CHAPTER 2: PLATE TECTONICS

What Is Plate Tectonics?

* Tectonics → study of the origin and arrangement of the broad structural features of the earth’s surface

(folds, faults, mountain belts, continent & earthquake belts)

* Basic idea of plate tectonics → the earth’s surface is divided into a few large, thick plates that move slowly

and change in size

* Intense geological activity occurs @ the plate boundaries where plates move away, past or towards one

another → 8 large plates and a few dozen smaller ones make up the outer shell of the earth ( crust & upper

part of mantle)

* Concept of plate tectonics was developed in the late 1960’s by combining 2 pre-existing ideas:

o Continental drift → continents move freely over the earth’s surface, changing their positions

relative to one another

o Sea-floor spreading → hypothesis that the sea floor forms at the crest of mid-ocean ridges, then

moves horizontally away from the ridge crest toward an oceanic trench

The 2 sides of the ridge are moving in opposite directions like slow conveyor belts

How Did The Plate Tectonics Theory Evolve?

Early case for continental drift

o Wegener → noted that South America, Africa, India, Antarctica & Australia has almost identical late

Palaeozoic rocks and fossils

Reassembled the continents to form a giant supercontinent = Pangaea

If continents are arranged according to Wegener’s Pangaea reconstruction: glaciations in

the southern hemisphere is confined to small area and the absence of widespread

glaciations in the northern hemisphere becomes easier to explain

Pangaea → Laurasia (Northern supercontinent) & Gondwanaland (Southern

supercontinent)

Laurasia (Northern supercontinent)

o North America & Eurasia

Gondwanaland (Southern supercontinent)

o Southern Hemisphere continents & India

Reconstructed old climate zones (Paleoclimatology) and the ancient sedimentary rocks

Discovered that paleoclimatic reconstructions suggested polar positions very

different to those at present – evident for changes in position of the poles overtime

Scepticism about continental drift

o Wegener presented the best possible case in the early 1900’s for continental drift but evidence

provided was not clear cut and he didn’t have a good mechanism to account for continental

movement

Proposed that → continents ploughed through the oceanic crust, perhaps crumpling up

mountain ranges on the leading edges of the continents where they pushed against the sea

floor

Geologists thought this violated what was known about the strength of rocks at the

time

Driving mechanism proposed by Wegener → combination of :Centrifugal force from the

earth’s rotation & Gravitational forces that cause tides

Too small to move continents

Geologists in the southern hemisphere where Wegener’s matches of fossils and rocks b/w

continents were more evident were ↑ impressed while geologists in the northern

hemisphere were not

Renewed interest in continental drift

o Work in the 1940s and 1950s set the stage from the revival of the idea of continental drift w/ new

investigations in the areas: study of the sea floor & geophysical research (especially in relation to

rock magnetism)

o Study of the sea floor

Oceans cover more than 70% of the earth’s surface → difficult to study

Samples of rock and sediments can be taken from the sea floor in several ways:

Rocks can be broken from the sea floor by a Rock dredge → open steel container

dragged over the ocean bottom @ the end of a cable

Sediments can be sampled with a Corer → weighted steel pipe dropped vertically

into the mud and sand of the ocean floor

Sea floor drilling → (both rocks and sediments) – revolutionized field of marine

biology

o offshore oil platforms drill holes in the relatively shallow sea floors near

shore

o a ship with a drilling derrick on its deck can drill a hole in the deep sea floor

far from land → the drill cuts long, rod-like rock cores from the ocean floor

Submersibles → small research submarines which take geologists to many parts of

the sea floor to observe, photograph & sample rock and sediment

Single-beam echo sounder → basic tool for indirectly studying the sea floor which measures

water depth and draws profiles of submarine topography

A sound signal send downward from a ship bounced off the sea floor and returns to

the ship – determining water depth from the time it takes the sound to make the

round trip

Multibeam sonar → uses a variety of sound sources to produce detailed shaded

relief images of the sea-floor topography

Sidescan sonar → measures the intensity of sound reflected back to the tow vehicle

from the ocean floor and provides detailed images of the sea floor and information

about sediments and bedforms

Seismic reflection profiler → works on essentially the same principles as the echo

sounders but uses a louder noise @ lower frequency

o Sound penetrates the sea floor and reflect from layers w/in the underlying

sediment and rock → records water depth and reveals the internal structure

of the rocks and sediments of the sea floor (i.e. bedding planes, folds and

faults, unconformities)

