Global Plate TectonicsGlobal Plate Tectonics

The Theory of Plate Tectonics was a revolutionary The Theory of Plate Tectonics was a revolutionary theory formulated in the 1960s to explain continental theory formulated in the 1960s to explain continental drift and the geologic history of the Earth drift and the geologic history of the Earth

Basic to the theory is the fact that the Earth’s Basic to the theory is the fact that the Earth’s diameter has remained stable for hundreds of diameter has remained stable for hundreds of millions of years and the idea that the Earth’s surface millions of years and the idea that the Earth’s surface is divided into platesis divided into plates

Global Plate TectonicsGlobal Plate Tectonics

Plate Tectonics = Sea Floor Spreading + Plate Tectonics = Sea Floor Spreading + Continental driftContinental drift

These two concepts came from separate sets of These two concepts came from separate sets of observationsobservations

The Plate Tectonic theory:The Plate Tectonic theory: combined these ideascombined these ideas answered “all” our questionsanswered “all” our questions provided incredible predictive POWER!provided incredible predictive POWER!

Plate Tectonics: The Unifying Plate Tectonics: The Unifying TheoryTheory

Oceans and continents are geologically and Oceans and continents are geologically and morphologically distinctmorphologically distinct

These These geologic differencesgeologic differences between rocks are between rocks are responsible for the differences between responsible for the differences between continents and ocean basins. Specifically:continents and ocean basins. Specifically: The The compositioncomposition of the crust of the crust underlying the underlying the

continents and the oceancontinents and the ocean The The thicknessthickness of the crust of the crust underlying the continents underlying the continents

and the ocean and the ocean

Geologic Differences Between Geologic Differences Between Continents and Ocean Basins Continents and Ocean Basins (Cont.)(Cont.)

Continental crust consists of Continental crust consists of granitegranite Igneous in originIgneous in origin Light in colorLight in color Mostly oxygen, aluminum, siliconMostly oxygen, aluminum, silicon

Oceanic crust consists of Oceanic crust consists of basaltbasalt Igneous in originIgneous in origin Dark in colorDark in color Rich in oxygen, silicon, and magnesiumRich in oxygen, silicon, and magnesium More dense than graniteMore dense than granite

Geologic Differences Between Geologic Differences Between Continents and Ocean Basins Continents and Ocean Basins (Cont.)(Cont.)

Essentially all seafloor is composed of volcanic Essentially all seafloor is composed of volcanic basalt basalt

Continents are composed of a variety of rocksContinents are composed of a variety of rocks The density of graniteThe density of granite

is 2.7 to 2.8 g/cmis 2.7 to 2.8 g/cm33

The density of basalt The density of basalt is 2.9 g/cmis 2.9 g/cm33

Plate Tectonics IntroductionPlate Tectonics Introduction

But what causes and maintains these differences?But what causes and maintains these differences? The Plate Tectonic Theory has unified much of The Plate Tectonic Theory has unified much of

geologygeology not just marine geology; all geologynot just marine geology; all geology allows us to understand, explain and predictallows us to understand, explain and predict

Is a new theory:Is a new theory: proposed a few decades agoproposed a few decades ago generally accepted in the 70’sgenerally accepted in the 70’s

We’ll consider continental drift, seafloor spreading, We’ll consider continental drift, seafloor spreading, and plate tectonics, and then apply them to and plate tectonics, and then apply them to understanding the earth’s morphology and dynamicsunderstanding the earth’s morphology and dynamics

Continental driftContinental drift

The theory of continental drift was proposed in 1915 The theory of continental drift was proposed in 1915 by by Alfred WegenerAlfred Wegener to explain the earth’s history. to explain the earth’s history.

The hypothesis was based on geologic and The hypothesis was based on geologic and paleontological (fossil) evidence.paleontological (fossil) evidence.

According to the theory a single continent, Pangaea According to the theory a single continent, Pangaea existed more than 200 million years ago.existed more than 200 million years ago.

Continental Drift (Cont.)Continental Drift (Cont.)

PangaeaPangaea is the name given to the is the name given to the large landmass large landmass It broke apart 100 to 150 million years ago. It broke apart 100 to 150 million years ago. The fragments became the continents.The fragments became the continents.

