Plate Tectonics

What is the plate tectonics theory?

What is the plate tectonics theory?

earth's surface is broken up into a number of plates that are in constant motion relative to each other through geologic time 

What is Mount Rundell?

• Made up of limestone 

• Limestone is formed by deposition of carbonate minerals in the ocean 

  • In shallow ocean basins 

  • Contain fossils that live in oceans, coral fragments 

• Original material was deposited below sea level and now it is up high

  • Large force 

• to early geologists, the apparent force that was involved in pushing up rocks to form mountains were unthinkable

What was the contracting earth theroy?

• The earth contracted or shrank over geologic time due to gradual cooling 

• cool=contracts 

• Contractions is how the mountain forms 

• wrinkles=ocean basins and mountains 

• No evidence that the earth shrank, and doesn't explain many things about t

Who was Frances Bacon?

• while reviewing the first maps of the coastlines of Africa and South America noted that the outlined of the continents appear as if they could fit together 

• In 1858 Antonio Snider-Pellegrini made the following “before and after” maps of South America and Africa, suggested they were together at some point  

  • The scientific community however dismissed these observations considering them a strange coincidence 

Who was Alfred Wegener?

• Meteorologist and Greenland Explorer

• Through field works noticed the connection between the different continents 

• Evidence from glacial deposits 

  • Contains sediments 

  • Melting, deposit materials in glacial deposits 

  • Were glaciers that existed in the past 

• Glaciers exist where its cold, top and bottom, high latitude  

  • Periods on earth where global glaciation occurs 

What is the “jigsaw” fit?

This “jigsaw” fit of continental margins is best when the continental shelves instead of continental edges are used for the match 

What was the glacial evidence?

• Evidence of permian glaciers (25o mya) found on 4 continents and some of this evidence is now found near the equator 

• Distribution of glacial deposits converge near south pole when the continents are regrouped 

• Suggests all these continents were at some point connected together and closer to the south pole 

What do glacial striations indicate?

• Glaciers carry large rocks at the base, create glacial striations, can indicate ice movement 

  • The distribution of glacial striations in glacial deposits makes more sense if the continents were once together 

    • Movement towards the interior of the continent 

    • Continents dispersed after initial assembly 

What is some other paleoclimate evidence?

• Predicted rocks preserving climate belts 

• Tropical coals 

• Tropical reefs

• Subtropical deserts 

  • Subtropical evaporites: mineral deposits that are formed through evaporation 

  • Have to have desert like climates over long periods of time for water to evaporate and deposit minerals 

How was the prediction proved?

• Looked at evidence for climatic belts in different rock deposits 

• Sand dunes, in subtropical regions 

• Coal deposits in an equatorial belt and coral reefs 

• Prediction that if continents were together, can see different climatic zone based on deposits 

What was the fossil evidence?

• Remains of extinct organisms 

• Identical fossils found on widely separated landmasses

• Looked at distribution 

  • Found fossils of an animal that can’t swim but was found on lots of continents (Lystosaurus)

• Look at pic 

• Seed too heavy to move by wind, continents were together

What was importnat about matching geologic units?

• distinctive rock assemblages match across the Atlantic

  • Rock found in africa and south america, match to what we find in other continents

  • geologic structures

  • rock types

  • rock ages

What was evidence found in mountain belts?

• Appalachian 

• Paladonia mountains-greenland and scandinavia 

  • Formed at same time as other but now separated 

• But were continuous in the reconstruction 

What is continental drift?

a hypothesis that continents were connected in the past, and then drifted apart

Why was Wegener’s idea rejected?

• Paleoclimatic evidence was explained movement of the poles rather than the continents 

• Fossil–explained by land bridges, rafts, or stepping stones

What was Wegener’s 1 problem with his hypothesis?

• Could not come up with the mechanism for how they would have moved 

• thought they plowed through the ocean floor and move-but not a viable explanation

What was the further evidence that came around after Wegenr’s death?

Geophysical exploration of Ocean basins

• magnetic patterns

• heat flow patterns

• topography

What can earths magnetic field be approximated as?

as a big bar magnet with a dipole—north and south pole

What are the 2 components of the earths magnetic field?

• the direction of magnetic ‘pointing’ (compass and detect at surface) 

  • angle between the magnetic north of the compass and the true north

• inclination of this with the earths surface

  • magnetic inclination goes from nearly horizontal at the equator to vertical at the magnetic pole

  • angle between the earth's surface and the magnetic field lines

What is Paleomagnetism?

