Evidence For Continental Drift. Introduction As Scientific Theories go, Plate Tectonics is a relatively new scientific concept, introduced in the 1960’s

Evidence For Continental Drift

Introduction As Scientific Theories go, Plate Tectonics is a relatively new scientific concept, introduced in the 1960s. It is the most revolutionary idea in Earth Sciences it has changed our notion of a generally quiet, unchanging planet (Uniformitarianism) into the understanding that the Earth is a dynamic planet.

Plate Tectonics the Unifying Theory of Earth Sciences The theory has unified the study of the Earth by drawing together many branches of the earth sciences, from paleontology (the study of fossils) to seismology (the study of earthquakes). It has provided explanations to questions that scientists had speculated upon for centuries -- such as why earthquakes and volcanic eruptions occur in very specific areas around the world, and how and why great mountain ranges like the Alps and Himalayas formed.

Look At The Globe By examining a globe, one can observe that many of the continents seem to fit together like a puzzle: the west African coastline seems to fit nicely into the east coast of South America and the Caribbean sea; and a similar fit appears across the Pacific. The fit is even more striking when the submerged continental shelves are compared rather than the coastlines. In 1858, geographer Antonio Snider- Pellegrini made these two maps showing his version of how the American and African continents may once have fit together, then later separated

Alfred Wegener In 1912 Alfred Wegener (1880-1930) noticed that fossil encyclopedias listed fossils that existed on opposite sides of the ocean - he found this idea to be highly unusual. Wegener then noticed that the continents seemed to fit together. By linking continents together, the fossil regions would be joined together. Wegener proposed that the continents were once joined into a single supercontinent which he called Pangaea (meaning "all lands"). He believed that over time the continents have drifted apart into their current distribution. Wegener is generally regarded as the father of the idea of Continental Drift Wegener was not only well known for his theory of Continental Drift (Kontinentalverschiebung) but also for his work in Meteorology. He studied polar air circulation in Greenland which contributed to the discovery of the Jet Stream. Wegener also collected ice drilling samples of the Greenland Glaciers. Wegener died while working in Greenland at the age of 50.

Continental Drift Wegener believed that Pangaea was intact until the late Carboniferous period, about 300 million years ago, when it began to break up and drift apart. Wegener's hypothesis lacked a geological mechanism to explain how the continents could drift across the earths surface as he proposed.

Background PowerPoint http://faculty.uml.edu/Nelson_Eby/87.201/Instructor %20pdfs/Plate%20Tectonics%20Class.pdf (University of Massachusetts at Lowell) http://faculty.uml.edu/Nelson_Eby/87.201/Instructor %20pdfs/Plate%20Tectonics%20Class.pdf

You Tube Video https://www.youtube.com/watch?v=X9_TIf2navc (21:41 - Wegeners Evidence) https://www.youtube.com/watch?v=X9_TIf2navc https://www.youtube.com/watch?v=NwqX4OLJRe4 (10:42 - Continental Drift) https://www.youtube.com/watch?v=NwqX4OLJRe4 https://www.youtube.com/watch?v=0KNqUwgqbZw (Bill Nye - 100 Greatest Discoveries - 47:00) https://www.youtube.com/watch?v=0KNqUwgqbZw

Continental Drift Wegeners Hypothesis was not widely accepted until the 1950s. The following evidence made his hypothesis more acceptable to the Scientific community: 1) Coastline Matching 2) Fossils collected in Africa and South America 3) Ocean Floor Exploration 4) Patterns of Seismic Activity (Earthquakes and Volcanoes) 5) Glaciation

1)Matching Coastlines Continents, especially if you include the continental margins, fit together very clearly as seen in the globe at right. The suggestion is that the continents were joined together and broke apart and moved into their present positions. More proof was needed to add strength to this hypothesis. Geologists looked to match rock formations on continents now separated by oceans. Similarly, paleontologists looked for similar trends with fossil assemblages.

11 Continental Fit and Mountain Ranges TLTH, fig. 1.15 Major time of mountain formation ended about 250 mya (million years ago), before Pangaea split Appearance of same rock sequences and mountains of the same age on continents now widely separated.

