Absolute-Dating Methods-4 types • The discovery of radioactivity • destroyed Kelvin’s argument for the age of Earth • and provided a clock to measure Earth’s age • A. Radioactivity is the spontaneous decay • of an atom’s nucleus to a more stable form • The heat from radioactivity • helps explain why the Earth is still warm inside • Radioactivity provides geologists • with a powerful tool to measure • absolute ages of rocks and past geologic events B. Fission Track Measurements (0.4 to 1.5 MYA) C. Carbon 14 dating-less than 70,000 years old D. Tree Ring dating-back as much as 14,000 years ago
Radioactive Decay • The different forms of an element’s atoms • with varying numbers of neutrons • are called isotopes • Different isotopes of the same element • have different atomic mass numbers • but behave the same chemically • Radioactive decay is the process whereby • an unstable atomic nucleus spontaneously changes • into an atomic nucleus of a different element • Three types of radioactive decay: • In alpha decay, two protons and two neutrons • (alpha particle) are emitted from the nucleus.
Radioactive Decay • In beta decay, a neutron emits a fast moving electron (beta particle) and becomes a proton. • In electron capture decay, a proton captures an electron and converts to a neutron.
Radioactive Decay • Some isotopes undergo only one decay step before they become stable. • Examples: • rubidium 87 decays to strontium 87 by a single beta emission • potassium 40 decays to argon 40 by a single electron capture • But other isotopes undergo several decay steps • Examples: • uranium 235 decays to lead 207 by 7 alpha steps and 6 beta steps • uranium 238 decays to lead 206 by 8 alpha steps and 6 beta steps
Concept of ‘Half-Lives’ • The half-life of a radioactive isotope • is the time it takes for one half • of the atoms of the original unstable parent isotope - to decay to atoms • of a new more stable daughter isotope • The half-life of a specific radioactive isotope • is constant and can be precisely measured • The length of half-lives for different isotopes • of different elements • can vary from • less than 1/billionth of a second • to 49 billion years • Radioactive decay • is geometric not linear, • so has a curved graph
Geometric Radioactive Decay • In radioactive decay, • during each equal time unit • one half-life, • the proportion of parent atoms • decreases by 1/2
Determining Age • By measuring the parent/daughter ratio • and knowing the half-life of the parent • which has been determined in the laboratory • geologists can calculate the age of a sample • containing the radioactive element • The parent/daughter ratio • is usually determined by a mass spectrometer • an instrument that measures the proportions • of atoms with different masses
Determining Age • For example: • If a rock has a parent/daughter ratio of 1:3 = a parent proportion of 25%, • and the half-life is 57 million years, • how old is the rock? • 25% means it is 2 half-lives old. • the rock is 57 x 2 =114 million years old.
What Materials Can Be Dated? • Most radiometric dates are obtained • from igneous rocks • As magma cools and crystallizes, • radioactive parent atoms separate • from previously formed daughter atoms • Because they fit, some radioactive parents • are included in the crystal structure • of certain minerals • The daughter atoms are different elements • with different sizes • and, therefore, • do not generally fit • into the same minerals as the parents • Geologists can use the crystals containing • the parents atoms • to date the time of crystallization
Igneous Crystallization • Crystallization of magma separates parent atoms • from previously formed daughters • This resets the radiometric clock to zero. • Then the parents gradually decay.
Not Sedimentary Rocks • Generally, sedimentary rocks cannot be radiometrically dated • because the date obtained • would correspond to the time of crystallization of the mineral, • when it formed in an igneous or metamorphic rock, • not the time that it was deposited as a sedimentary particle • Exception: dating the mineral glauconite, • because it forms in certain marine environments as a reaction with clay • during the formation of the sedimentary rock
Sources of Uncertainty • In glauconite, potassium 40 decays to argon 40 • because argon is a gas, • it can easily escape from a mineral • A closed system is needed for an accurate date • that is, neither parent nor daughter atoms • can have been added or removed • from the sample since crystallization • If leakage of daughters has occurred • it partially resets the radiometric clock • and the age will be too young • If parents escape, the date will be too old. • The most reliable dates use multiple methods.
