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Lecture

Lecture 4

5 Pages
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Department
Environmental Science
Course Code
EESA06H3
Professor
Nick Eyles

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Lecture 4 EESA06
Topics for lecture 4 and for quiz 3 on Thursday Feb 10
Introduction to mid-ocean ridges: modern and ancient fossil MORs (called ophiolites)
Why are MORs fractured? The role of transform faults
Iceland: walking over a mid-ocean ridge
Magnetic stripes recorded in ocean floor crust and the age of ocean floors
Basalt lava flows: fissure eruptions
Volcanic activity under glaciers: jokulhlaups!
oJokulh- mountain
oLaup- leap
Geothermal energy
Red Sea
Embryonic ocean
Continues to widen, one day will grow up into a mature, wide ocean like the Atlantic
In Iceland is where the mid-ocean ridge come up to sea level
Oceans only widen to a certain wide
Stages of Rift figure 2.19
As you move away from mid-ocean ridge, thickness of oceanic crust is thickens because it is cooling,
as you go away from ridge from either side
When you have thick slab of oceanic crust, it wants to sink back to mantle
Young oceanic crust- thin, ocean depth is not that deep
History of continent dictated by history of the ocean
Earth history is really controlled by ocean history
Like an accordion- opens and closes
Introduction to mid-ocean ridges: modern and ancient fossil MORs (called ophiolites)
Oceans closed and they push up on land- ancient pieces of ocean crust- ophiolites
Important because they contain minerals
Figure 2.8
Mid-ocean ridge, are literally underwater volcano, active all the time
If we add new volcanic material, pushing apart something
Iceland sitting right astride mid-ocean ridge, it is an island
The mid-ocean ridge is not strictly continuous, there are cracks running along ridges, called
fractures
oEnable ocean floor spreading on the curved earth surface
How oceans widen- rocks and a bike picture
Continuous intrusion of dikes at MORs
Orange strips on rocks- dikes
oResult of hot magma intruding onto older rocks along fractures
oThe rocks are being extended to accommodate the dikes
oOlder crust that is cooled earlier is being pushed apart- concept of plate movement
Result of continuous feeding of magma
oMany dikes are called sheeted dikes
Magma chamber
oTop of magma chamber are sheeted dikes
oWhen magma reaches ocean floor, it cools and find pillow magma, basalt magma was cooled
very quickly
oGabbro: chemically the equivalent of basalt, only difference is that it has larger crystals
within because it is cooling slowly
Igneous rocks cooling slowly
www.notesolution.com
oSheared periodotite
What the mantle is made out of
Made of mineral, olivine- green colour
When polished and cut as germ, known as peridot
oOcean crust are layers of pillow basalt, sheeted dikes, gabbro, sheeted peridotite
oGet distillation, basalt, gabbro are distilled from huge mantle plumes
Convection- plumes is feeding the ocean ridges
Tectonic forces acting to move plates- diagram
Hot mantle rocks rising up, it is plastic and begins to melt in areas of lower pressure thats when
basalt and gabbro comes off
Ridge push: process acting to drive plates across surface of earth
oOne of processes pushing accordion apart
Slab pull: old oceanic crust in free fall back into mantle
oThousands of km length and very thick
oAs cold slab falling down, pulling the ridge apart
Ridge push- dikes intruded and slab pull- other end of ocean floor sinks
Continental crust is the product of the melting of ocean crust plus sea water
What can we learn about MORs from ancient ocean crust? Fig 2.34
750 Ma and 350 Ma
oMega anum- millions of years
oGa- giga amun- billions
oKa- kilo amun- thousands
Shows time on the left
Closure of old ocean- Iapetus ocean
When close ocean, some ocean crust gets subducted, some gets pushed up on land- Himalayas:
result of destruction of younger ocean
Ancient ocean crust, now on land are called ophiolites
Sheeted Dikes
Thin-bedded chert aka flint
oSediment deposited slowly on top of ocean ridges
oFine sediments
oVery silica rich because result of green organisms
Lines running up hill and curved around- sheeted dikes
Discontinuous intrusion of basalt
Pillow Lava
Igneous rocks moving up along dikes
Unique to ocean crust because basalt have to be erupted underwater to form pillows
Fig 2.17
Oceanic crust made up of ultramaific and maific rocks
oMagnesium and iron
Continental crust made up of acidic rocks
oContains more silica
Proof about sea floor spreading
150 mya- Late Jurassic- maximum age
Anything red is relatively young, anything green or blue is old
Oldest outside mid ocean ridge, youngest rocks near middle of mid ocean ridge
Late Jurassic crust is being subducted near South America
Late Jurassic is oldest because Pangea broke up
oAll oceans are result of breakup of Pangea, they did not exist prior
www.notesolution.com

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Description
Lecture 4 EESA06 Topics for lecture 4 and for quiz 3 on Thursday Feb 10 Introduction to mid-ocean ridges: modern and ancient fossil MORs (called ophiolites) Why are MORs fractured? The role of transform faults Iceland: walking over a mid-ocean ridge Magnetic stripes recorded in ocean floor crust and the age of ocean floors Basalt lava flows: fissure eruptions Volcanic activity under glaciers: jokulhlaups! o Jokulh- mountain o Laup- leap Geothermal energy Red Sea Embryonic ocean Continues to widen, one day will grow up into a mature, wide ocean like the Atlantic In Iceland is where the mid-ocean ridge come up to sea level Oceans only widen to a certain wide Stages of Rift figure 2.19 As you move away from mid-ocean ridge, thickness of oceanic crust is thickens because it is cooling, as you go away from ridge from either side When you have thick slab of oceanic crust, it wants to sink back to mantle Young oceanic crust- thin, ocean depth is not that deep History of continent dictated by history of the ocean Earth history is really controlled by ocean history Like an accordion- opens and closes Introduction to mid-ocean ridges: modern and ancient fossil MORs (called ophiolites) Oceans closed and they push up on land- ancient pieces of ocean crust- ophiolites Important because they contain minerals Figure 2.8 Mid-ocean ridge, are literally underwater volcano, active all the time If we add new volcanic material, pushing apart something Iceland sitting right astride mid-ocean ridge, it is an island The mid-ocean ridge is not strictly continuous, there are cracks running along ridges, called fractures o Enable ocean floor spreading on the curved earth surface How oceans widen- rocks and a bike picture Continuous intrusion of dikes at MORs Orange strips on rocks- dikes o Result of hot magma intruding onto older rocks along fractures o The rocks are being extended to accommodate the dikes o Older crust that is cooled earlier is being pushed apart- concept of plate movement Result of continuous feeding of magma o Many dikes are called sheeted dikes Magma chamber o Top of magma chamber are sheeted dikes o When magma reaches ocean floor, it cools and find pillow magma, basalt magma was cooled very quickly o Gabbro: chemically the equivalent of basalt, only difference is that it has larger crystals within because it is cooling slowly Igneous rocks cooling slowly www.notesolution.com
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