The Dynamic Crust, Earthquakes and the Earth’s Interior
Why is the Earth’s crust described as being dynamic?
Crust- solid rock outer zone of Earth
The crust is part of the lithosphere.
The Earth’s crust is dynamic which means constantly changing. Earthquakes
Crustal movements along fault zones
Other evidence indicates that parts of the Earth’s crust have been moving to different locations for billions of years. Describe pieces of evidence that suggest minor changes in the Earth’s crust.
Displaced & Deformed Rock Strata
Sedimentary rocks appear to form in horizontal layers. However, observations of the Earth’s surface indicate that the original formations of rock have changed through past movements. Tilting
Earth movement resulting in a change in the position of rock layers, “rocks at an angle”
Bend in the rock layers produced during the mountain building process Faulting
Movement of rock along a crack (fault) in the crust
Displaced means “moved.”
Marine fossils- remains or imprints of once living ocean organisms such as coral, fish, etc. found in sedimentary rock Marine fossils found in layers of sedimentary rock in mountains, often thousands of feet above sea level. These marine fossils found at high elevation suggest past uplift of rock strata. Sinking or settling of rock strata
Rock layers that have been moved.
Horizontal Displacement (Faulting)
Earth shifts sideways along a transform fault in the crust
Vertical Displacement (Faulting)
Portion of Earth’s surface is either uplifted or subsides along a fault or crack. Benchmark
Permanent cement or brass marker in ground indicating a measured elevation.
Condition of balance or equilibrium in Earth’s crust.
Since the upper mantle acts like a very dense fluid, the crustal plates float on top of it. Any change in one part of the crust is offset by a corresponding change in another part of the crust. Example of Isostasy
If a piece of crust loses some of its material due to erosion, it becomes lighter and floats higher in the mantle. When the eroded material gets deposited, the crust is weighted down causing that area to sink lower into the mantle. Another isostatic example.
The deposition of 2 miles thick ice on NY during a glacial ice age caused the area to subside slightly. This forced other areas to rise higher in response to the settling under the ice. Later after the ice receded or melted, the region responded with gradual uplift causing minor seismic activity or earthquakes. Give examples of crustal activity and explain where the zones of crustal activity are located. Areas of Crustal Activity
Crustal activities such as earthquakes and volcanoes occur for the most part in specific zones or regions of the Earth. These regions are usually along the borders of continents and oceans. These zones mark boundaries or edges of large pieces of the Earth’s crust called crustal boundaries. ESRT p.5
What is an earthquake? Explain the difference between an epicenter and a focus of an earthquake.
Describe properties of the 3 types of earthquake waves and tell the difference between a seismograph and a seismogram. I. Earthquakes
Sudden trembling or shaking of ground usually caused by movement along a break or a fault releasing built up stress When an earthquake occurs, seismic waves are created and move out in all directions from the focus or point of origin. The earthquake’s focus or point of origin is usually deep below the Earth’s surface. The point on the Earth’s surface directly above the focus is called the epicenter. Describe properties of the 3 types of earthquake waves and tell the difference between a seismograph and a
seismogram. II. Earthquake Waves
Seismograph: Instrument that detects and records seismic waves. Earthquakes generate several kinds of seismic waves that can be detected by a seismograph. 3 types of seismic waves are p, s, & l waves.
Do not pass through the Earth.
Ripple along the surface of the Earth
Create the damage associated with earthquakes
Also called compressional because they cause the material through which they pass to vibrate back and forth (compress) in the same direction as the wave is traveling. Called primary because they move quickly through the Earth with a greater velocity than secondary waves and therefore are the first waves to reach a distant location. S waves
Also called shear waves because they cause the material through which they pass to vibrate at right angles (up & down) to the direction in which the wave is traveling III. Velocities of Waves
When traveling in the same material, primary waves travel at a greater velocity than secondary waves. So a seismograph will read the primary waves before the secondary waves arrive. A single seismogram showing the arrival times of p & s waves may be used to determine the distance to the earthquake and its time of origin. The greater the difference in arrival times of the primary and secondary waves, the greater the distance to the earthquake epicenter. Finding the Distance to an Earthquake’s Epicenter
To find out how far an epicenter was away from a location, a seismograph reading or seismogram is necessary that shows the arrival of both p and s waves. Determining the Exact Location of an Earthquake’s Epicenter Epicenter location is found by the comparison of differences in travel time of p &
s seismic waves. Knowing the separation time between arrival of both p & s waves gives the distance to the point on the Earth’s surface directly above the earthquake called the epicenter. Since only the distance to epicenter and not direction is known, a circle is drawn with the radius equal to the distance. The epicenter is on the circle.