Sources of Uncertainty • During metamorphism, some of the daughter atoms may escape • leading to a date that is too young. • However, if all of the daughters are forced out during metamorphism, • then the date obtained would be the time of metamorphism—a useful piece of information. • Dating techniques are always improving. • Presently measurement error is typically <0.5% of the age, and even better than 0.1% • A date of 540 million might have an error of ±2.7 million years or as low as ±0.54 million
Dating Metamorphism b. As time passes, parent atoms decay to daughters. a. A mineral has just crystallized from magma. c. Metamorphism drives the daughters out of the mineral as it recrystallizes. d. Dating the mineral today yields a date of 350 million years = time of metamorphism, provided the system remains closed during that time. Dating the whole rock yields a date of 700 million years = time of crystallization.
Long-Lived Radioactive Isotope Pairs Used in Dating • The isotopes used in radiometric dating • need to be sufficiently long-lived • so the amount of parent material left is measurable • Such isotopes include: Parents Daughters Half-Life (years) Most of these are useful for dating older rocks Uranium 238 Lead 206 4.5 billion Uranium 234 Lead 207 704 million Thorium 232 Lead 208 14 billion Rubidium 87 Strontium 87 48.8 billion Potassium 40 Argon 40 1.3 billion
2. Fission Track Dating • Uranium in a crystal • will damage the crystal structure as it decays • The damage can be seen as fission tracks • under a microscope after etching the mineral • The age of the sample is related to • the number of fission tracks • and the amount of uranium • with older samples having more tracks • This method is useful for samples between 1.5 and 0.04 million years old
3. Radiocarbon Dating Method • Carbon is found in all life • It has 3 isotopes • carbon 12 and 13 are stable but carbon 14 is not • Carbon 14 has a half-life of 5730 years • Carbon 14 dating uses the carbon 14/carbon 12 ratio • of material that was once living • The short half-life of carbon 14 • makes it suitable for dating material • < 70,000 years old • It is not useful for most rocks, • but is useful for archaeology • and young geologic materials
Carbon 14 • Carbon 14 is constantly forming • in the upper atmosphere • The carbon 14 becomes • part of the natural carbon cycle • and becomes incorporated into organisms • While the organism lives • it continues to take in carbon 14 • but when it dies • the carbon 14 begins to decay • without being replenished • Thus, carbon 14 dating • measures the time of death
The age of a tree can be determined by counting the annual growth rings • -in lower part of the stem (trunk) • The width of the rings are related to climate and can be correlated from tree to tree • -a procedure called cross-dating • -The tree-ring time scale now extends back 14,000 years 4. Tree-Ring Dating Method • In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans
What is an Earthquake? • Definition: a sudden motion or trembling of the Earth caused by the abrupt release • of slowly accumulated elastic energy in rocks. • A. Stress • -The force that is exerted against an object; the rocks of the Earth’s crust are • stressed by tectonic forces • -Whenever an object is stressed it changes its size and shape • B. Strain- The deformation that results from stress • 1. elastic deformation • -Stress applied slowly • -When stress its removed the object springs back to its original size and shape • 2. All rocks deform elastically when tectonic stress is applied; the sudden release • of that stored elastic energy when the rock fractures causes earthquakes • 3. elastic limit: the limit beyond which the rock cannot deform elastically • 4.plastic deformation: when a rock will not return to its original shape when the • stress is released • 5. brittle rupture-When a rock is deformed beyond the elastic limit, it may rupture • It breaks sharply and the fracture becomes a permanent feature of the rock Chp 9: Earthquakes
Chapter 9-Earthquakes: Izmit, Turkey 1999: est 17,000 people died, 240,000 buildings damaged Fig. 9-CO, p.240