2)Fossil Data Fossils within specific stratigraphic sequences were found on opposite sides of oceans. When the continents were pieced together, the fossil sequences merged. Mesosaurus, a reptile TLTH, fig, 1.13

Possible Explanations for Fossils being separated by oceans: Possible explanations?

3)Ocean Floor Exploration In the 1950s and 1960s, the first scientific exploration of the seafloor was undertaken. Among the data collected were depth measurements (using precision depth sounding), sea-floor sampling (and subsequent radiometric age dating), paleomagnetic readings. Glomar Challenger

Harry Hess - Sea-Floor Spreading https://www.youtube.com/watch?v=GyMLlLxbfa4&lis t=PL8aTbGWgTg6U95JvrN-g2mCnz7QNPs9r4 (Bill Nye - 4:01) https://www.youtube.com/watch?v=GyMLlLxbfa4&lis t=PL8aTbGWgTg6U95JvrN-g2mCnz7QNPs9r4 Geology professor Harry Hess acted as a naval transport captain during WWII. While travelling the Pacific Ocean, Hess discovered that there was a mountain range under the ocean. In 1953, Oceanographers discovered the mid-ocean ridge. The ridge is as high as 2500 m above the sea floor.

Harry Hess - Sea-Floor Spreading Hess followed up by studying the age of rocks and sediments on the ocean floor by using core samples and sonar data. He makes the surprising discovery that the sea floor is youngest at the mid-ocean ridges and that the crust gets older as you move away from the ridge. He also learns that the mid ocean ridge is a line of underwater volvcanoes where new crust is being formed. Hess discovered that new crust is produced at the mid ocean ridge and is pushed away from the ridge as new ocean crust (basalt) is created. Hess discovers that the Sea Floor is Spreading.

Sea Floor Spreading Hess reasoned that if: a)new crust is being created at mid-ocean ridges b)the Earth is not increasing in size Then crust must be destroyed somewhere else on the planet. Hess suggests the idea of subduction zones at the margins of some continents. Hess Sea-Floor Spreading resurrects Wegener waning Hypothesis of Continental Drift and leads to the larger idea of Plate Tectonics.

Ocean Floor Exploration Mapping of the ocean floor revealed the presence of mid-ocean ridges and trenches. Mid-ocean ridges constitute rift zones that form interconnected sub-sea mountain ranges that span the globe. The Glomar Challenger was the famous exploration craft used to sample the ocean floor.

Ocean Floor Exploration The following topographic features can be found on the seafloor mid-ocean ridge, continental slope, continental shelf, trenches, abyssal plains and volcanic islands.

Ocean Floor Exploration A cross-section of an ocean does not show a featureless topology. Exploration of the ocean floor showed the existence of mid-ocean ridges, continental shelves and deep trenches.

4)Patterns of Seismic Activity Seismic activity such as earthquakes or volcanoes do not occur in random locations. The vast majority of seismic activity occurs at the boundaries of plates (large sections of oceanic and continental crust). In fact, by plotting the location of major seismic events Trenches are deep troughs associated with active earthquakes zones and chains of volcanoes.

Location of Major Earthquakes

Location of Major Plates

Glaciation Patterns of Glaciation were matched across continents. When continents were joined the patterns made more sense.

Further work http://www.suu.edu/faculty/colberg/hazards/platetectonic s/platetectonics.html http://www.suu.edu/faculty/colberg/hazards/platetectonic s/platetectonics.html http://www.sciencecourseware.org/eec/Earthquake/ http://hyperphysics.phy- astr.gsu.edu/hbase/geophys/platevid.html http://hyperphysics.phy- astr.gsu.edu/hbase/geophys/platevid.html http://pubs.usgs.gov/gip/dynamic/developing.html http://www.ucmp.berkeley.edu/geology/tecmech.html http://scign.jpl.nasa.gov/learn/plate4.htm Sea floor spreading: http://www.suu.edu/faculty/colberg/hazards/platetectonic s/30_SeaFloor.html http://www.suu.edu/faculty/colberg/hazards/platetectonic s/30_SeaFloor.html

New Evidence Emerges 1) Paleomagnetism 2) Magnetic Pole Reversals 3) Magnetic Striping 4) Earthquake Mapping Still a mechanism is required

1)Paleomagnetism 1) Since the Earth has a liquid iron-nickel core, the planet generates a strong magnetic field (as seen in the diagrams at right) 2) When igneous rocks containing a magnetic element (iron, nickel and cobalt) cool from a liquid state, the grains of the element will align with the magnetic field.