• many rocks record and retain earths magnetism when they formed—paleomagnetism is the the study of this

• as rock cools, dipoles align with earths magnetic field. With more cooling, dipoles lock into this orientation

• Magnetite—Made up of iron-magnetic 

  • If a rock contains magnetite, will aline its north and south pole parallel to the earth's magnetic field 

  • At high temps the magnetic alignment isn’t there. But as it cools, align parallel 

• Go back and look at a rock formed millions of years ago and see the direction of the magnetic field 

How can paleomagnetic rocks be measured?

• by a magnetometer

• 3 features of earths magnetic field can be measured

  • direction of magnetic poles

  • inclination

  • strength of the magnetic field (In particular rocks)

Position of Paleo-pole for North America in Permian

• Looked at rocks 

• That pole direction of those magnetic-pointed to a pole position that was different than the earth's current direction 

• 2 ways to explain

  • Pole might have moved in the past 

  • The continent moved in the past 

Why is it more likely that the continents moved?

• If poles moved: than if you look at rocks same age at diff continents should point to same pole at the time, but had different pole position

• Continents must have moved 

What is the apparent pole wander path?

• percieved movement of the earths magnetic poles relative to a continnet while regarding the continnet being studied as fixed in position

  • why poles weren’t in the same position when measured by magnetism in rocks when they formed

• but it is now seen that the continents have moved

What is Bathymetry of Ocean Basins?

• the measure of depth of water in oceans, seas, lakes, and rivers. Bathymetry data is used to create maps (called "charts") of the seafloor

• Measurements using sonar–sound waves from ship down to floor and reflect back

• bathymetric profile along line X-X’ illustrates how mid-ocean ridges rose above abyssal plains—both are deeper than continental shelves

What is heat flow in ocean basins?

• Found that heat flow from mid-ocean ridge is really high

• Mid ocean ridges have high heat flow compared to oceanic crust away from them 

• But move away, heat flow becomes lesser and lesser 

  • Systematic pattern 

Who has Harry Hess?

• suggested that mid-ocean ridges where there are mountain chains and high heat flow is where the ocean floor is created by submarine volcanoes 

• Mid-ocean ridge birthplace of oceanic crust

  • Magma, forms new oceanic crust from magnetism–explains high heat flow  

• older crust pushed away with time as new crust is formed and as it cools has less and less heat flow

• Sea floor spreading model 

What is the seafloor spreading model?

• mid-ocean ridges represented tectonic plate factories, where new oceanic plate was issuing from these long volcanic ridges

• Provides mechanism for movement of ocean floor and continents 

  • New ocean floor forming between, continents have to move for ocean floor (move apart)

  • Somehow Oceanic crust is consumed, continents are brought together 

What was the evidence of sea floor spreading?

• Magnetic strength measurements on the ocean floor 

  • strange magnetic anomalies that formed a striped pattern of symmetrical rows on both sides of mid-oceanic ridges. What made these features unusual was the north and south magnetic poles within each stripe were reversed in alternating rows

  • Positive anomaly: strength is higher and Negative: strength is lower

What causes these anomalies?

• Magnetic anomalies show a striped pattern symmetric about mid-ocean ridge 

• Causes: moving symmetrically away from mid ocean ridge 

• Origin of magnetic anomalies has been attributed to changes in the dipole of earth's magnetic field in the past 

  • switch polarity at certain times 

  • Normal polarity-traveling south and goes up north (magnetic north and south)

What is the origin of magnetic anomalies in rocks?

• Positive: because magnetometer is measured the magnetism kept in the rocks and earth's current magnetic field. MF and sea floor have same polarity, the measured strength is sum of those 2 rocks formed when MF is in the same direction it is today 

• Negative: Sea floor magnetic some time in the past and the polarity is opposite of what it is today, cancel each other because of difference. When rocks form, Earth's polarity is opposite 

  • earth and the sea floor have opposite polarity

What is the origin of Magnetic Anomaly pattern from sea floor spreading?

• New oceanic crust formed at Mid ocean ridge will have neutral polarity

  • Ocean floor gets older as you move away from the mid ocean ridge

• Thicker band–retained that polarity for a longer period of time 

• Sea floor spreading:Ocean floor older as you move away from the mid ocean ridge

What does the thickness of sediments on the ocean floor indicate?

• thinner sediment layers are located close to mid-ocean ridges, indicating the ridges were younger than the surrounding ocean floor

  • This finding supported the idea that the sea floor was not fixed in one place.