Magnetic, gravity & seismic refraction surveys can also be made at sea & Deep sea cameras

can be lowered to the bottom to photograph the rock and sediment

Geophysical research

o Convincing new evidence about polar wandering came from the study of rock magnetism

Wegener’s world dealt with the wandering of earth’s geographic poles of rotation

Magnetic poles = close to geographic poles

The position of magnetic poles moves from year to year but the magnetic poles stay

close to the geographic poles as they move

Many rocks record the strength and direction of the earth’s magnetic field at the time the

rocks formed

Magnetite in a cooling basaltic lava flow acts like a tiny compass needle, preserving

a record of earth’s magnetic field when the lave cools below the curie point

Sedimentary rocks contain iron oxides and can also record the earth’s magnetism

Magnetism of old rocks can be measured to determine the direction and strength of

the magnetic filed in the past → paleomagnetism

b/c magnetic lines of force are inclined more steeply as the north magnetic pole is

approached → the inclination of the magnetic alignment preserved in the magnetite

minerals in the lava flows can be used to determine the paleolatitude as which the

flow formed

old rocks reveal very different magnetic pole positions to those at present → it was once

thought this was due to movement of the poles (polar wandering)

Now known that it is due to the movement of the tectonic plates

Polar wandering paths now used to reconstruct continental movement over time

Every continent shows a different position for the Permian pole → a single stood

still while continents split part and rotated as they moved

Wandering paths for north America and Europe = similar shapes but Europe path is to the

east of the north American path – continents were once joined

Recent evidence for continental drift

o Paleomagnetic evident revived interest in continental drift

o By defining the edge of a continent as the middle of the continental slope rather than the constantly

changing shoreline, a better fit has been found b/w the continents

o Most convincing evidence → greatly refined rock matches between now-separated continents

o Many of the boulders in south American glacial deposits have been traced to a source that is now in

Africa

o Now: there are an abundance of satellite geodetic data from the GPS that allow us to watch the

continents in real time

History of continental positions

o Rock matches show when continents were together – after the split of continents, the new rocks

formed are no longer similar

o Paleomagnetic evidence indicates the direction and rate of drift allowing maps of old continental

positions to be drawn

What Is Sea-Floor Spreading?

Hess → in 1962 - proposed that the sea floor moves away from the mid-oceanic ridge as a result of mantle

convection

o Contrasted with Wegener who thought that the ocean floor remained stationary as the continents

ploughed through it

Initial concept → sea floor is moving like a conveyor belt away from the crest of the mid-oceanic ridge,

down the flanks of the ridge, and across the deep-ocean basin, finally disappearing by plunging beneath a

continent or island arc

Spreading axis/center → the ridge crest, with sea floor moving away from it on either side

Subduction → sliding of the sea floor beneath a continent or island arc

Convection → a circulation pattern driven by the rising of hot material and/or the sinking of cold material

o Hot material – lower density → rises

o Cold material – higher density → sinks

o Slow convection circulation of a few cm/year is set up by temperature differences w/in the mantle

& convection can explain many sea-floor features as well as the young age of the sea-floor rocks

If convection drives sea-floor spreading → hot mantle rock must be rising under mid-oceanic ridges

Hess → showed how the existence of ridges and their high heat flow are caused by the rise of this hot

mantle rock

o Basalt eruptions on ridge crests are also related to this rising rock b/s the mantle rock is hotter

than normal & begins to undergo partial melting

o As hot rock continues to rise beneath ridge crests, the circulation pattern splits and diverges near

the surface → mantle rock moves horizontally (carrying the sea floor w/ it) away from ridge crests

on each side of the ridge (becoming cooler and denser, sinking deeper beneath the ocean surface),

causing tension at the ridge crest and cracking open the oceanic crust to form rift valleys and

associated shallow-focus earthquakes

Downward plunge of cold rock accounts for the existence of the oceanic trenches & their

low heat flow values

Also explains the large negative gravity anomalies associated with trenches

→sinking of the cold rock provides a force that holds trenches out of isostatic

equilibrium

o As the sea-floor moves downward into the mantle along a subduction zone, it interacts with the

stationary rock above it → causing the benioff zones of earthquakes associates with trenches

