneously. However, when a number of atoms are examined a statistical rate of decay can be determined. The time it takes for half of the atoms to decay is known as the isotopes half-life. Occurring in the nucleus, the half-life is essentially independent of the environment. Because of this radioactivity from radioactive elements can be used to date objects. Example: Potassium – Argon Dating One radioactive isotope often used in dating rocks is Potassium 40. Potassium 40 decays into Argon 40 via electron capture: K40 + e– —> Ar40 To see how let us consider an example... The half-life for this decay is 1.3 billion years. How might we use this decay to measure the age of a rock? Difficulties with radiometric dating: 1. Age of rock is much different than 1/2 life. Suppose a rock is much younger than the 1/2 life of a radioactive element. How many daughter nuclei will there be? e.g., suppose the 1/2 life is a billion years but a rock is only 100,000 years old (1/10,000 of a 1/2 life). Then we would have 1 daughter isotope for every 14,427 parent isotopes. 6 & 7-5 What if the age is much older than a 1/2-life? 3. Atoms may move into or out of rocks. This typically occurs at about 12 1/2-lives or so. For example, if an isotope decays into a gaseous element it may escape from the rock. Thus, a radioactive isotope is most useful for determining ages near its 1/2 life. e.g.: it can be a problem with K->Ar dating 2. When does the radiometric “clock” start? How would the age determination be affected if Argon had excaped from a rock? Radioactive dating techniques are most useful with igneous rocks. Their formation resets the radiometric “clock.” Clock may not be reset by formation of sedimentary or metamorphic rocks. 4. Contamination to begin with, what if a rock began with some daughter isotopes? Major Isotope Systems Used in Dating: An example where this might occur is the Rubidium–Strontium decay system. 1. Rubidium-Strontium (Rb87 -> Sr87) What would happen to the age determination if some daughter isotopes were present and this was not accounted for? 1/2-life: 47 billion years—useful for dating old rocks (10 million – 4.6 billion years) Strontium is a solid, thus not likely to escape. Occurs in potassium rich rocks—useful check on potassium ages. How can one tell what the initial concentration was? 2. Uranium, thorium to lead Long 1/2-lives (713 million to 14 billion years)—date old rocks (10 million years to 4.6 billion years). 6 & 7-6 3. Potassium-Argon (K40 -> Ar40) Carbon-14 Dating (C14 -> N14): 1/2 life of 1.3 billion years—useful for dating rocks of 100,000 years to 4.6 billion years. 1/2 life of 5730 years—can be used to date young objects (100 to 70,000 years old). Potassium is abundant making this system quite useful. Often used to date artifacts. Argon is a noble gas which does not normally bond with other atoms—rarely seen in a rock except as a decay product. However, this is also a bane—gas can easily escape from a rock. One must be careful especially with old, severely weathered, or metamorphic rocks. If carbon-14 has a such a short 1/2-life, why is it still around? Carbon-14 is continuously created by cosmic rays impacting atoms in the atmosphere. As long as an organism lives it continually recycles its carbon. When it dies it no longer does and the Carbon-14 clock starts to tick. Note that carbon-14 dating assumes that the rate of production of carbon-14 by cosmic rays has been constant. This may not be the case! Certainly isn’t today—atomic bomb tests have substantially increased the amount of carbon-14 seen currently. Other Absolute Dating Techniques While radiometric dating is the most widely used technique for absolute dating, other techniques are available. Fission-track dating—the decay of isotopes create high energy particles ==> upon leaving a rock tend to leave a path of destruction behind. The more fission tracks seen, the older the rock. 6 & 7-7 Dendrochronology—due to the variations between winter and summer, trees create alternating dark and light rings, one set per year. Ice core samples Similar to varves, glacial ice shows a seasonal cycle. These can be used to date trees and events during a tree’s lifetime. Can be used to date climate variations on Earth. Varve chronology—inflow into lakes may also follow a seasonal cycle leading to sets of sediments layed down each year. Lichenometry—lichen tends to grow on rocks at a relatively constant rate. (Though it depends on the rock and climate). By counting the varves we can determine the timing of various events (e.g. the formation of the lake, times of drought...) Can be used to date relatively young rocks (~9000years). The larger the lichen colony the older the rock. Surface Exposure Dating Sometimes we are not only interested in the age of a rock but how long it has been in its current environment. For example: How long has a rock been on the surface of the planet? Cosmis radiation is constantly bombarding the surface of the planet. This radiation can create new isotopes not normally seen on Earth (cosmogenic isotopes). It can also leave tracks of destruction (much like fission tracks). A technique known as surface exposure dating can be used to determine this: Such particles can only penetrate the near surface (upper meter or two or so). Thus the abundance of these cosmogenic elements and/or tracks is an indication of how long the rock has been within a meter or so of the surface. 