Paleomagnetism 3) As a result the direction of the magnetic field at the time of their solidification is preserved. 4) These rocks can be used to pinpoint the direction of magnetic north at the time of solidification.

Paleomagnetism 5) Rocks that move across the globe due to continental drifting can be traced back to their original location. 6) This allows geologists to track the movement of continents over time. https://www.youtube.com/watch?v=uGcDed4xVD4 (Video showing the movement of continents over the past 600 million years and 100 million years into the future). https://www.youtube.com/watch?v=uGcDed4xVD4

2)Magnetic Pole Reversals https://www.youtube.com/watch?v=BCzCmldi aWQ (2:35) https://www.youtube.com/watch?v=BCzCmldi aWQ The direction of the Earths magnetic poles has flipped many times in the past. We know there have been about 170 magnetic pole reversals during the last 100 million years, and that the last major reversal was 781,000 years ago (see chart at right - black represents normal polarity).

2)Magnetic Pole Reversals These shifts leave traces in rocks. When lava cools, metal oxide particles within the rock become frozen in the direction of the prevailing magnetic field. It is believed that changes in the convection of the liquid outer core cause these flips to occur (as well as changes in strength of the Earths magnetic field) It is believed that the rate of reversals is increasing (possibly due to the increase in the size of the solid inner core) The layers of (iron-rich) lava show differing magnetic pole directions.

3)Magnetic Striping 1) The most important application of this property was done during Ocean Floor Mapping. 2) Magnetic detection devices called magnetometers (developed during WWII to detect submarines) measure the strength and direction of the magnetic fields found in iron-bearing rocks such as basalt.

3)Magnetic Anomalies 3) Geologists noted a pattern of stripes on the ocean floor of normal and reverse polarity. Most importantly, the patterns mirrored each other on opposite sides of a mid-ocean ridge. 4) Watch this brief YouTube video to see how this works: https://www.youtube.com/watc h?v=YIAXiE8RedA https://www.youtube.com/watc h?v=YIAXiE8RedA

3)Magnetic Anomalies 5) The magnetic striping leaves a distinctive pattern across all oceans and can be used to date ocean crust and sediments. 6) The oldest ocean crust is less than 250 million years old.

4) Earthquake and Volcano Mapping The Earths major tectonic plates and the location of volcanoes. Note that the vast majority are located on or near a plate boundary.

Earthquake Depths This depth vs distance chart (cross section) shows the focus of earthquakes along an ocean crust-continental crust boundary. This evidence gives the idea that slabs of ocean crust descend underneath continental crust. Crust is subducted into the Earths mantle.

Earthquake Depths The diagram shows a cross-section of the subduction zone off Japans east coast. Ocean crust is subducted under more ocean crust. Melting of the descending slab is responsible for the chain of volcanoes that form the Japanese islands. Suggest the rock type that forms these islands.

Putting the Evidence Together to Create a Theory https://www.youtube.com/watch?v=X9_TIf2navc (start at 18:00) https://www.youtube.com/watch?v=X9_TIf2navc https://www.youtube.com/watch?v=0KNqUwgqbZw (Bill Nye - start at 15:25) https://www.youtube.com/watch?v=0KNqUwgqbZw

Mantle Convection Mantle Convection Video (short 0:45) https://www.youtube.com/watch?v=p0dWF_3PYh4 https://www.youtube.com/watch?v=p0dWF_3PYh4 The mechanism that Wegener needed to drive continental drift is the convection of the mantle. The mantle is semi-molten and convects very slowly - this is the reason continents drift at an average rate of 5 cm/year. Rising mantle plumes drive sea-floor spreading and are also responsible for mid-plate hot spots like Hawaii and Yellowstone.