• as you move away from the mid ocean ridge the thickness increases (the sea floor gets older as you move away from the mid ocean ridge)

• Accumulation time-older so more time to accumulate 

• Supports idea of getting older

How to determine the age of oceanic crust?

• Determine age using radiometric dating and magnetic stripes 

• 180 mya for the oldest ocean floor rocks–far NW of pacific ocean 

What are the key observations of the earth?

Surface of the earth is dynamic–not static, movement of continents and oceans  

When did the plate tectonic theory evolve?

• evolved in the 1960’s and became an integral model for the working of earth

  • Wegner–evidence supporting continental drift 

  • Hess/Dietz–the sea floor spreading hypothesis 

What are plates?

• Earth's outer layer is broken into rigid plates that constantly move relative to each other 

• There are ~20 tectonic plates 

  • Half a dozen large plates–pacific plates

  • And small plates-Indian and Iranian 

• A plate may contain both oceanic and continental crust

What constitutes a plate?

• Rigid lithospheric plates moves on top of the plastic asthenosphere (move and act like a liquid–convects)

  • Riding on top of asthenosphere 

• 150-200 km thick under continents, 100 km thick under ocean 

• What drives the motion of the plates?

  • Because the mantle below-asthenosphere-is a convecting mantle 

  • Because it's moving and drags the plate with it 

  • Mid ocean ridge making new crust and gets dragged along 

What drives the motion of the plates?

• Because the mantle below (asthenosphere) is a convecting mantle 

  • it's moving and drags the plate with it 

• Mid ocean ridge making new crust and gets dragged along  (Convection of the asthenosphere)

• In simple terms, convection is the idea that dense, cold things sink, and buoyant, warm things rise. In the earth the cold sinking things are slabs (subducting plates) and the warm things are plumes, or just rising material from deeper in the mantle

What are the 3 kinds of plate boundaries?

• Divergent boundary 

  • Mid ocean ridge-creation of new crust-moving away from each other 

• Convergent boundary 

  • Moving towards each other 

• Transform boundary 

  • Plates are moving past each other 

  • Oceanic

  • Continental 

• Any time plates move you have earthquakes 

Example of a divergent boundary

• Mid ocean ridge

• Occurs in every ocean basin we have 

Example of a convergent boundary

• Western margin of south america 

• South american plate and Nazca plate 

  • Denser plate goes down-Nazca

  • But if density is similar-collision 

• Why earthquakes in Chile 

Example of a transform boundary

• Ocean

  • Mid ocean ridge-form zig zag line—55,000 km 

  • Offsets of the ridge 

  • Look at picture 

  • Some move past each other 

• Continental 

  • San Andreas fault in california 

    • North American plate and pacific plate

  • Causes major earthquakes 

  • Relative motion usually 

What is continental rifting (divergent boundary)?

• A location where the continental crust is being stretched

•  Stretch the crust, thin the crust and the mantle upwells and creates oceanic crust in between continents 

• And in extreme cases the crust will break and split the continents and create a new ocean 

• Produce new plate boundary 

Ex of continental rifting

• The arabian plate is rifting from the african plate 

  • Rifting has progressed to sea-floor spreading in:

    • The red sea

    • The gulf of Aden 

• Rifting continues along the east african rift 

  • Thinned crust

  • Elongate trough

  • Volcanoes 

How do we know where plate boundaries are?

Plate boundaries can be identified by:

• Map the Concentration of earthquakes 

  • Concentrate around plate boundaries 

  • Happen when large scale movement occurs 

• Using other features that develop at these boundaries 

How fast do plates move?

• Plates move continuously at a rate of 1-5 cm/year

  • Slow on a human time scale; extremely rapid geologically 

• Rates vary 

• About the speed at which our fingernails grow 

What is earths internal heat?

• Earth's temp comes from the sun

• Internal heat comes from radioactivity, earth's primordial heat (from when the earth formed, collisions that formed the planet–accretion), gravitational energy released during core formation(slowly escaping)

• Temp increases with depth (geothermal gradient)

  • Average 25 degrees celsius/km–lithosphere-conductive  

  • 0.3 degrees celsius/km-in asthenosphere–convective

  • Difference due to convection–heat transferred through vibrations 

• Pressure increases with depth 

What is the geothermal gradient?