How old is the sea floor?

o Determined through isotopic dating and sediments (fossils)

Discovered that all the rocks and sediments from the deep sea floor proved to be <200

million yrs old → they formed during the Mesozoic and Cenozoic eras

The earth = 4.55 billion yrs old →deep sea floor covering more than half of the earth’s

surface preserves less than 1/20th of its rock and sediment

Young age of the sea floor rocks → explained by Hess’ sea-floor spreading

New, young sea-floor is continuously being formed by basalt eruptions at the ridge crest → basalt is carried

sideways by convection and is subducted into the mantle at an oceanic trench

o Old sea floor is being destroyed at trenches & new sea floor is being formed at the ridge crest

Young sea floor has little sediment b/c basalt is newly formed

Old sea floor farther from the ridge crest has been moving under a constant rain of pelagic

sediment – building up a thicker layer as it moves along

o Youngest sea floor = at the ridge crest

o Older sea floor = at the trenches

Measuring Plate Movement In Real Time

GPS data can be used to measure plate movement direction & on a global scale

1970s → space geodesy – a space based technique measuring points on the earth’s surface

Most common space-geodetic techniques:

o Very long baseline interferometry (VLBI)→ uses pairs of radio telescopes pointed toward a common

quasar

o Satellite laser ranging (SLR)

o Global positioning system (GPS) → most useful of the 3 techniques for studying movement of the

earth’s crust and measure relative motion b/w plates

Plate motions are recorded on a yearly basis around the world → rates and directions of

plate movement measured by a GPS are close to the those calculated on the basis of

geological data

Measurement of relative plate movement provides valuable data regarding potential

earthquake activity along active plate margins

GPS data is currently being used to monitor interactions b/w the pacific plate and

surrounding continental plates to learn about the earthquake and volcanic activity in the

pacific ring of fire

What Are Plates And How Do They Move?

Plate → large, mobile slab of rock that is part of earth’s surface

o Surface may be made up entirely of sea floor (Nazca plate) or of both continental and oceanic rock

(NA plate)

o Most small plates are entirely continental while large plates contain some sea floor

o Plates = part of lithosphere which consists of rocks of the crust and the uppermost mantle

Age of Lithosphere → ↑es with both age and thickness w/ distance from the crest of the mid-

oceanic ridge

Oldest → far from crest of the mid-oceanic ridge (100km thick)

Youngest → close to mid-oceanic ridge (10km thick)

Continental lithosphere = thicker

o 125km thick to as much as 200-250km thick beneath the oldest, coldest and

most inactive parts of the continent

Interior of a plate = inactive tectonically

Plates move away from the mid-oceanic ridge crest

If plate is made up of mostly sea floor (Nazca and pacific plates) → plate can be

subducted down into the mantle forming an oceanic trench and its associated

features

If the leading edge of the plate is made up of continental rock (SA plate) it will not

subduct because it is less dense than oceanic rock and too light

Athenosphere→ zone of low-seismic wave velocity hat behaves plastically b/c of ↑ed temp and pressure

below the rigid lithosphere

o Plastic Athenosphere acts as a lubricating layer under the lithosphere allowing plates to move

o Made up of upper-mantle rocks

o Below the Athenosphere = more rigid mantle rock

Plate boundaries

o Divergent plate boundary → boundary b/w plates that are moving apart

o Convergent plate boundary → lies b/w plates that are moving toward each other

o Transform plate boundary → one at which 2 plates move horizontally past each other

How Do We Know That Plates Move?

Magnetic anomalies and seismicity of fracture zones convinced most geologists that plates do indeed move

Paleomagnetic Evidence

o Earth’s magnetic field has periodically reversed its polarity in the past → north magnetic pole and

south magnetic pole exchange positions → Magnetic Reversals

Normal polarity → magnetic lines of force flow from the south pole to the north pole and

compass needles point to the north

Cooling dikes are normally magnetized

Reversed polarity → lines of magnetic force run flow from the north pole to the south pole

and compass needles point south

Dikes that cool when the field is reversed are reversely magnetized

o Paleomagnetism → study of ancient rocks can tell us about changes in the earth’s magnetic fields in

the past

o Lava flows contain abundant magnetic minerals and can be isotopically dated → stacked

continental lava flows have been used to construct a magnetic polarity time scale which measures

the pattern of magnetic reversals over time

o Reverses ever 500 000yrs and takes 10 000 yrs for a magnetic reversal to develop

o Strength of the earth’s magnetic field also changes over time

Anomaly → deviation of magnetic strength from the average

Magnetometer → instrument that measures the strength of the earth’s magnetic field