C. Deformation of Tectonic Plate • 1. Segmented lithosphere that moves relative to each other by gliding over the • asthenosphere • 2. As the plates move they slip past one another along immense fractures that form • the boundaries between adjacent plates • 3. The slippage is not smooth and continuous, but occurs as rapid jerks as one plate • suddenly slips, causing an earthquake. • EARTHQUAKES, FAULTS, AND TECTONIC PLATES • A. Fault • 1. A fracture in rock along which movement has occurred in the past • 2. Earthquakes start on old faults that have moved many times in the past and will • move again in the future. • 3. If new stresses develop in a region of the crust, new faults form and earthquakes • can occur in places that were previously earthquake free. • 4. fault creep • a. Slow movement of plate past one another at a continuous snail-like pace • b. The movement occurs without violent and destructive earthquakes • c. seismic gap: fuzzy area on graph- little or no seismic activity Chp 9: Earthquakes
Light reflecting off rock surface- fault plane, slickensided… p.266a
B. Tectonic Plates • 1. Most earthquakes occur along the faults separating tectonic plates • 2. Less commonly, earthquakes occur where thick piles of sediment • have accumulated • a. Causes of earthquakes in plate interiors are not as well • understood • b. Some interior quakes occur where thick piles of sediment • have accumulated • c. Other interior quakes may be caused by plate movements • d. Rebound: “ Isostasy” over central US because of glacial • covering Chp 9: Earthquakes
Relationship between earthquake epicenters and plate boundaries!! approx 80% occur within circum-Pacific. Each dot represents one epicenter Fig. 9-4, p.247
EARTHQUAKE WAVES • A. Body Waves • 1. Seismic Waves • -waves that travel through rock • -initiated naturally by earthquakes • -can also by produced artificially by explosive charges detonated on or beneath • the Earth’s surface • seismology: the study of earthquakes and of the structure of the Earth’s interior • from evidence provided by seismic waves • body waves • -travel through the interior of the Earth • -start from the initial rupture point, or focus, of an earthquake. • -Epicenter: point on the Earth’s surface directly above the focus • a. primary wave- also called p-wave • -formed by alternate compression and expansion of the rock—like a slinky. • -Transmitted in both liquids and solids • -Travels at speeds of 5-7 km/sec in the crust and 8 km/sec in the upper mantle • -The first wave to reach an observer. • b. Secondary wave--also called S-wave • -Form when shearing forces are transmitted • -Travel at speeds between 3-4 km/sec in the crust • -Move only through solids Chp 9: Earthquakes
Rocks are deformed as high energy from earthquake passes through- rocks are deformed, store energy, then bend as a result. When internal strength is exceeded, rocks fracture-causing an earthquake. They rebound to their original shape. b) fence in Marin County Ca, offset 5m Fig. 9-1, p.243
Surface waves due to earthquake Pwave= parallel to direction of movement Swave= perpendicular Fig. 9-8, p.251
5. Secondary wave--also called S-wave • -Form when shearing forces are transmitted • -Travel at speeds between 3-4 km/sec in the crust • -Move only through solids • B. Surface waves • 1. L-Waves: “Surface Waves” • -Two types of L-waves • a. Up-and-down motion • b. Side-to-side vibration • Surface waves cause the most property damage because of the ground motion • associated with them • C. Measurement of seismic waves • -seismograph: a device that graphs seismic waves • -seismogram: the record of an earth vibration • An earthquake measuring station generally has at least three seismographs • -two horizontal seismographs • a. oriented to measure east-west movements • b. oriented to measure north-south movements • -one vertical seismograph Chp 9: Earthquakes
Focus of earthquake: Where rupture begins and energy is released. The location on the surface of the earth, vertically above the focus is known as the Epicenter. Seismic waves move out in all directions… Fig. 9-3, p.247
Types of S waves: Rayleigh and Love waves: a. Rayleigh: move material in elliptical path parallel to wave direction b. Love: move material back and forth in a horizontal plane, perpendic- ular to the motion of the earthquake wave. VERY dangerous to homes Fig. 9-9, p.251
LOCATING THE SOURCE OF AN EARTHQUAKE • Time-travel Curves • 1. Graphs used to quantify the general relationship between distance from an • earthquake epicenter and arrival times of the different types of quakes • 2. to create a time travel graph one must know both when and where the earthquake • occurred • 3. graph can then be used to measure the distance between a recording station and an • earthquake whose epicenter is unknown • EARTHQUAKES AND HUMANS • A. Measurements of earthquake strength • 1. Mercalli Scale: a qualitative scale of earthquake intensity and measures the • effects of an earthquake at a particular place • 2. Richter scale • a. A quantitative scale of earthquake magnitude based on measurements • made by a seismograph • b. first refined by Charles Richter in 1935 • c. The magnitude is determined by measuring the amplitude of the largest wave • recorded by a seismograph. • Adjustments are made for the distance from each recording station to the earthquake Chp 9: Earthquakes