To find the actual location of the epicenter you must find the distance from 3 different seismograph stations. Why not 2? Draw 3 circles around the 3 seismograph stations and where they intersect is the earthquake’s epicenter. The earthquake occurred at a point somewhere below the epicenter and that internal point is called the focus. Scientists wanting to improve accuracy of finding the true epicenter will find the distance from more than 3 seismograph stations. Compare and contrast the 2 scales for determining the strength of an earthquake. a) The Modified Mercalli Scale
Based upon the damage inflicted by an earthquake.
This intensity scale ranges from I to XII with I being felt by few people to XII resulting in total devastation. Modified Mercalli Scale Continued
Although this scale is still used, it is not very precise. Why? Damage inflicted by earthquakes depends on many factors besides the strength of the earthquake such as location, type of land, building design & structure, etc. b) The Richter Scale
A Magnitude scale used to describe the amount of energy released by an earthquake. Richter scale magnitudes range from 0 to 9.
Each number step up the scale indicates a release of 32 times more energy than the previous step. Earthquakes that are less than 2.5 are not usually felt by people. Approximately 20 major earthquakes in the magnitude 7.0-7.9 occur every year and each 5-10 years an earthquake of 8.0 or more will devastate a portion of Earth. Give examples of dangers to humans from volcanic and earthquake activity. Dangers to Humans from Earthquakes and Volcanoes
Tell at least 4 of these hazards.
Fires (Ruptured gas or power lines)
Collapsing buildings/Falling Debris
Broken bridges and roads
Tsunamis (Seismic Sea Waves)
Lava flows melt and burn
Volcanic ash & poisonous gases make it difficult to breathe Large submarine (under water) earthquakes or those that occur along a coastline may result in tsunamis or seismic sea waves. Describe differences between p and s wave transmission through the Earth and how it creates a shadow zone. VII. Transmission of Earthquake Waves
The velocity of an earthquake wave varies according to density of the material through which it is traveling. The greater the density of the material, the greater the velocity. As seismic waves travel through materials of different densities, the velocity of the seismic waves will change. This change in velocity of the wave causes the wave to be bent or refracted. Since the density of the Earth gradually increases with depth, seismic waves tend to increase in their velocity and continually refract (bend) as they travel down into the Earth. Difference in P and S Wave Transmission
Compressional or p waves are transmitted through all phases of matter; solid, liquid or gas. However, shear or s waves are only transmitted through solids. This difference provides valuable information for scientists about the composition and interior structure of the Earth. S waves that penetrate the Earth to the depth of the outer core disappear. Since these waves are not transmitted by the outer core, the material of the outer core is assumed to be liquid. Earthquakes generate p & s waves that move out from the earthquake through the Earth in all directions. Seismographs that are located within 102 degrees from the epicenter record both p & s waves. Those seismograph stations that are farther away than 102o do not record any s waves because they are not transmitted through the core. A band that runs approximately 102o to 143o away from the epicenter records neither p nor s waves. Describe a model of the Earth’s crust and interior. Describe characteristics of both the crust and interior. Crust & Interior Properties
There are 4 major Earth zones, three solid ones and one liquid. The 3 solid zones are the crust, mantle and inner core.
The only liquid zone is the outer core.
See ESRT p.10
The crust of the Earth compared to other zones is relatively thin, only a few kilometers in average depth. The average thickness of the continental crust is greater than the average thickness of the oceanic crust. Crustal Composition
The continental crust is composed mainly of felsic igneous rock like granite that is low in density. The oceanic crust is composed mainly of mafic igneous rock like basalt that is high in density. Interior Structure
Crust sits on top of mantle.
