Unit 6: Plate Tectonics

differences between continental drift hypothesis and plate tectonic theory

Continental Drift

• Continents move independent of oceanic crust, even plough through oceanic crust

• no known why/how

Plate Tectonic

• Tectonic plates with continents and ocean crust move as one slab across asthenosphere

• driving mechanism: convection including mantle plumes, slab pull & ridge push

similarities between continental drift hypothesis and plate tectonic theory

• plates move around

• pangea happened

• tell you that the earth’s surface changes and moves

number of tectonic plates

7 big (definite) + 13 small = roughly 20

Mid-ocean Ridge

• form at divergent plate boundaries

• form as result of seafloor spreading

• warm mantle rises and fills in the separated part between divergent boundaries

Pole Reversal

when the magnetic north pole switches from being at the geographical south pole to being at the geographical north pole, or the opposite

Frederick Vine and Drummond Matthew

scientists that interpreted the data that showed bands of equal with and polarity as products of steady creation of new ocean crust over geologic time

Reverse Polarity

when the magnetic north pole is geographical south pole

Normal Polarity

when the magnetic north pole is geographical north pole

Paleomagnetism

• Earth is a giant magnet with a magnetic field and poles

• Every 200-300k years magnetic poles of Earth reverse places

• Today we are at Normal Polarity but when the South pole is the magnetic pole, it’s called Reversed Polarity

evidence that supports seafloor spreading

• bands of equal width and polarity

• mid ocean ridges

• age of ocean floor

Basalt

• makes up oceanic crust

• denser than continental crust

Granite

• makes up continental curst

• less dense than oceanic crust

Peridotite

• makes up mantle

• denser than continental AND oceanic crust

Collisional Mountain Range

result of convergent cc boundary

Continental Volcanic Arc

formed from formation of mountain ranges on the continental crust side due to convergent O-C boundaries

Convergent Boundary

• →←

• when plates move together

Divergent Boundary

• ←→

• plates move apart

• Place where new crust is created at rift valleys or mid-ocean ridges

• Mild volcanic eruptions and shallow earthquakes

Transform Boundary

• ↑↓

• tectonic plates move horizontally past each other

• Earthquakes (shallow)

• No up or down motion

• No volcanism

• No subduction

O-C convergent boundary

• Oceanic crust subducts and pushes continental crust upward

• forms mountain ranges on continental crust side (continental volcanic arcs)

• forms trench in the ocean floor where subduction begins

• explosive volcanoes and earthquakes (shallow and deep)

C-C convergent boundary

• Overriding of one continental plate over another

• Formation of collisional mountain ranges

• Earthquakes are common (shallow and deep)

• No volcanoes

• No subduction of continental crust here

O-O convergent boundary

• older oceanic crust is usually more dense and subducts

• forms of a volcanic island arc

• Formation of trench in the ocean floor where subduction begins

• explosive volcanoes and earthquakes (shallow and deep)

Lithosphere

• mechanical layer of earth

• crust and the upper part of the mantle

• rigid, brittle, elastically (snaps)

• solid

Asthenosphere

• mechanical layer of earth

• upper part of the mantle

• between lithosphere and mesosphere

• has plasticity (flows)

• solid

Mesosphere

• mechanical layer of earth

• part of mantel below asthenosphere and above outer core

• peridotite

• solid but flows slowly

Outer Core

• mechanical layer of earth

• below mesosphere, above inner core

• liquid

Inner Core

• mechanical layer of earth

• most inner layer, below outer core

• solid

Crust

• Compositional layer of earth

• oceanic and continental

• thin, solid, brittle

• outermost layer

• oceanic and continental

• basalt and granite

Core

• Compositional layer of earth

• iron nickel alloy

• inner and outer

• innermost layer

• densest and hottest layer

Mantle

• Compositional layer of earth

• solid but has plasticity,

• upper and lower

• between core and crust,

• has convection currents

• peridotite

Moho

• a discontinuity where seismic waves accelerate

• boundary between the crust and mantel

• thicker under continents

moho full name

Mohorovičić Discontinuity

discontinuity

where seismic waved change velocity

Pillow Basalt

when basaltic lava erupts underwater and cools quickly to form round bulges

Ridge push

• gravity driven force that results from elevated position on oceanic ridge

• slabs of lithosphere slide down flanks of ridge

• less common than slab pull

Slab pull

subduction of oceanic lithosphere into asthenosphere

Hot Spots

• rising plume of hot mantle

• plumes stay, plates move, volcanic island chains created

• reason why some volcanos are not on plate boundaries

• located randomly in mantle

Tectonic Plate

pieces of rigid solid lithosphere that move around on top of the asthenosphere

Harry Hess

• navy admiral turned seafloor-studying geologist

• gained interest and questions about young oceanic crust and lack of sediment on seafloor from navy travels

