Chapter 10 What to know Scarp Characteristics of faults Displacement Where displacement is greatest Earthquake scales Seismicity MOR Convergent Tsunami What Causes Earthquakes To Happen Seismicity earthquake activity This can occur because of o The sudden formation of a new fault fracture o A sudden slip on an existing fault o A sudden change in the arrangement of atoms in the minerals of comprising rock o Movement of magma in a volcano o The explosion of a volcano o A giant landslide o A meteorite impact o Underground nuclear bomb testing Hypocenter focus or center of the earthquake Epicenter The point on the surface of the earth above the hypocenter Faults occur due to the hypocenter which cause the damage in an earthquake Foreshocks a minor tremor before a major earthquake How Does Earthquake Energy Travel Travels in the form of waves Seismic Waves Body waves travel through the interior of the earth Surface waves along the earth s surface Compressional waves waves where particles move parallel in the wave Shear waves same as above but move perpendicular Four basic types of seismic waves 1 P waves primary Compressional body waves 2 S waves secondary shear body waves 3 R waves Rayleigh name of a physicist surface waves that cause the ground to ripple up and down 4 L waves love a seismologist surface waves that cause the ground to ripple back and forth in a snake like movement How Do We Measure and Locate Earthquakes Seismograph systematically records ground motion from an earthquake happening anywhere on earth Mercalli Intensity Scale defines the intensity of an earthquake Today we use the Richter Scale Where and Why Do Earthquakes Occur Seismic Belts where earthquakes occur because they don t occur everywhere on the world Divergent Plate Seismicity Takes place at shallow depths less than 10 km These earthquakes don t affect anything because they are deep in sea Only a few places worry about these ex Iceland Convergent Plate Seismicity Several kinds of earthquakes take place here Intermediate and deep focus earthquakes Wadati Benioff Zone deep sloping band of seismicity How Do Earthquakes Cause Damage Ground shaking and Displacement Seiche the water s rhythmic movement Predicting the Big One Seismic Risk predicting earthquakes Annual Probability the probability of an earthquake is about 1 in 510 or 2 Chapter 12 Numerical age Relative age Fossils record Radioactive decay 12 1 Introduction Unconformities 3types gap in geologic record Dating Clasts metamorphism and igneous rock Geologic time is the span of time since Earth s formation Deep time is the immense span of geologic time 12 2 Time A Human Obsession Nothing important here 12 3 The Concept of Geologic Time During the Renaissance scientists began to speculate that geologic time might far exceed historical time Archbishop James Ussher added up generations from the Old Testament and determined that the Earth was formed on October 23 4004 BCE Nicolaus Steno observed shark teeth in the mountains concluded these were ancient animals in loose sediment this then hardened into rock James Hutton the principle of uniformitarianism states that physical processes we observe today also operated in the past and were responsible for the formation of the geologic features we see in outcrops It implies that the present is the key to the past there is no vestige of a beginning no prospect of an end Relative age is based upon order of formation qualitative method younger vs older relationships Numerical age is the actual number of years since an event quantitative method age is a given number 12 4 Principles for Defining Relative Age Principle of uniformitarianism physical processes we observe operating today also operated in the past Factors such as climate change can affect erosion rates and occasional catastrophic events may trigger global extinctions Principle of superposition In a sequence of sedimentary rock layers each layer must be younger than the one below Layer at bottom is oldest Principle of original horizontality The surfaces on which sediments accumulate are fairly horizontal When we see folds and titled beds we are seeing the consequences of deformation that postdates deposition Principle of original continuity Sediments generally accumulate in continuous sheets Principle of cross cutting relations If one geologic feature cuts across another the feature that has been cut is older If a fault cuts across and displaces layers of sedimentary rock then the fault must be younger than the layers And vice versa Principle of inclusions If an igneous intrusion contains fragments of another rock the fragments must be older than the intrusion If a layer of sediment deposited on an igneous layer includes pebbles of the igneous rock then the sedimentary layer must be younger Principle of baked or chilled contacts The rock that has been baked must be older than the intrusion Since an intrusion injects into cooler rocks the margin of the intrusion cools rapidly and is finer grained than the interior Geologic history is defining the relative ages of events that took place there Fossil assemblage is a group of fossil species found in a specific sequence of sedimentary rock The predictability of fossil distribution which allows geologists to arrange fossil species in a progression from older at the bottom to younger at the top has been codified as the principle of fossil succession Range is the interval of a sequence of strata in which a specific fossil species appears Because of the principle of fossil succession we can define the relative ages of strata by looking at fossils 12 5 Unconformities Gaps in the Record An unconformity is a gap between two different rock sequences representing an interval of time during which new strata were not deposited and or eroded Angular unconformity cuts across the underlying layers the layers below have a different orientation from the layers above form where rocks are either folded or tilted by faulting before being uplifted and eroded Nonconformity sedimentary rocks overlie intrusive igneous rocks and or metamorphic rocks Typically find pebbles of the igneous or metamorphic rock in the lowest bed of the sedimentary sequence Disconformity no new sediment accumulates and some of the pre existing sediment gets eroded away Later sea level rises and a new sequence of sediment accumulates over the old The boundary between the sequences is a disconformity Even though the beds above and below the disconformity are
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