Wegener’s theory did not consider Wegener’s theory did not consider how the continents drifted apart.how the continents drifted apart. This was his downfallThis was his downfall he envisioned continents plowing through seafloorhe envisioned continents plowing through seafloor

PangaeaPangaea

Continental drift (cont.)Continental drift (cont.) There are several lines There are several lines

of evidence for of evidence for continental drift, continental drift, including:including:

1: “map fit”1: “map fit” when you reposition when you reposition

the continents on a the continents on a globe the all fit globe the all fit togethertogether

This was done by Sir This was done by Sir Edward BullardEdward Bullard

the fit is stunning!the fit is stunning! LOTS of strange LOTS of strange

theories to explain this theories to explain this prior to PTprior to PT

Continental Continental DriftDrift

Later map fits Later map fits were betterwere better

This one uses the This one uses the 1,000m isobath, 1,000m isobath, which is a good which is a good metric for where metric for where

the the continent continent ends.ends.

Note that we aren’t Note that we aren’t talking about talking about Plate Plate BoundariesBoundaries (yet). (yet).

Continental Drift (cont.)Continental Drift (cont.)

Wegener’s bold new hypothesis was ignored by most Wegener’s bold new hypothesis was ignored by most for more than 50 years.for more than 50 years.

Many objected to the idea that granitic crust could Many objected to the idea that granitic crust could plow into basaltic crust.plow into basaltic crust.

To understand the concept of continental drift, we To understand the concept of continental drift, we can look at the motion of the continents over time in a can look at the motion of the continents over time in a moviemovie

Drift 1 Drift 2

Continental drift (cont.)Continental drift (cont.)

2: glacial scarring:2: glacial scarring: glaciers scratch the rocksglaciers scratch the rocks these patterns “match” on opposing continentsthese patterns “match” on opposing continents must have happened before breakup of original continentmust have happened before breakup of original continent

Glacial Scarring Glacial Scarring is pretty easy to is pretty easy to spot where it spot where it has been has been preserved.preserved.

Continental DriftContinental Drift When you look at When you look at

specific trends, it specific trends, it is clear that the is clear that the continents were continents were connected during connected during the glaciationsthe glaciations

This map was This map was made by made by Wegener in 1915!Wegener in 1915!

No wonder he No wonder he believed in CD!believed in CD!

Continental drift (cont.)Continental drift (cont.) 3: geologic 3: geologic evidenceevidence Rock typeRock type

ageage mineralmineral

ogyogy tectonic tectonic

trendstrends faultsfaults failed failed

riftsrifts

Continental DriftContinental Drift

Rock AgesRock Ages

Continental drift (cont.)Continental drift (cont.)

4: Fossils4: Fossils identical fossils found on continents which are now separateidentical fossils found on continents which are now separate must have been connected at come point in the distant past:must have been connected at come point in the distant past:

The “land bridge” hypothesisThe “land bridge” hypothesis can determine age of separation by dating fossilscan determine age of separation by dating fossils

Continental drift (cont.)Continental drift (cont.)

4: Fossils4: Fossils These fossils were keyThese fossils were key they existed on 4 continents which are currently widely separatedthey existed on 4 continents which are currently widely separated indicated a previous link AND changes in climate and / or positionindicated a previous link AND changes in climate and / or position

Continental DriftContinental Drift 5) Polar “Wandering”5) Polar “Wandering”

using either climatologic or using either climatologic or magnetic evidence, you can magnetic evidence, you can show that RELATIVE show that RELATIVE positions of poles have positions of poles have changedchanged

part of this is due to real part of this is due to real motion of poles but results motion of poles but results from different continents from different continents disagree, so...disagree, so...

most of it is due to motion most of it is due to motion (rotation) of continents (rotation) of continents relative to magnetic polesrelative to magnetic poles

If you backtrack continents to If you backtrack continents to their former positions:their former positions:

the discrepancies go awaythe discrepancies go away you an plot the actual you an plot the actual

position of the poles through position of the poles through timetime

Sea Floor Sea Floor SpreadingSpreading This is a separate body of evidence This is a separate body of evidence

which was later combined with which was later combined with continental drift to produce the unified continental drift to produce the unified concept of Plate tectonicsconcept of Plate tectonics includes creation and destruction of includes creation and destruction of

sea floorsea floor didn't really involve the concept of didn't really involve the concept of

plates and didn't connect to continental plates and didn't connect to continental motion.motion.

Seafloor SpreadingSeafloor Spreading

Vening Meinesz found large negative isostatic gravity Vening Meinesz found large negative isostatic gravity anomalies at trenches, and invoked convection as an anomalies at trenches, and invoked convection as an explanation for such the cause of such downwarps or explanation for such the cause of such downwarps or tectogenes.tectogenes.

Harry Hess, a Naval officer (1962), and Robert Dietz (1961) Harry Hess, a Naval officer (1962), and Robert Dietz (1961) finally put it all together when sufficient marine geological and finally put it all together when sufficient marine geological and geophysical data were gathered.geophysical data were gathered.

perfected ideas relating to the transport of crust by convective perfected ideas relating to the transport of crust by convective currents, proposed that new crust is generated at the Mid currents, proposed that new crust is generated at the Mid Ocean Ridges (MOR's).Ocean Ridges (MOR's).

