Geology 101 539 Class 3 Test 1 Tuesday September 9 2014 Outline Recap Rates and history of plate movements o Paleo magnetism recorded in rocks o Seafloor as a magnetic tape recorder The grand reconstruction o Seafloor isochrones o Reconstructing the history of plate movements o Mantle convections mantle plumes Mechanism of plate tectonics Plate Tectonics Involves the outer portion of earth crust and topmost part of the mantle down to the asthenosphere o About 20 distinct plates o Tens of km in thickness o In relative motion with respect to each other Plate motion causes earthquakes builds mountain ranges and volcanic activity 3 Type of Plate Boundaries Divergent Boundaries o Oceanic spreading center o Continental rift zone new oceanic lithosphere Rifting and spreading along a mid ocean ridge create Mid Atlantic ridge Rifting and spreading zones on continents are characterized by parallel rift valleys volcanism and earthquakes Great Rift Valley Convergent o Ocean Ocean Convergence Where oceanic lithosphere meets oceanic lithosphere one plate is sub ducted under the other and a deep sea trench and volcano is formed o Ocean Continent convergence Peru Chile trench When oceanic lithosphere meets continental lithosphere the oceanic lithosphere is sub ducted and a volcanic mountain belt is formed and the continental margin o Continent Continent Convergence Where two continents converge the crust crumples and thickens creating high mounts and a wide plateau Himalayas Transform Fault Boundaries o Continental Transform Fault o Mid Ocean Ridge Transform Fault Plates slip horizontally past each other San Andres fault Mid Ocean ridges are typically offset by transform Eurasian plate faults Suggested Video Secrets in Stone o The role of paleomagnetism in the evolution of plate o http online wr usgs gov calendar 2004 jul04 html tectonic theory Seafloor As a Magnetic Recorder Produced by the convection of the liquid Fe Ni alloy in the outer core Similar to the ones produced by a bar magnet or a current coil Types o Bar magnet o D c energized coil o Geodynamo NO GEOMAGNETIC FIELD Assume during class experiment o Magnetic minerals represented as magnetic needles o When lavas crystallizes each magnetic moment orientation o Random orientation of the magnetic materials o Every crystal is going to move wherever we want o Move from liquid to solid phase direction they were moving get frozen The needles will point whatever direction it wants Completely random is preserved o Lets say a few million years pass GEOMAGNETIC FIELD PRESENT NORMAL DIRECTION o Magnetic minerals orient their magnetization in the o When lavas crystallizes each magnetic mineral remains direction of the geomagnetic field aligned GEOMAGNETIC FIELD PRESENT REVERSED DIRECTION o New lavas preserve magnetization in the new reversed o Tape recorders are similar direction Very thin plastic ribbon with a thin layer on it like a credit card strip Magnetic field that preserves information Charges one side going one direction Plays the way the magnetic strip flows Paleomagnetism in a nutshell o 1 Geologists examine magnetization of volcanic rocks Their findings show reversals of Earth s magnetic field o 2 Geologists then determine the absolute age of the rock Every sample is characterized by its magnetization and age construct a reversal time scale by studying many volcanoes and lava flows age vs magnetization o 3 Large number of rock samples allows geologists to Determining Seafloor Age By Magnetics Paleomagnetism in a nutshell o 4 Measure magnetic field offshore using ship borne instruments Match the magnetic patterns to those determined on land and assign its age crust rocks Zebra stripes represent normal and reverse magnetized ocean Magnetic dating of the sea floor o The magnetic time scale was used to date the magnetic anomalies on the seafloor The magnetic chrons of the last 5 million years represented here as blue and brown bands were named prominent scientists who studied earth s magnetic field Seafloor is very young oldest seafloor is 200 Ma Atlantic ocean is growing in size What part of the Atlantic Ocean is the oldest ocean splitted first North part of south part o When the Americas started splitting which part of the North Part approximately 60 million years before How old is the oldest part of the Pacific Ocean o Lies West of Marianas Trench 170 180 million years old The plate of the Pacific Ocean is wider because it is growing where are the oldest rocks on the planet o Not in the ocean They are youngest in the ocean Older than 180 million years old are located on the continents faster The Grand Reconstruction Geologists use plate motions and the study of different types of 24 of Earth s history rocks in continents to reconstruct geological past Mechanism for Plate Tectonics Mantle Convection o Whole Mantle Convection o Stratified Convection Plastic upper mantle asthenosphere Gravity field as the driving force Ridge push slab pull How o Mid Ocean ridge o Gravity pushed the plate sliding off the mid ocean ridge o Sub ducting plate o The sinking lithospheric slab pulls on the sub ducting plate o Deep sea trench Mantle Convection Anomalies Hotspots and mantle plumes Hotspot is a location on the earth s surface that has experienced active volcanism for a long period of time Question Who proposed the theory of continental drift o A Charles Darwin o B Harry Hess o C Alfred Wegener o D Tuzo Wilson o Trench o Subducted Plate o Mountain Range o A Continent o B Subduction zone o C Spreading Center Identify the zones below with their corresponding number on the diagram What type of plate boundary is shown in the diagram When a deep sea trench is ocated next to a continent where would you expect to find active volcanoes o A on the ocean side o B In the deap sea trench o C on the continent side of the trench Which of the following is an example of a transform plate boundary o East African rift o San andreas On a map of the seafloor the boundaries between normally magnetized oceanic crust and reversely magnetized oceanic crust are called o isochrons