Marine Magnetic Anomalies

o Most magnetic anomalies on these floor are arranged in bands that lie parallel to the rift valley of

the mid-oceanic ridge

o Alternating (+) and (-) anomalies form a stripe like pattern parallel to the ridge crest

o Morley-Vine-Matthews Hypothesis

Pattern of magnetic anomalies was symmetrical about the ridge crest → pattern on 1 side of

the ridge was a mirror image of the pattern on the other side

Same pattern exists over different parts of the mid-oceanic ridge

Pattern of magnetic anomalies at sea matches the pattern of magnetic reversals already

known from studies of lava flows on the continents

Proposed explanation of magnetic anomalies → Dikes

There is a continual opening of tensional cracks w/in the rift valley on the mid-

oceanic ridge crest which are filled by basaltic magma → cools to form dikes

o Cooling magma in the dikes records the earth’s magnetism at the time the

magnetic minerals crystallize

o dikes preserve a record of the polarity that prevailed during the time the

magma cools → extension produced by the moving se floor then cracks a

dike in 2 and the 2 ½’s move in opposite directions down the flanks of the

ridge

o new magma intrudes the newly opened fracture which cools, is magnetized

and forms a new dike (repeat cycle)

How Fast Do Plates Move?

Morley-Vine-Matthews Hypothesis

o Allows us to measure:

Rate of sea-floor motion (plate motion)

Since magnetic reversals have already been dated from lava flows on land. The

anomalies caused by these reversals are also dated and can be used to discover how

fast the se-floor has moved

Sea floor age

By matching the measured anomaly pattern with the known pattern of anomalies,

the age of the sea floor (& therefore rocks) in the region can be predicted

Fracture zones & Transform Faults

o Mid-ocean ridges are offset along fracture zones

They were once continuous across fracture zones but have been offset by strike slip motion

Earthquakes occur only in those segments b/w offset sections of ridge crest

Transform Fault → portion of a fracture zone b/w 2 offset portions of ridge crest (Wilson)

Only in that section do rocks move in opposite direction - rocks move away from

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What Happens At Plate Boundaries?

Divergent plate boundaries

o Plates move away from one another → can occur in the middle of the ocean or in the middle of a

continent

Result of divergence at plate boundaries is to create or to open new ocean basins

o Marked by → rifting, basaltic volcanism and uplift

RIFTING →continental crust is stretched and thinned

Extension produces shallow-focus earthquakes on normal faults & a rift valley forms

as a central garben (down-dropped fault block)

Faults act as pathways for basaltic magma which rises from the mantle to erupt on

to the surface as cinder cones and basalt flows

UPLIFT → caused by the upwelling of hot mantle beneath the crust

Surface is elevated by the thermal expansion of the hot, rising mantle rock and of the

crustal rock as it is warmed below

How a continent might rift to form an ocean (fig. 2.19)

Crust is initially stretched and thinned

Numerous normal faults break the crust and the surface subsides into a central

garben

Shallow earthquakes and basalt eruptions occur in the valley which has high heat

flow

o i.e. African rift valleys → valleys = garbens that may marks site of future

break up of Africa

continental crust on the upper part of the plate separates and sea water floods into

the linear basin b/w 2 divergent continents

rise of hot mantle rock beneath thinned crust causes continued basalt eruptions that

create true oceanic crust b/w the 2 continents

center of narrow ocean → marked by a rift valley w/ it’s typical high heat flow and

shallow earthquakes

after widening of new ocean → uplift of continental edges may occur

o continental crust thins by stretching and faulting → surface initially

subsides

o At the same time → hot mantle rock wells up beneath the stretched crust

o Rising diaper of hot mantle causes uplift by thermal expansion and

weakening of crust

Figure 2.19B

o New ocean = narrow & tilt of adjacent land is away from new sea so rivers

flow away from sea

Sea water that has flooded into the rift may evaporate leaving salt

behind overlying continental sediment

o Plates continue to diverge, widening the sea → thermal uplift creates a mid

oceanic ridge in the center of the sea

Flanks of the ridge subside as the sea-floor rock cools as it moves

Trailing edges of ridge also subside as they are lowered by erosion

and as mantle rock beneath them cools → subsidence continues until

edges of continents are under water

o Thick sequence of marine sediment blankets the thinned continental rock,

forming a passive continental margin

o Sediment forms a shallow continental shelf which may contain a deeply

buried salt layer

o Continental rise is formed as sediment is carried down the continental slope

by turbidity currents and other mechanisms (Atlantic ocean = at this stage)