Seafloor SpreadingSeafloor Spreading Evidence: 1) Evidence: 1)

TopographyTopography Ewing and Heezen at Ewing and Heezen at

Lamont Doherty Lamont Doherty Geological (now Geological (now “Earth”) Observatory “Earth”) Observatory discovered the discovered the continuity of the MOR's' continuity of the MOR's' central position.central position.

Deepest “should” be in Deepest “should” be in middle but wasn’tmiddle but wasn’t

Trenches at very edgesTrenches at very edges

Sea-Floor Spreading Sea-Floor Spreading (Cont.)(Cont.)

The midocean ridge was discovered in the mid-20th The midocean ridge was discovered in the mid-20th century from soundings of the deep sea-floorcentury from soundings of the deep sea-floor

It forms a continuous mountain range girdling the It forms a continuous mountain range girdling the earthearth ~60,000 km long~60,000 km long ~3 to 4 km high~3 to 4 km high

Geologically activeGeologically active Volcanism, Volcanism, earthquakes, earthquakes, faultingfaulting

Sea-Floor Spreading Sea-Floor Spreading (Cont.)(Cont.)

Although ridge systems resemble Although ridge systems resemble mountain chains they are formed in mountain chains they are formed in different ways.different ways. Mountain chains Mountain chains

form by form by

compressioncompression Ridge systems Ridge systems

form by form by tensiontension

Sedimentary RocksSedimentary RocksSqueezed By CompressionSqueezed By Compression

This happens any place the crust is squeezedThis happens any place the crust is squeezed the pressure (compression) wrinkles the rocksthe pressure (compression) wrinkles the rocks the Appalachian Mountains are a good examplethe Appalachian Mountains are a good example

Sea-Floor Spreading Sea-Floor Spreading (Cont.)(Cont.)

Ridge systems form by Ridge systems form by tensiontension as the crust moves away, new lava fills the gapas the crust moves away, new lava fills the gap

Seafloor Seafloor SpreadingSpreading

2) 2) Age of Age of deepsea deepsea sediments and sediments and rocks:rocks: Oldest deep Oldest deep

ocean crust is ocean crust is 170 my old 170 my old

Age of basalt Age of basalt crust older crust older proceeding proceeding away from away from MOR crest.MOR crest.

Derived mostly Derived mostly from sediment from sediment datadata

Seafloor SpreadingSeafloor Spreading 3) Nature of ocean crust rocks:3) Nature of ocean crust rocks: Essentially all seafloor is composed of volcanic Essentially all seafloor is composed of volcanic

basalt basalt Continents are composed of a variety of rocksContinents are composed of a variety of rocks The density of graniteThe density of granite

is 2.7 to 2.8 g/cmis 2.7 to 2.8 g/cm33

The density of basalt The density of basalt is 2.9 g/cmis 2.9 g/cm33

Seafloor Seafloor SpreadingSpreading

4) Paleomagnetism4) Paleomagnetism Vacquier, Mason, Raff discovered LMA's in NE Pacific Vacquier, Mason, Raff discovered LMA's in NE Pacific

in late 1950's (Scripps) TRANSin late 1950's (Scripps) TRANS originally thought to result from alternating bands of originally thought to result from alternating bands of

sediments and volcanic rocks.sediments and volcanic rocks. Origin explained by Vine and Matthews of Origin explained by Vine and Matthews of

Cambridge in 1963 in terms of seafloor Cambridge in 1963 in terms of seafloor spreading theory. spreading theory. Symmetry and parallelism to MOR crest.Symmetry and parallelism to MOR crest. LMA's mapped over world ocean by Heirtzler, Pitman, LMA's mapped over world ocean by Heirtzler, Pitman,

Dickson, LePichon, Herron (Lamont)Dickson, LePichon, Herron (Lamont)

Seafloor SpreadingSeafloor Spreading 5) Seismicity (earthquakes):5) Seismicity (earthquakes):

occurs in defined patterns, in continuous chainsoccurs in defined patterns, in continuous chains

Seafloor SpreadingSeafloor Spreading 6) Volcanoes: 6) Volcanoes:

generally co-occur with earthquakes; same patternsgenerally co-occur with earthquakes; same patterns

Seafloor SpreadingSeafloor Spreading The infamous The infamous

“Ring of Fire” in “Ring of Fire” in the Pacific is an the Pacific is an expression of this expression of this distributiondistribution

both earthquakes both earthquakes and volcanoesand volcanoes