Modern seismographs record earthquake waves electronically-see a. b. A horizontal motion seismograph. c. A vertical motion seismograph. Fig. 9-2, p.246
The Richter magnitude scale measures the amount of energy released by an earthquake at its source. 1.Measure maximum amplitude of largest seismic wave, mark on right hand scale. 2. Difference in arrival time of P and S waves, in seconds, is marked on left hand scale. 3. Draw a line between the two points-magnitude of earthquake is where the line crosses center scale. Fig. 9-14, p.257
d. Scale is logarithmic • 1.An increase of one unit on the scale represents a 10-fold increase in • the amplitude of a recorded earthquake wave • 2.An increase of one unit on the scale corresponds approximately to a • 30-fold increase in energy related during the quake • B. Earthquake Damage • 1. Ground motion • -Motion of ground is dependent upon the rock and the soil type • - Some types of building materials and design features are more able to • withstand an earthquake than others • 2. Permanent alteration of landforms • Scarps: a short, steep cliff formed by the vertical displacement of land by an • earthquake (ex: Balcones Escarpment) • 3. Fire • 4. Landslides • 5. Tsunamis: Seismic sea wave caused by displacement of the sea floor Chp 9: Earthquakes
San Francisco, Marina district-fires caused by ruptured gas lines... Fig. 9-18, p.261
a. fault scarp: block on right moved up with respect to one at bottom b. Earthquake triggered landslide ,seen in distance, which dammed lake Fig. 9-22, p.268
30,000 people were killed in 1993 quake in India Fig. 9-17b, p.261
Earthquake damage in Pacific: Kobe Japan; Oakland, Ca; Northridge Fig. 9-6, p.249
Tsunami crashes into street in Hilo, Hawaii in 1946-caused by Earthquake in Aleutian Islands….159 people died in Hilo Fig. 9-19, p.264
EARTHQUAKE PREDICTION • Long-Term Prediction • 1. Motion along a fault • a. On segments of the fault where faultcreep occurs, the plates slip past • one another smoothly and without major earthquakes • b. In other segments of the fault, the plates “hop” past one another in a • series of small jumps causing numerous, small, non-damaging earthquakes • c. In still other segments of the fault, plates become locked for tens to • hundreds of years and then produce catastrophic earthquakes when they • break free. • 2. Seismic gap • a. An immobile region of a fault bounded by moving segment • b. Rock within the seismic gap is accumulating elastic deformation and will • eventually fracture producing a major earthquake • 3. Historicalstudies of earthquake activity make it relatively easy to • identify zones of high earthquake hazard. Chp 9: Earthquakes
Earthquake predictions-look for gaps in activity…. Fig. 9-24, p.269 3 gaps evident in seismic activity along San Andreas fault
B. Short-Term Prediction • 1. A reliable early warning system; a signal or group of signals that immediately • precedes an earthquake • 2. Foreshocks • -Small earthquakes that precede a large quake by an interval ranging from a • few seconds to a few weeks • -Only about ½ of the major earthquake sin recent years were preceded by a • significant number of foreshocks • -Measure change in the shape of the land; distortions of the crust may precede major • earthquakes. • 3. release of radon preceding an earthquake • 4. strange animal behavior preceding an earthquake • C. Social and Economic Factors in Earthquake Prediction • Short-term earthquake prediction is not only a formidable scientific problem but it • also involves political, social, and economic issues Chp 9: Earthquakes
3 seismograph stations are • needed to locate the epicenter • of any earthquake. • Procedure is as follows: • P-S time interval plotted • on time distance graph for • each station. This • determines the distance • each station is from the • epicenter. • 2. A circle with that radius • is drawn from each station, • the intersection of the • 3 circles locates • the epicenter. Fig. 9-11, p.253
Seismogram showing the a. arrival order and pattern of different waves. b. Seismogram for 1906 San Francisco earthquake. Recorded in Germany. c. A time-distance graph showing the average travel times for P and S waves Fig. 9-10, p.252