• published seafloor spreading hypothesis with Tharp’s + Heezen’s research

• revived interest in Wegner’s pangea

Alfred Wegener

• german meteorologist and interdisciplinary scientist

• wrote origins of continents and oceans

• proposed pangea

• made continental drift hypothesis

Marie Tharp and Bruce Heezen

• mapped the seafloor using new sonar technology

• found that seafloor was not flat

• found that a huge mountain ranged runs through middle of atlantic

Sonar

technology that uses sound waves to measure depth. one measures how long it takes them to bounce back

Rift Valley

• result of divergent plate boundary

• second stage of rifting (before formation of new ocean)

• often forms large deep lakes

Seafloor Spreading

• As the plates move away from each other, asthenosphere rises to surface and melts (lower pressure). Once it cools, new ocean crust is created.

• The continents “grow” apart as new crust forms between them

• driven by convection in mantle

why the plates move

convection currents in mantle

Subduction

• sinking of lithosphere into the asthenosphere

• denser plate pushed under

• happens at convergent boundaries

Trench

• forms due to subduction

• form at O-O and O-C boundaries

Volcanic Island Arc

form at O-O convergent boundaries from the melting of the subducted oceanic lithosphere

Plate Tectonics Theory

• Tectonic plates with continents and ocean crust move as one slab across asthenosphere

• driving mechanism is convection including mantle plumes, slab pull & ridge push

• Evidence: moving plates supported by age of ocean floor, shape of ocean floor, geomagnetic reversals on ocean floor, hotspot chains, locations of earthquakes and volcanoes

Pangea

supercontinent proposed by Wegner that split 200 millions years ago

Continental Drift Hypothesis

• Continents move independent of oceanic crust, even plough through oceanic crust

• No science based driving mechanism

• Evidence: if they used to be together and now they are not… Coastlines matchup, Fossils, Mountain ranges and rocks and glacial evidence

Upwelling

the force of convection that pushes a divergent boundary up and out

Failed rift

• when a plate starts to tear apart and for some reason it stops

• leaves scar stuck at the beginning of the rifting process

kinds of convergent boundaries

• C-C

• O-C

• O-O

where trenches form

O-C convergent and O-O covergent boundaries

where volcanic island arcs form

O-O convergent boundaries

where continental volcanic arcs form

O-C boundaries

where mid-oceanic ridges form

divergent boundaries

where rift valleys form

divergent boundaries

where narrow seas form

divergent boundaries

where oceans form

divergent boundaries

where collisional mountain ranges form

C-C convergent boundaries

where earthquakes occur

transform, divergent, and all convergent boundaries

where volcanos occur

all convergent boundaries

boundaries that create lithosphere

divergent boundaries

boundaries that destory lithosphere

all convergent boundaries

Compositional layers of earth (in to out)

core, mantle, crust

mechanical layers of earth (in to out)

inner core, outer core, mesosphere, asthenosphere, lithosphere

how fast tectonic plates move

how fast fingernails grow

Wegener’s 4 lines of evidence for the Continental Drift hypothesis

• continents fit together like puzzle pieces

• fossils match and solves fossil problems

• rocks and mountain ranges match

• solves paleoclimate problems (glacial striations)

 the great weakness of the continental drift hypothesis

there was no explanation/science to why they moved or how

what Harry Hess contributed to our knowledge about plate tectonics

mid-Atlantic Ridge was a spreading center (divergent boundary), a place where two “plates” move apart and it was driven by convection in mantle.

• why so little sediment accumulated on the ocean floor

• why there are fossils no more than 180 million years in oceanic crust

what Vine and Matthews contributed to our knowledge about plate tectonics

supported Hess’ sea floor spreading by interpreting the magnetic stripes as products of steady creation of new ocean crust over geologic time

what  Tharp and Heezen contributed to our knowledge about plate tectonics

they made a map of the depth of the seafloor which found that it was not flat and that there was a mountain range running down the middle of each ocean

5 pieces of evidence for plate tectonics theory

• Shape of the Ocean Floor

• Age of the Ocean Floor

• Locations of Volcanoes and Earthquakes

• Hotspots

• Paleomagnetism

how the moho was discovered

• in 1909, dude realized that velocity of a seismic wave relates to density of material is moves through

• he interpreted the acceleration of seismic waves observed within Earth's outer shell as an interior compositional change

• acceleration must be caused by a higher density material being present at depth

• calculations determined that the oceanic crust and continental crust are underlain by a material which has a density similar to an olivine-rich rock such as peridotite.

slope of O-O convergent boundary

steep

slope of O-C convergent boundary

not steep

steepest sloped convergent boundary

O-O of similar age and densities

how steepness of subducting plate slope of is affected

if the two plates are of similar densities, then the slope will be steeper