• the increase in temperature with depth in the earth

• mid ocean ridge is a high geothermal gradient

What is decompression melting? (divergent boundary)

• Granite has a low melting point

• involves the upward movement of Earth's mostly solid mantle. This hot material rises to an area of lower pressure through the process of convection. Areas of lower pressure always have a lower melting point than areas of high pressure

• occurs at divergent boundaries where tectonic plates separate (melts the lithosphere)

  • rifting movement causes the buoyant magma below to rise and fill the space of lower pressure. The rock then cools into new crust.

What does the melting point of a rock depend on?

• Temperature

  • increases with depth, so melting is more likely to occur at greater depths

• Pressure

  • increases with depth, but increased pressure raises the melting temperature, so melting is less likely to occur at higher pressures

• Water

  • The addition of water changes the melting point of rock.

What occurs at Mid-Ocean Ridges?

• only places on earth that create new oceanic lithosphere

• Decompression melting in the rift zone changes asthenosphere material into the new lithosphere, which oozes up through cracks in the oceanic plate

• mantle upwells causing decompression melting to produce basalts

• magma crystallized and adds material to each plate

What happens at continental rifts?

• when lithosphere stretches and thins the asthenosphere rises and melts the lithosphere and magma erupts

• Generates the new oceanic floor between rifting continents–east africa 

• Depressurizing the mantle 

What are ocean continent convergent boundaries?

• as the oceanic lithosphere moves away from mid-ocean ridge its cooling down and the cooling increases the density of basalt lithosphere 

• when a dense oceanic plate meets a more buoyant plate, like a continental plate, and descends into the mantle (subduction)

  • denser plate dives under continental lithosphere

• Recycles oceanic lithosphere back into mantle—why earth's volume basically stays the same 

• Subduction zones-and are destructive boundary 

• Downgoing plate=subducting plate and Overriding plate=top plate 

How does subduction produce magma?

• As the denser tectonic plate subducts under the less-dense tectonic plate, hot rock from below can intrude into the cooler plate above. This process transfers heat and creates magma

  • the water in oceanic crust, when it gets heated, water is released and water flushes the mantle and the mantle is melted and produces basaltic magma 

• the immense heat and pressure push volatile materials like water and carbon dioxide into an area below the continental plate and above the descending plate

• The molten magma is more buoyant than the lithospheric plate above it and migrates to the Earth’s surface where it emerges as volcanism

What are the unique features associated with subduction?

• volcanic arcs-produced by magma that’s produced underneath the overriding plate (develop on overriding plate)

• Accretionary prisms–sediments that were deposited on the ocean floor, and as the plate is subducted, the sediments are sprayed on to the continent

  • Wedge shaped accumulation of sediments that is scraped off subducting plate and added to the overriding plate  

• Trench–form where the oceanic plate is bending down 

  • Long topographic depressions formed where the subducting plate bends (deepest trench is 11km deep) 

• Subduction zones are regions of intense earthquakes 

What do the Andes Mountains represent?

a continental arc developed on south american plate due to subduction of Nazca plate west of south american continental margin 

What are ocean-ocean convergent boundaries?

• 2 oceanic plates converge on each other 

• Colder/older plate (makes it denser) and will subduct under the warmer and younger plate 

  • Pacific plate under philippine plate 

• Sometimes forms trenches and islands or volcanoes on overriding plate 

  • Island arc–volcanic arc that forms when the 2 plates are oceanic 

• Japan

  • Pacific subducts under philippine plate, philippine plate subducting under eurasian plate

  • Japan is an island arc

What is a continent-continent convergent boundary?

• When 2 continental plates collide the crust crumples and thickens creating high mountains and a wide plateau–like the Himalayas  

  • Neither want to subduct because they are the same density 

• No magma melting in these zones

• Thicken the crust–melting occurs–magmas are melting the crust instead of mantle (granitic magma)

• Ex: Collision of India and Asia–continent-continent convergence 

What happens at transform boundaries?

• Lithosphere slides past; crust not created or destroyed 

  • Many transforms offset spreading ridge segments

  • Some transformations cut through continental drift

• Relative motion, no magnetism

  • But still causes earthquakes

• No magma production 

What drives plate motion?

• Mantle convection-upper mantle hotter, move like fluid

• Ridge push

  • occurs at mid-ocean ridges as the result of the rigid lithosphere sliding down the hot, raised asthenosphere below mid-ocean ridges—Push oceanic plate 

  • Caused by gravitational push provided by the high topography of mid ocean ridges 

• Slab pull

  • subduction

  • Downward pulling force resulting from basalt transforming to eclogite 

    • Rate increases as more is produced