Mantle accounts for the greatest part of the volume of the Earth. The crust-mantle boundary is called the Mohorovicic Discontinuity or the Moho. Below the mantle is the liquid outer core and the solid inner core. Interior Composition
Evidence from the behavior of seismic waves and metallic meteorites suggests that the inner portion of the Earth is a high density combination of the metallic elements iron (Fe) and nickel (Ni). Characteristics of Earth’s Interior
The density, temperature and pressure of the Earth’s interior increases with depth. (ESRT p.10). The density ranges from 2.7g/cm3 for the continental crust and 3.0g/cm3 for the oceanic crust to 12.7 g/cm3-13.0g/cm3 for the inner core. Compare theories of continental drift and plate tectonics. Give evidence that support the idea that continents have moved. I. Plate Tectonics Theory
Theory that Earth’s lithosphere is made of a number of solid plates that move in relation to each other. ESRT p.5
Theory that continents are now, as well as in the past, shifting positions. Wegener noted that the present continents appear to fit together as fragments of an originally larger landmass, much the same way the pieces of a jigsaw puzzle fit together. This is especially true if the edges of the continental shelves are used as the boundaries. However, over the years new evidence has been collected that indicates that approximately 200 million years ago, the major continents were connected and since that time the continents have been moving generally apart. The following diagrams show the Inferred Positions of the Continents over the last 458 million years. Label the Geologic Period for each diagram. Diagrams found in ESRT on page 9. Evidence to Support Idea that Continents Have Moved
Many rock layers and fossils can be correlated across ocean basins. Rock types along with mineral composition and the fossils found in those rocks match up. A good example of this are rocks and fossils found on the east coast of South America match those found along the west coastline of Africa. Diamonds found in eastern Brazil are very similar to those found in western Africa. More Evidence for Continental Movement
Some mountain chains appear to be continuous from continent to continent. Example: Appalachians and Caledonian
More Evidence for Continental Movement
Rock and fossil evidence indicates ancient climates much different from those of today. Examples: glacial deposits in tropical regions or coal deposits in Arctic More Evidence for Continental Movement
Rocks of the ocean basins are much younger than continental rocks. The most conclusive evidence comes from the ocean basins.
Explain evidence for sea floor spreading from both igneous ocean rocks and the reversal of magnetic polarity. Evidence to Suggest Sea Floor Spreading
There is much evidence to indicate that the ocean floors are spreading out from the mid-ocean ridges. The two major pieces of evidence are related to the age of igneous ocean materials and the reversal of magnetic polarity. a) Igneous Ocean Rocks
The ocean crust is made up mainly of basalt that is formed when magma (molten rock) rises, cools, solidifies and crystallizes into igneous rocks of the mid-ocean ridges. Evidence shows that igneous rocks along the center of the mid-ocean ridge is younger (more recently formed) than the igneous rock found farther from the mid-ocean ridge. The age of igneous rock has been accurately determined using radioactive dating techniques. This suggests that as new ocean crust is generated at mid-ocean ridges, the ocean floor widens. Reversal of Magnetic Polarity
The strips of basaltic rock that lie parallel to the mid-ocean ridge show matched patterns of magnetic reversals. Check out this animation! Over thousands of years, the magnetic poles of Earth reverse their polarities. The magnetic north pole changes to the magnetic south pole and vice versa. When the basaltic magma flows up in the middle of the ridge and begins to cool, crystals of magnetic minerals align themselves with the Earth’s magnetic field. This alignment of minerals in the rock leaves a recording of magnetic polarity for the Earth at the time of rock formation. When the Earth’s magnetic field is reversed, the new igneous rocks formed during the reversed polarity period have their minerals aligned in an opposite direction from the previously formed rocks. These changes in magnetic orientation are found in rock on both sides of the mid-ocean ridge, indicating that the development of the ocean floor is form the center of the mid-ocean ridges outward. Describe the 3 types of plate motion. Identify plate boundaries. Lithospheric Plates and Plate Boundaries
Three kinds of plate motion are associated with plate boundaries; convergent, divergent and transform. a) Convergent Plate Boundaries
Convergent Plate Boundaries- plates collide with each other Ocean Plate Meets Continental Plate
If an oceanic plate collides with a continental plate, the denser ocean plate made of basalt dives down (subducts) into the mantle forming a subduction zone with an ocean trench formed at the surface. At the subduction zone, old crust is consumed by the mantle to create more molten material. The overriding continental plate made of granite forms mountains. An example is
the Andes of South America. Ocean Plate Meets Ocean Plate
If two oceanic plates converge, the older, denser plate will subduct also forming a trench on the surface along with a chain of islands called an island arc. An example of this convergent subduction zone is the Northern and Western boundaries of the Pacific Ocean. Continental Plate Meets Continental Plate
If a continental plate collides with another continental plate, the edge of both plates are crumpled up forming folded mountains. An example of this type of convergent boundary is the Himalayas of India. b) Divergent Plate Boundaries
Divergent Plate Boundaries- plates move apart
A divergent boundary allows heat and magma to flow up from below forming parallel ridges made of new crustal material. An example of a divergent plate boundary like this is any mid-ocean ridge. c) Transform Plate Boundary
Transform Plate Boundary- plates grind slowly past each other At this type of boundary, crust is neither formed nor consumed. An example is San Andreas Fault in California.
