Geology: Plate Tectonics

Continental Drift

"It is just as if we were to refit the torn pieces of a newspaper by matching their edges and then check whether the lines of print ran smoothly across. If they do, there is nothing left but to conclude that the pieces were in fact joined this way." -Alfred Wegener. He is the father of continental drift and tectonic plates. It released the continents from the Earth's core and transformed them into icebergs of gneiss (granite) on a sea of basalt. It let them float and drift, break away, cracks, rifts, trenches remain; where they collided, ranges of folded mountains appear. Plates collide. Not accepted because he was German and fought in WWI. Also he was a meteorologist. Continental slopes fit and fossils read across - believed convection cells (fluids) that helped create tectonic movement. Died in Greenland and was buried in the ice sheet.

Continental Drift - Continental Fit

1. Continental Fit: based on fit of continents along the continental slope not coastlines. This is where the continental crust meets the oceanic crust. Similar metamorphic range rock belts - regional metamorphism - index minerals (same temperature and pressure). Position at which continental meets oceanic is static throughout time - matches. Coastlines are where the water meets the land. Shelf edge in Yukaton is where the Gulf of Mexico opened up. Coastline is far to the north where water meets the land - sea levels rise and fall all the time. Continental slope.

Hot Spots - Mantle Plumes

A hot spot is a volcanic center, 100 to 200km across and persistent at least a few tens of millions of years, that is thought to be the surface expression of a rising plume of mantle material (mantle plume). An aseismic ridge is a submarine ridge that is a fragment of continental crust, names so to distinguish it from the seismically active mid-ocean ridge. A trail of extinct and progressively older volcanoes delineating plate movement over a hot spot. Aseismic ridge means it has no earthquake activity and it is different than the mid-ocean ridge. Can get velocity of plate movement, look at radioactive isotopes to get date of cooling and crystallization. Anonymous - can't explain. Mantle plumes of heat burning hole in crust. Hawaii is in the middle of plate. Hotspot - area in upper mantle where hot material rises up, burns hole in crust, spews molten lava, eventually coming to ocean's surface and forming an island. Plate moves over the top of hotspot and as it does so, creates a ridge of volcanoes. The point in the middle of the plate is relatively stationary through time. New volcanic mountains/islands created as plate moves. Plate's been drifting Northwest for millions of years now. Hotspot is where volcano's erupting. Plate movement is not constant through time and can change direction. Hotspot beneath yellowstone plate movement moved North America over hotspot. Continent moves over the hotspot.

Divergent Plate Boundaries

Accreting Plate Boundaries, spreading ridges. A boundary between two crustal plates that are moving apart, with new oceanic-type lithosphere being created at the seam. Underlying mechanism of plate tectonics is mantle convection where density changes from heat differences. 3 ways that plates can move are spreading apart, coming together, and sliding past each other. In divergent plate boundaries, plates are spreading apart, accretion is adding material to the surface of the planet (usually new oceanic crust). Constructive. Plates pull apart and new material comes to the surface. Center at seam has mountain range underneath the ocean - longest chain of mountains on the planet. Spreading centers are divergent plate boundaries, in oceans now, oceans started out on continents. Rift zones. Split continent and spread it apart. Raise the ocean floor, hot material is low density and buoyant. Rift valley collapses and fills with water.

Alexander du Toit

Alexander du Toit - South Africa (1878-1948). Provided much of the support for Wegeners theory. Wrote Our Wandering Continents. Proposed a northern landmass 250 million years ago: Laurasia. England (excludes people that are different from them) lacks fossil evidence and coastlines don't seem to match up. Did geologic mapping of South Africa mountain rocks extend across coast to South America and had since parted. Laurasia and Gondwanaland = Pangea. Mapping of Africa provides evidence from the South.

Alfred Wegener

Alfred Wegener - Germany (1880-1930). Credited with the hypothesis of Continental Drift. Wrote The Origins of Continents and Oceans. Proposed that all landmasses were originally united in a single supercontinent that he named Pangea. Pangea is the 1st one that people know about. Southern and Northern were combined. Proposed the theory of Continental Drift to account for the breaking up of Pangea and the migration of the continents. Dedicated life to getting Continental Drift accepted. Spent life in very uninhabitable places to piece together puzzle, give him credit posthumously. Early maps are very similar to Pangea and how it broke up. Father of continental drift and plate tectonics. Spends most of time in Northern continents.