6 & 7-8 Geologic Time Scale The largest blocks of time are the eons. Geologic history is organized into different blocks of time. The earliest eon is the hadean which extends from the planets formation to ~3.8 billion years ago. The blocks of time are divided on the basis of major events which have occured in the past. Period before origin of life—very few rocks are seen at the surface from this age. The major divisions are split on the basis of biological events (e.g. the appearance of multicelled organisms). The archean marks the first appearance of life and extends to about 2.5 billion years ago. However nonbiologic events do divide some of the finer time periods. The proterozoic marked the beginning of multicelled organisms and runs to ~545million years ago. Together, the hadean, archean, and proterozoic are sometimes referred to as the precambrian. The phanerozoic (visible life) eon marks a great diversification in life and runs to the present. The eons are further subdivided into eras. The phanerozoic is split into 3 eras: The paleozoic (ancient life) 245 – 545 million years ago. The mesozoic (middle life) 65 – 245 million years ago. The cenozoic (recent life) 65 million years ago to the present. The age of the Earth I’ve said the age of the Earth is 4.6 billion years. How do we determine this age? Oldest rocks on Earth are dated at 3.96 billion years by radiometric dating techniques. But the Earth exhibits active geology—thus we believe the oldest rocks have been destroyed. Lunar rocks have been dated at ~4.55 billion years Meteorites give a similar age. Eras are then further subdivided into periods. Astronomy suggests the sun is about this age. Periods are divided into epochs. 6 & 7-9 Earthquakes Unfortunately, faults blocks do not slide smoothly by each other. Friction between the blocks will lock them into place for a time. Rocks on either side of the fault continue to move and deform elastically. Even if there isn’t a fault initially in place one can still get an earthquake: Stress on rock may become large enough to fracture it. Rock suddenly loses strength. ==> slippage along new fault. As they deform they build up strain energy. Stress along the fault builds. Eventually, the stress along the fault becomes large enough to overcome the friction. Fault slips ==> Earthquake! Measuring the Strength of Earthquakes: Richter Scale In terms of the energy released by an earthquake, the difference is even more severe. When earthquakes are reported, one will often be given the earthquake’s “magnitude.” A gain of one in magnitude indicates a 33× increase in energy released. The Richter scale is based on the amount a seismograph needle will be deflected and is logarithmic: A 2 magnitude earthquake makes 10 times the displacement of a magnitude 1 earthquake. A magnitude 3 ten times that of a magnitude 2 (100× that of a magnitude 1) A magnitude 4 is ...? e.g. A magnitude 2 earthquake releases 33× the energy of a magnitude 1 earthquake. Magnitude 3 earthquake releases 33× the energy (or 33×33 ≅ 1100× that of a magnitude 1). And so on... 6 & 7-10 Relieving Stress for the “big one” The Moment-Magnitude Scale “Maybe this will relieve stress for the big one” While the Richter Scale is often cited it does have limitations: An oft heard refrain when spoken about a moderate earthquake. 1. It was calibrated using crustal rocks in California and thus is less accurate elsewhere. Is it true? 2. While theoretically the scale doesn’t have an upper limit, it cannot effectively measure earthquakes of more than magnitude 7.0 or so. Suppose we look at a magnitude 6 earthquake. Compare this to the big one: 8 magnitude. How many “small” magnitude 6 earthquakes would need to occur to releasr the same energy as one magnitude 8 earthquake? 3. It is based on the deflection of a needle on a Wood-Anderson seismograph—more modern seismographs are now available. This has led to the adoption of another scale: The Moment-Magnitude Scale Seismic Waves The energy released by an earthquake generally depends on: Before an earthquake occurs, the rocks are stressed. Those nearest to the fault are pulling on those a bit further out. These then pull on those still further out and so on. When the fault goes, stress is released all along the line. Rocks fall back, then rebound pushing/pulling on rocks further away causing them to react. In this way the energy from the earthquake is propagated away from the earthquake site. The motion that occurs as rocks move in reaction to the earthquake are called seismic waves. 6 & 7-11 Body Waves Body waves are waves that are transmitted through the interior of the Earth. There are two varieties: The motion of particles in a P-wave are in the direction of travel of the wave. Particles are continually compressed then extended then compressed... Because of this P-waves are sometimes called pushpull waves. P-Waves P (or primary)-waves are the fastest travelling body wave (~7km/s). ==> they are the first to arrive at a location. S-waves Surface Waves The second type of body wave is called an S (or secondary) wave. Surface waves travel on or near the Earth’s surface. Similar to water waves travelling on the surface. S-waves travel more slowly than P-waves (3.5km/s). Hence they arrive later. As with water waves, surface seismic waves die out quickly with depth. Surface waves are slower than P- or S-waves. Unlike P-waves, in S-waves the particles move perpendicular to the direction of travel of the wave. However, they tend to result in larger motions of the ground. S-waves are also sometimes called shear waves. Further, because they are slower, they take longer to pass a given location. For these two reasons surface waves tend to be more destructive than body waves. 