Sea Floor Spreading at Mid Ocean Ridges - New Crust Forms Why is ocean crust subducted under continental crust? (Why does continental crust float higher in the mantle)

Sea Floor Spreading at Mid Ocean Ridges - New Crust Forms Ocean crust (dark blue) and continental crust (grey) sit on a semi-molten layer called the lithosphere (brown). https://www.youtube.com/watch?v=Kpoko_l34ZE (good video showing mantle convection and discussing mantle layers. - 1:15) https://www.youtube.com/watch?v=Kpoko_l34ZE

Mantle Hot Spots - Hawaii Some volcanism occurs in the middle of plates. Rising mantle plumes are hot enough to punch through the mantle and produce volcanoes that rise from the ocean floor and in the case of Hawaiis big island, rise well above the surface.

Mantle Hot Spots - Hawaii These mantle plumes or hot spots remain stationary while the oceanic plate moves over the hot spot. As a result, the volcano loses its source of magma and becomes extinct and is carried (westward in the case of the Hawaiian Islands) and a new volcano forms. What type of rock do we expect to find in Hawaii? What type of volcano do we expect?

Mantle Hot Spots - Hawaii The older extinct volcanoes are eroded into smaller and smaller islands (often ringed by a lagoon) and eventually disappear beneath the surface and are called seamounts.

Mantle Hot Spots - Hawaii This oceanic map shows the Hawaiian island chain and the Emperor seamounts that were created by the same hot spot. The oldest seamount is 65 million years old. The oldest Hawaiian island is 7 mya. The change in plate direction occurred 42 mya Notice that the plate direction changed at some point in the past.

Mantle Hot Spots Yellowstone happens to be hot spot located beneath a continent. What type of rock do we expect to find at Yellowstone? What type of volcanism? https://www.youtube.c om/watch?v=bYv6V5EJ AKc (Great short video - Hawaiis hot spot volcanoes - 3:05) https://www.youtube.c om/watch?v=bYv6V5EJ AKc

Subduction Zones - Crust is Subducted and remelted into the Mantle When heavier basaltic ocean crust (3.1 g/cm 3 ) is forced beneath lighter (and thicker) granitic continental crust (3.1 g/cm 3 ), the subducting slab melts to produce volcanoes 50 - 300 km from the plate boundary. An ocean trench is formed where the slab begins its descent. The deepest trench (Marianas Trench) is almost 11 km deep. https://www.youtube.com/watch?v=K 8J7t3KYr9chttps://www.youtube.com/watch?v=K 8J7t3KYr9c (Model using sand to simulate mountain growth near suduction zones - very accurate)

Subduction Zones The west coasts of North and South America are these types of subduction zones Volcanoes occur from Alaska down the Rocky Mountains to the Andes Mountains. This whole region is at risk from volcanoes, earthquakes and tsunamis. The Cascadia Zone has been hit with massive tsunamis in the past. What type of rock do we expect? What type of volcanism?

Subduction Zones Most of the largest earthquakes (and subsequent tsunamis) are produced at subduction zones. https://www.youtube.com/watch?v =f-nIb8JkFrg (Subduction zone earthquakes - 2:50) https://www.youtube.com/watch?v =f-nIb8JkFrg https://www.youtube.com/watch?v =Yukp0bPkQxs (45 minutes - Cascadia Megaquake) https://www.youtube.com/watch?v =Yukp0bPkQxs https://www.youtube.com/watch?v =j0YOXVlPUu4 (Japanese tsunami) https://www.youtube.com/watch?v =j0YOXVlPUu4 https://www.youtube.com/watch?v =w3AdFjklR50 (Japanese tsunami - short video) https://www.youtube.com/watch?v =w3AdFjklR50

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Evidence For Continental Drift. Introduction As Scientific Theories go, Plate Tectonics is a relatively new scientific concept, introduced in the 1960’s