Antonio Snider-Pellegrini

Antonio Snider-Pellegrini - France (1802-1885). Wrote: Creation and Its Mysteries Revealed. Publicly (so gets credit) suggested that all of the continents were linked together in the past and later split apart. Based idea on similar fossils in 300 million year old coal in Europe and North America. Changed evidence to make it fit his theory. Dismissed his ideas because they overlapped and didn't fit perfectly. Coal boom - mapped where it was found in sedimentary rocks. Coal belts from England into North America that seemingly crossed the ocean. Ancient life deposited there. Believed rocks might make own fossils themselves.

Mid-oceanic ridge system

Buoyant magma rises and buckles the ocean floor creating faults, pipes of magma deliver basalt to the ocean floor where it cools into pillow basalts, pipes cool and spread away creating sheeted dikes. Accreting and constructive boundaries have sheeted dikes which are vertical fractures filled with mafic magma that freeze over time to form phaneritic igneous rocks. Old material is out by the continents. In ocean, new material is at the center from the lava. Pillow basalts with obsidian crust at the surface. Low-pressure zone is wear the rock melts easily and rises to surface - creates mountain chain.

Definitions

Continental Drift: Theory that the continents were joined as a single landmass that broke apart with the various fragments (continents) moving (drifting away) with respect to one another. It's not too far-fetched. Plate Tectonics: Theory holding that large segments of Earth's outer part move relative to one another. Segments=plates. Plate: An individual segment of the lithosphere (crust and top of the upper mantle, rigid plate) moves over the asthenosphere (plastic). Not just crust, reacts like a rock would. Compositional layers divided based on density. Differentiated into layers based on density. Nonferromagnesium crust made of Al, Na, and K are on the outside. Ferromagnesium rocks are in the mantle (Olivine). It's higher density and hotter temperature in the core and lower density and cooler temperature on the outside. Mantle is primarily Iron and very solid because of pressure. Portions are liquid with low pressure. Convection cells drive movement of plate. Glaciers are distinct from plate tectonics. Isolated continents floating through oceanic crust. Granitic continents attached to oceanic crust drifting of continents through time along with oceanic crust. Float on top of Earth, in asthenosphere. Contains oceanic and continental crust. Lithosphere has structural and mechanical layers based on stress. The asthenosphere is a weak layer, just below upper mantle and it behaves like a plastic.

Edward Suess

Edward Suess - Austria (born in England) (1831-1914). Documented similarities between 250 million year old fossils in India, Australia, South America, and South Africa, as well as evidence of ancient glaciation. Believed continents were connected by land bridges (over ocean, allow movement of animals and plants between continents), which explained fossil similarities. Proposed the term Gondwanaland (Gondwana) and Tethys Ocean. Gondwanaland were these southern continents as all one land mass. Tethys Ocean is the ocean between Northern and Southern continents that disappeared. Similar fossils found in all these southern continents. Fossil ferns today in tropical climates were found in Antarctica which means either climate or position is changing through time.

Plate Tectonics Summary

Evidence for Continental Drift: Fossils, Mountains, Glacial (South Africa - spread out radially - put together then reads across lines), Apparent Fit. Types of Plate Boundaries: Convergent, Divergent, Transform. Mechanisms of Plate Movement: Ridge Push, Slab Pull, Mantle Convection (overall, drives everything).

Frank Taylor

Frank Taylor - USA (early 20th century). Suggested that current continents where once part of larger polar continents. Suggested that mountains formed by lateral movement of the continents through "equatorial creep." Believed that continental movement was induced by tidal forces generated when "the Earth captured the Moon - 100 MYA." Suggested that the Mid-Atlantic Ridge marked the boundary where an ancient supercontinent broke apart.

Continental Drift - Glacial Evidence

Glacial evidence is seen in striations (carve out underlying rocks) show direction of flowing glacier. Glacial striations tell us direction of movement. Wegener lived in Greenland for a while. Glaciers are not static features. They are slow-moving rivers of ice moving uphill to downhill. These striations found in Madagascar, South India, and Africa which connects them back to polar glacier in South Africa. All connected over South pole where glacier is growing.