Shallow focus earthquakes are very common at transform boundaries. Plate Tectonic Map (ESRT p.5)
Although plate motion is only a few centimeters a year, the interactions of the boundaries result in earthquakes, volcanoes and mountain building on a grand scale showing that the Earth is a dynamic system. Explain how mantle convection cells are thought to be the method for moving crustal plates. Mantle Convection Cells
Although forces exist within the Earth that are powerful enough to move the lithospheric plates, the scientific community is not in total agreement on the specific mechanism (method) involved. Convection cell- stream of heated material that is moving due to density differences Evidence suggests that convection cells exist within a part of the mantle called the asthenosphere because of the occurrence of heat flow highs in areas of mountain building and heat flow lows in areas of shallow subsiding basins. These convection cells may be part of the driving force which causes continents to move. What are hot spots? How are they formed?
Hot Spots- places on Earth’s surface with unusually high heat flow Most hot spots occur along active plate margins but some are found within the plates. Hot spots are thought to be caused by magma rising up from the mantle producing sites of active volcanism. Wow! That was Dynamic!
Prepare for Chapter Test…Good Luck!!!
* How strong is an earthquake?
Do you live near an active fault?
Earthquake and tsunami
What is inside the earth?
* What is an Earthquake?
* An earthquake is a shaking of the ground caused by the sudden breaking and movement of large sections (tectonic plates) of the earth’s rocky outermost crust. The edges of the tectonic plates are marked by faults (or fractures). Most earthquakes occur along the fault lines when the plates slide past each other or collide against each other. * The shifting masses send out shock waves that may be powerful enough to alter the surface of the Earth, thrusting up cliffs and opening great cracks in the ground and • cause great damage … collapse of buildings and other man-made structures, broken power and gas lines (and the consequent fire), landslides, snow avalanches, tsunamis (giant sea waves) and volcanic eruptions. * How strong is an Earthquake
* Earthquakes are measured in two different ways:
* Earthquake magnitude
* Earthquake magnitude is a measure of the energy released by an earthquake, or its “size”. Because earthquakes vary a lot in size, earthquake magnitude scales are logarithmic. For a one-step increase in magnitude the amount of energy released increases about 32 times. So a magnitude 7 earthquake is 32 times bigger than a magnitude 6 earthquake, and a magnitude 8 earthquake is 1000 bigger. * Earthquake intensity
* Earthquake intensity describes how much ground shaking occurred, or how “strong” an earthquake was, at a particular location. Earthquake waves weaken as they travel away from the earthquake source, so an earthquake generally feels less strong the further away from the source you are. * Earthquake intensity
* The intensity of an earthquake is determined by observing the effects of the earthquake in different places. Houses, buildings, and other structures are inspected. People are interviewed about what they saw (the cabinet fell over), how they felt (I was frightened), or what they did (I ran out of the house). * The Modified Mercalli (MM) intensity scale
* MM 1Not felt.