6 & 7-12 Where do Earthquakes Occur? Around the pacific we have transform boundaries and convergent boundaries leading to widespread seismic activity. Earthquakes do not occur at random locations. Most (~80%) earthquakes occur in a ring around the Pacific plate. At the midoceanic ridge we see mostly normal faulting. All evidence of plate tectonics. Another group occurs in the Mediterranean–Himalayan region. A final group of earthquakes occurs along the midoceanic ridge. Each of these zones indicate a region on plate boundaries. Depth of Earthquakes Most earthquakes are shallow (within 70km of the Earth’s surface). Though some (~15%) occur at greater depths. Coping with Earthquakes Earthquakes are a fact of life, or are they? People have proposed setting earthquakes off artificially before they can build up large energies. All large earthquakes occur near the surface. For example, by pumping water into the ground. Deepest earthquakes occur at ~700km depth—occur at subduction zone boundaries within the subducting plate. Given our current knowledge, such an effort might be foolhardy... Why? What if we set off the “big one” by accident? Think of the lawsuit! 6 & 7-13 Thus, we are not likely to control earthquakes in this way in the near future, but maybe someday. In the mean time, what can we do to minimize their destructive effects? Earthquake Prediction We can then be careful when building in those areas: Strict building codes. Avoid placing critical facilities in such areas (e.g. nuclear power plants). Unfortunately, unlike volcanoes, geologists have not been very successful in predicting when earthquakes will occur. While we cannot predict earthquakes in the shortterm we can make long term predictions. How? “Seismic Zoning” Interior of the Earth: Seismology Some types of land are safer than others. Most of what we know about the Earth’s interior comes from seismology—the study of how seismic waves propogate within the Earth. Loosely consolidated sediment is not a good place to build. Such sediments may suffer liquefaction during an earthquake: Loose sediments also tend to focus seismic waves. Seismic waves can be used to “sound out” the Earth’s interior. Much like how sonar is used to look for underwater objects (submarines) or examine the ocean bottom. The velocity of seismic waves depends on the physical properties of the rocks they are moving through. Strength of rocks: Solid bedrock is a better place to build. Density: 6 & 7-14 Reflection and Refraction of Waves When a seismic wave reaches a boundary between regions of significantly different properties it will be reflected and refracted. As a familiar example consider the reflection and refraction of light waves. Same holds for seismic waves. Crust Velocity of seismic waves is seen to increase significantly through the crust. What does this suggest about how the properties of rocks vary with depth in the crust? Crust-Mantle boundary Seismic waves are reflected and refracted from a zone at some depth below the surface. What does this suggest? Mantle As the temperature of a rock approaches the melting point what happens to its strength? The mantle is the largest segment within the Earth accounting for 80% of the Earth’s volume. As one continues into the surface, seismic wave velocities continue to increase. However, at about 100 – 350 km depth the velocities of seismic waves are seen to decrease. What does this suggest about the strength of rocks at this depth? Why does this occur? What happens to the melting point as the pressure increases? 6 & 7-15 Below the asthenosphere the pressure again dominates, and the rocks’ temperatures become less than the melting point. What does this suggest about the rigidity of rocks here and thus the velocity of seismic waves? Lower Mantle The lower mantle extends from a depth of 700km to 2900km. Compression within this region increases the density of rocks from ~4g/cm3 to 5.5g/cm3. How would you expect this to affect the seismic wave velocities? Rock rigidity also increased by phase transitions which occur at depths of 400 – 700 km depth. Yet, the opposite is seen! What does this indicate about the rocks? These phase transitions mark the boundary between the upper and lower mantle. Core-Mantle Boundary Indicates a region of substantially reduced seismic wave velocities. At a depth of 2900km we see evidence of the reflection and refraction of seismic waves indicating another major boundary. In fact, S-waves can’t go through this region at all. Liquids can’t support shearing motion of S-waves. This boundary has major effects on the seismic waves we see. ==> suggests the outer core is liquid! P-waves are not seen at angles between 103° and 143° from an earthquake. Obviously the region also has much lower rigidity ==> reduced P-wave velocities. P-wave shadow zone. Analysis of seismic data also indicate this region to be one of substantially increased density. S-waves are not seen at all beyond 103° from an earthquake. How do we explain this? ==> Metal, mostly iron and nickel. 6 & 7-16 Solid Inner Core One final boundary is seen within the core. At this boundary (at a depth of ~5100km) the velocity of P-waves is seen to suddenly increase. What does this suggest about the rigidity of the material here?