Divergent boundary on sea floor = on the crest of a mid-ocean ridge

If spreading rate is slow (Atlantic ocean) – the crest has a rift valley

Divergent boundary at sea is marked by the same features as a divergent boundary on land:

Tensional cracks

Normal faults

Shallow earthquakes

High heat flow

Basaltic eruptions → basalt forms dikes w/in cracks and pillow lavas on the sea

floor creating a new oceanic crust on the trailing edge of plates

Mid-oceanic Ridges → Giant undersea mountain ranges that extend around the world

o Rift valley of tensional origin commonly runs down the crest of each ridge

As hot water rises in the rift valley, cold water is drown in from the sides to take its place →

creates a circulation pattern in which cold sea water is drawn downward though the cracks

in the basaltic crust of the ridge flanks and then moves towards the rift valley where it re-

emerges on the sea floor after being heated

As sea water circulates, it dissolves metals and sulphur from the crustal rocks

When hot metal-rich solutions contact the cold water, metal sulphide particles are

precipitated in the cold sea water at black smoker mounds which build chimney like

mounds around hot springs

o Small and widely scattered on the sea floor

Metals released are predominantly: iron, copper & zinc and less manganese, gold and silver

Hot metallic solutions also found on some divergent continental boundaries

o Marked by:

Lines of hot springs that carry and precipitate metal sulphides

o Biological activity

Many organisms found in the hot springs → live on bacteria that thrive by oxidizing

hydrogen sulphide from hot spring

o Geological activity on ridges

heat loss ↓es away from the ridge crest

Basaltic eruptions occur in and near the rift valley on ridge crests

Iceland → part of the mid-Atlantic ridge

Transform Boundaries

o Occur when 1 plate slides horizontally past another plate, the plate motion can occur on a single

fault or on a group of parallel faults

o Marked by: shallow-focus earth quakes, in a narrow zone for a single fault and broad zone for many

parallel faults

o Most common type of transform fault occurs along fracture zones and connects 2 divergent plate

boundaries at the crest of the mid-oceanic ridge

Spreading motion at one ridge segment is transformed into the spreading motion at the

other ridge segment by strike-slip movement along the transform fault

o Transform faults can connect:

Ridge to a trench (convergent and divergent boundary)

2 trenches (2 convergent boundaries)

Ridge – ridge

o What is the origin the offset in a ridge-ridge transform fault?

Result of irregularly shaped divergent boundaries

(READ REST IF YOU WANT)

Convergent plate boundaries → 2 plates move towards each other

o Character of the boundary depends on the type of plates that converge

Plate capped by oceanic crust can move toward another plate capped by oceanic crust

1 plate subducts under the other

Oceanic plate converges with place capped by a continent

Dense oceanic plate subducts under the continental plate

Both plates carrying continents

Collide and crumple but neither is subducted

o Ocean – ocean convergence

When 2 plates capped by sea floor converge – one plate subducts under the other

Subducting plate bends downward, forming the outer wall of an oceanic trench which

usually forms a broad curve convex to the subducting plate

Oceanic trench → narrow, deep trough parallel to the edge of a continent or an island arc

Continental slope on an active margin forms the landward wall of the trench – steepness

increasing with depth

As one plate subducts under another, a Benioff zone of shallow, intermediate and deep-

focus earthquakes is creates within the upper portion of the downgoing lithosphere

As the descending plate reaches depths of at least 100km, magma is generated in the

overlying asthenosphere

Magma works its way upward to erupt as an island arc → curved line of volcanoes

that form a string of islands parallel to the oceanic trench

Distance between the island arc and the trench can vary

Subduction angle = steep → close to trench

Subduction angle = gentle → far from trench

Inner wall of a trench (toward arc) consists of an accretionary wedge or a subduction

complex of thrust faulted and folded marine sediment and pieces of oceanic crust

A relatively undeformed forearc basin lies b/q the accretionary wedge and the volcanic arc

