MAR 110 Lecture #8 Introduction to Oceans Hazards REVIEW …MAR 110: Lecture 8 Outline –Review #1...

MAR 110: Lecture 8 Outline –Review #1

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MAR 110 Lecture #8

Introduction to Oceans Hazards REVIEW #1


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External Energy Source-the Sun The sun is the source of nearly all the energy used on earth, mostly in the form of heat which drives both oceanic and atmospheric processes. (UWaC)

Internal Energy Source-Radioactive Decay Radioactive decay deep inside the earth produces geothermal energy (heat) as well as create convection currents within the mantle that drive plate tectonics. (?)


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Interior Earth Structure (left) The Earth’s interior is composed of multiple spherical layers: inner

core, outer core, mantle, and a relatively thin crust. (right) The properties of the mantle vary from the brittle lithosphere (part of which is the earth’s crust) to the relatively fluid asthenosphere to the more rigid mesosphere. The denser layers are deeper. (ItO)

Hurricane Structure Warm moist air enters at the bottom of a hurricane and spirals upward through convection. As the air moves upward it cools and releases moisture, strengthening the storm. (?)


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Ocean Basin Geographic Zones The geographic zones of the North Atlantic are identified in the bird’s eye view of the sea floor above. Below is shown a vertically exaggerated profile of the ocean sea floor features along the transect A-B. (LEiO)

Earth Crustal Configuration The relatively thin oceanic crustal layer and the thicker continental crustal layers “float” on the mantle below…they are said to be isostatic equilibrium. It is their layer thickness differences that cause the continental crust to rise higher out of the mantle relative to the thinner oceanic crust. The different densities of the rock material only make slight differences. The resulting ocean basin depression catches the water that runs off of the land. Note the ~ 20 times (20x) vertical exaggeration of this profile. (ItO)


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Plate Tectonics Dynamics Schematic of the major plate tectonic features in the upper 450km of the Earth. Convection under the ocean ridge creates the lithospheric plates that move, collide, and subduct to great depth, where they melt and rise to form volcanic islands. (ItO)

Plate Movement and Ocean Hazards The Earth’s crust is composed of twelve major plates (named above), whose boundaries are moving as indicated by the orange arrows. Note the regions of plate divergence -usually along mid ocean ridges - and plate convergence -usually associated with ocean trenches. Earthquakes (black dots) silhouette some of these boundaries, with greater concentrations along active convergent zones, and lesser concentrations along active divergent zones. Along with the earthquake hazard is also the possibility of the tsunami hazard. Volcanism is also associated with the earthquake hazard. (ItO)


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Earthquake Distribution (Top)The locations of historical earthquakes that are “shallow” (less than 100km deep) mark the location of (1) divergent spreading ridges in mid-ocean and (2) convergent deep trench zones around the edges of ocean basins. (Bottom) The locations of deep earthquakes are usually associated with plate subduction. (e.g., circled locates the Sumatra earthquake – 26 December 2004) (?)


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Sea Floor Magnetic Anomaly Formation This schematic illustrates the reason for the observed sea floor magnetism. (top) Iron particles within the magma (molten rock) align magnetically with the Earth’s prevailing magnetic lines of force (normal polarity – dark brown). When the hot magma hits the cold ocean waters of the mid-ocean ridge, it solidifies to form solid rock and in so doing “freezes” the magnetic orientation of its iron particles forever. Ocean crust formation proceeds for many years, pushing symmetrical bands of older rocks further away from the ridge (middle) Every million years or so the magnetic polarity of the whole Earth changes; yielding bands of rocks with a “reversed” polarity (R, light brown). (bottom) This idealized record of magnetism in these rocks shows the alternating bands of small departures in magnetism – consistent with the actual measurements shown next. (ItO)


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Plate Tectonics Schematic Magma rises to form new ocean crust at the oceanic spreading center along the mid-ocean ridge (MOR), which exhibits shallow earthquakes (dots). The oceanic crust, exhibiting magnetic stripes, moves away from the MOR across the ocean basin. Upon collision with a continent, the oceanic crust subducts under the edge of the continental plate.

(LEiO)

Continental Drift A time series showing the configuration and movement (arrows) of the continents at (top) 200 million years ago; (middle) 65 million years ago; and (bottom) today. (?)


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Earthquake Scenario The epicenter of an earthquake is the location of the earthquake’s source on the surface of the earth, while the focus is the three dimensional location. (?)

Earthquakes on Mid Ocean Ridge Transform Faults The earthquakes (red dots) near mid ocean ridges usually occur in the short transform faults connecting sections of the ridge. A few weak earthquakes do occur within the divergent zone due to the block faulting that occurs as the plates pull apart. (?)