Where did Alfred Wegener die?

He died on the Greenland ice sheet, and was buried in the ice.

Continental Drift - Fossil Evidence

If there were land bridges, then the continents must have been close together. In land masses now separated by oceans, you get your Lystrosaurus which could not swim across oceans. Remains of terrestrial animals were found on different continents. Lystrosaurus is the cow of Pangea, a reptile-like mammal. The Mesosaurus is a lazy, freshwater creature that lies on banks of streams and wait for things to come drink and then eats them. They can't tolerate oceanic conditions because of the high saline content, and need rivers, streams, or land bridges. Fern fossils are found in many different continents, even Antarctica. They reproduce via spores which can become wind-borne.

Western United States

It has 3 Plate Boundaries: Divergent, Convergent, and Transform. Divergent example is Juan de Fuca plate off the coast of Washington and Oregon. Convergent slides beneath Cascades mountain ranges. Transform is lateral movement - transform at right angles to divergent boundaries. Andres (San Andres Fault) in North America SouthEast and Pacific Northwest are sliding against each other.

Earth's Magnetic Field

Magnetism is the physical property resulting from the spin of electrons in some solids and moving electricity. Dipolar refers to objects possessing two unlike magnetic poles. Magnetic Field is an area in which magnetic substances are effected by lines of magnetic force emanating from a magnet. This is why our compass needles point North. Thin - young crust. Thick - farther towards the continent (ocean constantly rains out sediments - accumulate critters) ran - older. How can they move through dense oceanic crust? Basalt is the igneous rock that contains magnetite, a magnetic mineral that aligns itself with magnetic fields when it crystallizes; records remnant magnetism in the rocks. North pole moves around slightly - sometimes flips (solid inner, liquid outer core). Iron-rich minerals/rocks such as magnetite, little needles in basalt act like compass needles, lock in Earth's remnant magnetism (show us what direction the rock was facing).

Driving Forces

Mantle Convection, Ridge Push, and Slab Pull. Gravity pushes down material. Heat source brings new material to the surface. Ridge Push is gravitationally high ridge pushes downward. Slab Pull is when a subducting plate edge drags down the plate (pulled down over overlying materials). Drags (dives into mantle and starts to melt). Plates don't move at constant velocity - depends on other plates' movement.

Plate Tectonics

More exciting stuff. Convection cells drive movement of plate. Reason why rock cycle occurs in first place. Why mountains/volcanoes/earthquakes occur where they do. Distribution of life.

More divergent plate boundaries

Opening up new oceans - Eastern Africa - East Africa Rift Zone. Linear lakes are formed where divergence takes place - forming islands - like Madagascar. Divergence boundaries to be seen today: Iceland (volcanic island being rifted apart by Mid-Atlantic Ridge). Iceland is gaining land still. North American and Eurasian plate. Mid-Atlantic rift zone is 50,000 miles long, longest mountain chain on planet.

Seven major plates

Plates do interact with each other and move relative to each other. Moving past, spreading apart, pushing together (mountain building). Plate boundaries often are associated with earthquakes, volcanoes, trenches, and mountains. North America + Greenland, Canada, North Pole and bits of North Asia are all on the North American pole. Tiny plate is the Arabian plate. What plate under continents is pretty straight-forward. Caribbean and then the South American plate. Smaller plates are named by places where they're found or by who found them.

Magnetic Reversals

Polarity reversals - polarity weakens until it is nonexistent, then reappears in the opposite direction. Normal polarity runs from South to North. Reversed polarity runs from North to South. Flip randomly (stable for random intervals). When polarity switches from north to south the compass needles point in the opposite directions. Reverse, normal, reverse, normal: flipped many times in the past. Seen in lava flows. Polarity weakens, gives out entirely and returns in other direction. Solar flares wipe out planet. Magnetic field keeps us safe. Record of lava flows - basaltic lava cools on surface and locks in direction, lava flow flip then points to the South.

Continental-Continental Plate Convergence

Portions of India subduct under Eurasia. India used to be attached to the oceanic crust and it collided with Eurasia which is connected to oceanic crust. Continental vs. Continental buckles land surface - raises Himalayan Mountains and Plateaus. These result in particularly nastly landforms.