Trench side = forearc

Other side = backarc

Trench positions change with time

o Ocean – Continent Convergence → forms an active continental margin, trench, benioff zone,

magmatic arc & young mountain belt on the edge of the continent

When a plate capped by oceanic crust is subducted under the continual lithosphere → an

accretionary wedge and forearc basic form an active continental margin b/w the trench and

the continent

Benioff zone of earthquakes dips under the edge of the continent which is marked by

andesitic volcanism and a young mountain belt

Magma that is created by ocean-continent convergence forms a:

Magmatic arc → broad term used both for island arcs at sea and for belts of igneous

activity on the edges of continents

o Surface expression of a magmatic arc is either a line of andesitic islands or a

line of andesitic continental volcanoes

There are large plutons in thickened crust under volcanoes

o Hot magma rises from the subduction zone thickens the continental crust

and makes it weaker and more mobile that cold crust → regional

metamorphism takes place here

Crustal thickening causes uplift – young mountain belt forms

o Continent – continent convergence → forms a young mountain belt in the interior of a new larger

continent

2 continents may approach each other and collide – but they must be separated by an ocean

floor that is being subducted under one continent and that lacks a spreading axis to create

new oceanic crust

The edge of one continent will initially have a magmatic arc and all other features of ocean-

continent convergence

As the sea floor is subducted, the ocean becomes narrower and narrower until the

continents eventually collide and destroy or close the ocean basin

Oceanic lithosphere = heavy and can sink into the mantle but continental lithosphere is less

dense and cannot sink

o Backarc Spreading

Regional extension occurs w/in or behind many arcs

It can tear an arc in two, moving the 2 halves in opposite directions

Spreading creates new oceanic crust that is similar but not identical to the oceanic crust

formed at the crest of mid-oceanic ridges

o Oceanic crust and Ophiolites

Oceanic crust differs significantly from continental crust – it is both thinner and of a

different composition

Top layer → marine sediment

Layer 2 → pillow basalt overlying dikes of basalt

o Pillow basalts form when hot lava erupts into cold water

Lowest layer → sill-like gabbro bodies

Ophiolites are distinctive rock sequences found in may mountain chains on land

Thin top layer → marine sedimentary rock

2nd layer → zone of pillow basalt which is underlain by a sheeted-dike complex that

probably served as feeder dikes for the pillowed lava flows

o Sheeted dikes – thick layer of pod-like gabbro intrusions underlain by

ultramafic rock like peridontite – top part of which has been

metamorphosed

Ophiolites are not typical of sea floor but a special type of sea floor formed in

marginal ocean basins next to continents by the process of backarc spreading

o Convergent Boundaries and Ore deposits

Sea floor spreading carries metallic ores away from ridge crests to be subjected beneath

island arcs or continents at convergent plate boundaries

Slivers of ancient oceanic crust (ophiolite)exposed on land may contain these rich ore

minerals in relatively intact form

Volcanism at island arc → can produce hot spring deposits on the flanks of the andesitic

volcanoes

Island arc ores usually contain more lead an d fold compared to spreading centers

Subduction of the sea floor beneath a continent produces broad belts of metallic ore

deposits near the edge of the continent

Hot spring deposits from the ridge crest are subducted with oceanic crust and could be

remobilized to rise into the continent above

Metals may also derive from the continental crust itself or the mantle below it and may be

concentrated by the heat of a rising blob of magma or by hydrothermal circulation

How Do Mountain Ranges Form?

Orogenies and plate convergence

o Orogeny → episode of mountain building characterized by intense deformation of the rocks in a

region

o Mountain belts form along plate margins, especially where plate convergence compresses the crust,

causing uplift and deformation

o Ocean – continent convergence

Refer above

o Arc-continent convergence

As arc and continent converge, intervening ocean is destroyed by subduction

Arc, like a continent is too buoyant to be subducted → continued convergence may cause

the remaining sea floor to break away from the arc and create a new site of subduction and

a new trench seaward of the arc

Direction of new subduction is opposite to the direction of original subduction (FLIPPING

SUBDUCTION ZONE) but may still supply the arc w/ magma

Arc is now welded to the continent - ↑ing its size

o Continent – continent convergence

Mountain belts form when an ocean basic closes and continents collide (ex. Those found

w/in continents)

Rest are examples = read if you want

* read about the Rocky Mountains (pg. 53)

How do Plates Change Over Time?