* MM 2Felt by peeple at rest on upper floors of buildings. * MM 3Felt indoors, like a small truck passing; hanging objects swing slightly. * MM 4Felt indoors by many, like a heavy truck passing; hanging objects swing, windows rattle. * MM 5Felt outdoors, sleepers awakened, small objects and pictures move. * MM 6Felt by all, crockery breaks, furniture moves, weak plaster cracks. * The Modified Mercalli (MM) intensity scale
* MM 7Difficult to stand, noticed by car drivers, furniture breaks, weak chimneys break at roof line, plaster, loose bricks and tiles fall. * MM 8Driving is difficult, ordinary masonry is damaged, chimneys and towers fall, some liquefaction. * MM 9General panic, poor masonry destroyed, ordinary masonry and foundations damaged, liquefaction and landslides. * MM 10Most masonry structures destroyed. Some well-built wooden structures
and bridges destroyed. Dams and embankments damaged, large landslides. * MM 11Few buildings left standing.
* MM 12Damage nearly total.
* What is a fault?
* A fault is a break in the rocks that make up the Earth’s crust, along which rocks on either side have moved past each other. * The direction of movement along the fault plane determines the fault type. * 3 Major Faults
* Do You Live Near an Active Fault?
* An active fault is one that has moved in the past and is expected to move again. Put in another way, an active fault has generated earthquakes before and is capable of causing more in the future. * Scientists use different ways to find out if a fault is active. One is by checking the country’s historical records. Historians always write about destructive events such as earthquakes. * Another is by studying the vibrations, past and present, that come from faults. Still another way is by observing the surroundings. For example, a fault may cross a road and because of that, the road is displaced. * Do You Live Near an Active Fault?
* Or a fault may cut across a stream and the stream channel is then shifted. Or a fault may slice through mountains and form cliffs. This is not to say that anyone can spot an active fault. Scientists need a lot of training to do that. * But along some faults, the effects may be dramatic. Suppose a house was built on a fault. As the ground shifts little by little, parts of the house will be affected. The floor will crack, doors will not close, and the roof may start to leak. * Obviously, it is important to know the location of active faults. As far as possible, no important structures should be built near or on them. Tsunami
* What is a tsunami?
* A tsunami is a series of waves usually caused by an undersea earthquake that displaces the ocean floor. But a tsunami is not really a “wave” that moves up and down; it’s actually the ocean moving sideways as a massivesurge or a wall of water. It’s also knownas a tidal wave. The Japanese word tsunami means “harbor wave.”A tsunami can generate waves for 12 to 24 hours. And the first wave is not always Japan, 2011 The Boston Globe the biggest! A tsunami travels across the open ocean at over 500mph, the speed of a jet airplane. As it reaches shallower water and approaches shore, it slows down but grows in height. A tsunami can happen at anytime of day or year. How do earthquakes generate tsunamis?
* Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth’s crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created. * What is a Tsunami Earthquake
* A tsunami earthquake is an earthquake that triggers a tsunami of a magnitude that is very much larger than the magnitude of the earthquake as measured by shorter-period seismic waves. Such events are a result of relatively slow rupture velocities. They are particularly dangerous as a large tsunami may arrive at a neighbouring coast with little or no warning. a tsunami earthquake is that the release of seismic energy occurs at long periods (low frequencies) relative to typical tsunamigenic earthquakes. Earthquakes of this type do not generally show the peaks of seismic wave activity associated with ordinary events. A tsunami earthquake can be defined as an undersea earthquake. * What is inside the Earth?
* Earth’s Layers
* CrustThe crust is the first layer of the earth. It is split up into
two parts the continental
crust, and the oceanic crust.
The mantle is the second layer of the earth. It is split up into two different parts, the lithosphere (which is the top part) and the asthenosphere (which is the bottom part). * Earth’s Layers
* Outer coreThe outer core is a liquid made up of iron and nickel. The depth of the outer core is 2, 890. This is one of thethree layers that is putting pressure on the inner core. * Inner coreThe Inner crust is the second thinnest layer. The inner core is hotter than the surface of the sun. The inner core is made out of iron and nickel. It is 5159 to 6378 km thick. * Earth’s Layers
* The Earth is formed of three concentric layers: the core, the mantle and the crust; these are separated by transition zones called discontinuities. * Mohorovicic discontinuity
* Gutenberg discontinuity
* How the seismic waves travel
* The shaking starts from the focus and spreads out. You can get an idea of how this happens by throwing a pebble into a pond. See the ripples that move out in circles? The vibrations from the focus are something like that. * The vibrations are more properly called seismic waves. As seismic waves travel through the body of the Earth, they behave in different ways, depending on what they encounter along way
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