Plate Configuration


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Waves Broadcast the Earthquake Occurrence It is possible to pinpoint the location of the epicenter of an earthquake by compiling the data from seismic recorders to find the P wave shadow zone that is caused by the way seismic waves travel through the earth. (NG 1965)


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West Coast Spreading Center & Transform Faults The North American (NA) plate is overriding the older Pacific Plate spreading center. The famous San Andreas fault is one of the spreading center transform faults. Thus Southern California and the Baja peninsula are moving northwestward relative to the NA plate. Thus in 15 million years or so, Los Angeles will not “fall into the ocean”, but rather will be located west of most of northern California – (ItO)


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Tsunami Shoaling The height of a tsunami grows dramatically as it comes a shore. Usually the water at the shore receeds to feed the growth of the offshore tsunami wave.

Tsunami Evolution

Figure 12.17 Underwater Faulting & Sea Surface Distortion The underwater faulting illustrated above causes some vertical displacement of the whole column of water above it. (?)


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Tsunami-Related Destruction – Atlantic Ocean 1755 A tsunami wave that was generated by an earthquake in the Azores pummeled Lisbon, Portugal in 1755.

Sumatra Tsunami The location of the origin of the 26 December 2004 Sumatra tsunami (red bullseye). The location in the Indian Ocean of the devastating wave 2 hr, 3.5 hr and about 10 hr after it was generated. (NG Apr05)


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Mid Ocean Ridge Magma As new crust is formed along the ridge axis the older crust is pushed to either side forming bands of rock parallel to the ridge. The upward moving magma in the rift zone forces its way through the oceanic crust, forming underwater volcanoes where the magma breaks through as pillow lava. (?)

An Underwater Earthquake in Pacific NW is Overdue! (top) The Juan de Fuca (JF) plate is subducting below the North American (NA) plate. The stresses associated with the “locked” configuration of the JF/NA plate collision zone are causing an extreme distortion (i.e. strain) of the NA plate. (middle) Sometime in the future (who can say when) the stresses of the distorted configuration will exceed the friction between the two plates and the fault will rupture – allowing the NA plate edge to “spring” upward and outward. The rapid sea floor movement could produce a tsunami that would propagate both shoreward and seaward. (SciAmer 12/1995)


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Juan de Fuca Plate Subduction The plate subduction in the northeast Pacific (offshore of Washington and Oregon) generates a chain of volcanic mountains known as the Cascades. ashore , (NG)


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Krakatoa 1883 . (NH)

Subduction Zone Volcanism Northeast Pacific Deep magma chambers - formed by the friction between the subducting Juan de Fuca plate and the North American plate – and feed the active volcanoes and spawn earthquakes in the Cascade Range. An active erupting volcano produces volcanic “bombs” (small to large pieces of solidified magma) and at times huge amounts of smoke (fine particles) and somewhat larger ash which tends to settle back to earth in the region. Besides slower lava flows, this kind of volcanic action can also produce superheated, high-speed pyroclastic flows that in turn can trigger landslides and mudflows. On occasion when the magma conduits are blocked major explosions can occur like mt St Helens in the ‘80s.(NG)


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Tsunami “Sleigh Ride” The 1886 eruption of Krakatoa triggered a series of tsunamis which were some of the largest volcanically induced tsunamis in history. The waves were so large that they carried heavy iron clad ships such as those shown-and their moorings-several miles inland. The tsunami carried several heavy iron ships a couple miles inland and deposited them in the jungle. Many other ships were carried back out and deposited on the beaches as debris or washed all the way out to sea. (SW?) The only known eruption to create tsunamis on a comparable scale was the eruption of Santorini around the year 1450BC that covered the entire island of Crete in over a meter of water and destroyed the Minoan civilization. (NH)

Krakatoa Explosion - Followed by Tsunami ! The series of 1883 Krakatoa explosions triggered a series of tsunami. The first few were minor (couple of feet), but the final one was so powerful that it carried an iron ship two miles inland. The tsunami triggered by the final explosion was unusual in that it “skipped” over islands and other landmasses to hit the coast on the opposite side of the landmass before the wave had a chance to refract/reflect around the island/landmass. There were reports of the wave, which traveled around the world, reaching England. (SW)


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Krakatoa Explosion “The Sound Heard ‘Round the World” The sound of the largest explosion was heard all around the Indian Ocean and in other parts of the world including Cuba. The sound was described as ships firing their guns. Even where nothing was heard, barometers recorded air pressure changes; which traveled around the world seven times and “echoed” for fifteen days. (SW?)

Tsunami Measurement


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Mantle Convection Interior heat escape causes convection cells, with hotter rising plumes and the cooler sinking plumes to form. The lithospheric plates at the surface move with the lateral flow that connects the plumes. The movement of the electrically-conducting mantle material in the convection cells creates the earth’s magnetic field (?)

Hot Spot Island Formation Isolated stationary mantle hotspots can produce chains of volcanic islands and seamounts in the mid-ocean - like the Hawaiian island chain (right). (ItO, LEiO)


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Hot Spot Island Formation (C) The moving plate drags volcano #2 away from magma chamber rendering it extinct, while forming a new volcanic island (#3). (C) The moving plate drags volcano #3 away from magma chamber rendering it extinct, while forming a new volcanic island (#4). (ItO)

MAR 110 Lecture #8 Introduction to Oceans Hazards REVIEW …MAR 110: Lecture 8 Outline –Review #1...

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