Plates move and so do plate boundaries

o Convergent boundaries migrate and can also jump – subduction can stop in one place and begin

suddenly in an new place

o Transform

boundaries can change position

A → stable craton under which a hot spot develops causing crust to dome upward and fracture creating a

rift valley or garben

B → sea floor spreading center with an erupted basaltic ocean floor is flooded to form a new ocean

o Separated continental margins on each side cool and become denser subsiding beneath sea level

o Sediments accumulating on theses subsiding continental margins begin to form continental shelves

C → ocean basin widens through addition of igneous mantle material via a mid-ocean ridge system

characterized by young volcanoes and steep slopes – continental shelves are now well developed

D → subduction zone forms when ocean basin is enlarged sufficiently to allow the basaltic crust to cool and

sink down into the asthenosphere

E → when most of the original oceanic crust has been subducted the 2 continents that were originally

separated begin to collide

o Sea floor sediments are scraped off the descending plates form accretionary wedges characterized

by intense folding and faulting

F → continent oceanward of the subduction zone is obducted on to the other continent and a mountain

range is created

What Causes Plate Motions?

Any mechanism for plate motion has to explain why:

o Mid-ocean ridge crests are hot and elevated while trenches are cold and deep

o Ridge crests have tensional cracks

o Edges of some plates are subducting sea floor while the edges of other plates are continents which

do not subducting

Convection in mantle → has been proposed as the mechanism but is complicated

Why do plates diverge and sing?

o Ridge push → as a plate moves away from a divergent boundary, it cools and thickens

Cooling sea floor subsides as it moves and this subsidence forms the broad aide slopes of

the mid-oceanic ridge

o Slab pull → most important force in moving an oceanic plate away from a ridge crest

Cold lithosphere sinking at a steep angle through hot mantle should pull the surface part of

the plate away from the ridge crest and then down into mantle as it cools

o Trench-suction → subducting plates fall into the mantle at angles steeper than their dip then

trenches and the overlying plates are pulled horizontally seaward toward the subducting plates

Reason for plate motions → properties of the plates themselves and the pull of gravity

How Are Mantle Plumes and Hot Spots Related?

Mantle plumes → narrow columns of hot mantle rock that rise through the mantle form thermal boundary

layers at the base of the mantle (or upper mantle)

o Stationary w/ respect to moving plates and to each other

o May form hot spots of active volcanism at the earth’s surface

Rising mantle plume contains a hot, mushroom-shaped plume head and a narrow tail

Plume head forms a broad hot spot when it reaches the top of the mantle and causes uplift

and stretching of the crust and eruption of flood basalts

When the tail rises to the surface, a narrower hot spot forms a volcano

Continued plate motion over the hot spot creates a trail or chain of volcanoes

How can 2 plumes split a continent and begin plate divergence?

o Local uplift causes rifting over each plume

o Rifts lengthen with time until he land is torn in two

o 2 halves begin to diverge from being dragged along from below by the outward radial flow of the

plume

o Along the long rift segments b/w plumes, rifting occurs before uplift

Some plumes rise beneath the centers of oceanic plates

Seamounts, Guyots and Aseismic Ridges

o Seamounts → conical undersea mountains that rise 1000m or more above the sea floor

Some rise above sea level to form islands

They are scattered on the flanks of the mid-oceanic ridge and on other parts of the sea floor

(including abyssal plains)

Rocks dredged from seamounts are nearly always basalt → thought that most seamounts

are extinct volcanoes

Only a few are actually active volcanoes on the crests of the mid-oceanic ridges

o Guyots → flat-topped seamounts found in the western pacific ocean

Thought that guyots were cut by wave action → they must have subsided after erosion took

place

Evidence of subsidence comes from the dredging of dead coral reef corals from

guyot tops

Are hundreds of meters below sea level

o Aseismic ridges → Guyots and seamounts on the sea floor aligned in chains along with some other

ridges on the sea floor

Are submarine ridges – not associated with earthquakes