Oceanic-Continental Plate Convergence

Predictable configuration. Jaun de Fuca vs. Pacific. Cascades on Northwest Coast of North America. Volcanoes around Seattle, Washington, and Oregon. Oceanic will subduct under continental crust. This is always true, because oceanic crust is more dense. Creates a mountain range (long, linear mountain belts), for example the Andes Mountains, Cascades, St. Helens, and Adam's.

Seafloor Spreading and Support of Theory

Seafloor Spreading: Harry Hess (1962) published a paper on the theory of seafloor spreading - theory that continents and ocean crust move together. Seafloor spreading fueled by thermal convection cells in the mantle. He discovered the structure of the ocean floor as a Captain he used a bathometer to figure out high places in coastline. Ocean floor is Fe-rich with Basalt. Sends out a ping that submarines can hear, which makes them an open target, endangering lives, but scanned the floor anyway. Support of theory is seen in paleomagnetism, age of oceanic crust, thickness of sediments, and age of sediments. Ridges are young and Continents are old. It spreads and now magma is formed in the middle. Igneous rocks lock in radioactive isotopes and you can look at the decay to determine age. Reverse, normal weakens then ridge then normal then reverse weakens. They are mirror images of each other. Mountains on ocean floor were discovered, it was originally thought to be flat. Raised portion - ridge - shallow area in middle of Atlantic Ocean. Inject molten rock up causing sea floor to spread, possibly moving continents. Heat source is the convection cells. Convection cells bring magma up to the surface at the mid-ocean ridge. At middle of ridge should be the young rocks which are pushed farther and farther away over time, until they get close to the continents and are then old rocks.

Continental Drift - Similar Rock Sequences/Mountains

Similar Rock Sequences/Mountains are not likely to be coincidence. On the edges of the continents, they have the same age rocks.

Theory of Plate Tectonics

Simple Earth composed of a rigid lithosphere, including oceanic and continental crust, as well as the adjacent upper mantle, which consists of numerous plates. Plates vary in thickness from 100 to 250 km thick. 100km is the thin oceanic crust by the ridge. There are seven major plates: Eurasian, Indian-Australian, Antarctica, North America, South America, Pacific, and African. Convection in mantle makes new basalt material at the ridge which pushes apart oceans and plates (continents).

Continental Drift: Early Concepts

Sir Francis Bacon - England (1561-1626) Noted the similarity of the shorelines between western Africa and eastern South America. Postulated, but not accepted (reluctantly accepted). Entire surface of Earth is in motion, dynamic on longer time-scale. Shorelines fit together, but didn't say anything. Were once one continent that split apart; don't match perfectly, weren't mapped out correctly either.

Oceanic-Oceanic Plate Convergence

This is one of 3 types of convergent plate boundaries. This one formed the Islands of Japan. Marianas Trench is the deepest portion of the ocean. Dark-dark blue at the deep portions at convergence. Oceanic and Oceanic crust slamming together. Either one could subside the other. Usually the one with the major continent on it does NOT subduct. Water-rich basalt slides down into asthenosphere, spews out lava on ocean floor and builds a chain of volcanoes - volcanic island arc.

Transform Plate Boundaries

Transform Faults occur by sliding past on surface of planet. A plate boundary that ideally shows pure strike-slip displacement. Conservative boundaries. Plates sliding past each other. Dynamic Metamorphism. Conserving Earth material - nothing is being destroyed or created.

Convergent Plate Boundaries

Zones of Plate Destruction. A boundary between two crustal plates that are moving together, where one plate subsides under another plate. Benioff Zone - dipping plane of earthquake foci that defines the plane of subduction. Crushed together, destroying old earth, recycling rocks to create new rocks all occur when plates smash together. Associated with tall mountains (Mt. Everest is the tallest at 30,000 ft. above sea level), deep trenches (drags down material with it), volcanoes, and earthquakes. Earthquakes and tsunamis. Marianas Trench is the deepest place on Earth. 1 plate dives beneath the other; the more dense plate - oceanic crust, will dive under. Plate subducts beneath another plate - it is not gentle - tends to get stuck, frictional force impedes movement. Finally have a slip creating a massive megathrust earthquake with a quick drop of the subducting plate. Volcanism is associated. Benioff zone is a spot where earthquakes are located beneath the surface.