©1996, West Publishing Company (Modified by Asper, 2006) Slide 1 Lesson #4 How did the Oceans get to be the way they are?

©1996, West Publishing Company (Modified by Asper, 2006)©1996, West Publishing Company (Modified by Asper, 2006) Slide Slide 11

Lesson #4

How did the Oceans get to be the way they are?



Origin of the oceans

• We weren’t there when the earth was formed so this is all theory– Theory:

guess, test, evaluate, repeat– Fact: observable

• Origin theories are pretty good– Fit the observations– Predict what you’ll find in new places– But expect them to change as new facts are observed

http://www.astro.psu.edu/~bmiller/astro11/pics/earth_from_space.jpghttp://www.astro.psu.edu/~bmiller/astro11/pics/earth_from_space.jpg


Faith vs. Science• Different sets of rules:

• Science: must be testable, subject to the “scientific method”– Question: theory, hypothesis– Test– Evaluate– Repeat, repeat, repeat…

• No theory ever becomes fact

• Faith: must be un-testable

• Problems happen when you mix these up

www.britanica.com


The Origin of the Earth

• This is the best theory we have– works very well– but expect it to change as

we learn more

• The universe is very large– nearest star is 27 million,

billion miles away– 4.5 light years (light travels

at 186,000 miles / second)

• How do we measure distance in space?– triangulation– light intensity


A Light-Year is a Measure of Distance

• A light-year is equal to– The distance light travels in one year

– exactly 9,460,730,472,580.8 kilometers– about 5,878,630,000,000 miles– Based on a year of 365.25 days; 86,400

seconds a day; 31,557,600 seconds a year– A speed of light of 299,792,458 meters per

second• 670,616,629 miles per hour


The Origin of Earth• The universe was formed roughly 15 billion years ago• The solar system formed roughly 5 billion years ago• formed from a nebula, which is a revolving mass of particles• some collected into stars (such as our sun)• others collected into planets (such as the earth) which continued to

revolve around it


The Origin of the Earth

• The planets formed as the particles condensed• conditions on each planet are determined by:

– distance from the sun

– amount of mass


Earth’s Structure

• Like other planets, the earth was formed through this process

• As it cooled, the heavier elements (iron, nickel) settled to the bottom (center) and the lighter elements (aluminum, silicon) floated to the top

• this resulted in the creation of concentric shells – Each with a distinct chemical composition– Layered on the basis of density


The Origin of the Oceans

• As the earth continued to cool, water and gases were given off– these accumulated in the

oceans and atmosphere respectively

– originally all of the water was in gaseous state due to the hot earth

– this processes took a long time and continues today

• The composition of the original ocean and atmosphere was determined by this process


The Origin of the Oceans

• Important Events in the Earth’s History• Each event paved the way for the next• Man is a relative newcomer


Earth’s Structure (Cont.)

• The earth’s interior is made up of several layers.

• The physical characteristics of each layer are determined by:– The chemical composition of the rocks– Temperature– Pressure

• These layers are can be defined two ways:– By acoustic velocity

• Crust, Mantle, Core

– By physical characteristics (rigidity, viscosity)• lithosphere, asthenosphere, mesosphere


Earth’s Structure (Cont.)(according to velocity)

• The crust is the outermost layer.– Relatively thin– Low density

material: aluminum, silicon, oxygen


Geologic Differences Between Continents and Ocean Basins (Cont.)

• Continental crust consists of granite– Igneous origin, solidified from molten material.

– Light in color.

– Mostly oxygen, silicon, aluminum.

• Oceanic crust consists of basalt– Volcanic origin, solidified from

material erupted onto Earth’s surface.

– Dark in color.

– Rich in oxygen, silicon, magnesium and iron.


Geologic Differences Between Continents and Ocean Basins (Cont.)

• The density of granite is– 2.7 to 2.8

g/cm3

• The density of basalt is– 2.9 g/cm3

Basalt Granite


Oceanic Crust vs. Continental Crust

• Continental crust is thicker than oceanic crust– Also more variable– Much thicker under mountains



• The mantle lies below the crust.– Relatively thicker– Higher density

material: magnesium, iron, silicon, oxygen



• The core is below the mantle.– Thickest of all

layers

– Very dense material: iron, nickel• Outer: molten• Inner core: solid



• These three layers are determined by the speed at which sound travels through them– Crust– Mantle– Core

• But for plate tectonics, we need to consider their physical properties


Earth’s Structure According to Physical properties

• Lithosphere– Upper layer

– rigid

• Asthenosphere– Just below the

lithosphere

– “plastic”

• Mesosphere



• The lithosphere consists of the crust and uppermost mantle.– Hard,

– Rigid,

– Brittle

Molten



The asthenosphere is below the lithosphere.– A portion of the

mantle– Rocks are

semi-melted.– Behavior is

plastic.



The mesosphere consists of the lower mantle.– Higher pressure

causes mantle rocks to change structure.

– More solid than the asthenosphere.

– No plastic flow, but still some flow.



• We know all of this through indirect evidence:– mass of earth

– transmission of sound

– samples

• NO direct samples– too deep

– too hot

– too much pressure


Lithosphere• The crust

“floats” on the mantle beneath it

• Like an iceberg, there is more below the surface than above


Isostasy

• This demo shows the effect of height and density on how an object floats

• This applies to icebergs and to the crust

• Isostasy Demo


Isostasy explains the distribution of Earth’s topography

•Continental crust is lighter than ocean crust, so• It floats higher than oceanic crust

•Continental crust is thicker than ocean crust, so • It stands higher than ocean crust.• It has deeper roots than ocean crust.


Plate Tectonics

• The plate Tectonic theory revolutionized geology.

• It explains:– earthquakes– distribution of volcanoes– character of the

seafloor– age of the seafloor– shape of the ocean

basins– just about everything

http://nzphoto.tripod.com/volcano/PacificPlate.gifhttp://nzphoto.tripod.com/volcano/PacificPlate.gif


Plate Tectonics

• Plate tectonics theory is based on two types of data

• 1) Continental Drift– Data about the continents, their shape, and

what’s on them– All based on observations above sea level

• 2) Seafloor spreading– Data about the sea floor, its shape, and

what’s on it– All based on observations below sea level


Continental Drift

• The theory was proposed in 1915 by Alfred Wegener (a meteorologist) to explain Earth’s history.

• The theory states that a single continent, Pangaea, existed more than 200 million years ago.

• It broke up and the continents drifted apart

http://vaidehipatil.wordpress.com/category/design-2/illustration/


Evidence for Continental drift

• There are several lines of evidence for continental drift, including:

• (1) “map fit”– when you

reposition the continents on a globe the all fit together

– the fit is stunning!


Continental drift (cont.)

• (2) Glacial scarring– Glaciers gouge the rocks

– Regions of glacial scour “match” on opposing continents

– Must have happened before breakup of original continent

Glacial striations

Glacial striations

Glacial pot holes

Glacial scarring



• (3) Geologic evidence (rock type).

– Age of rock.– Mineralogy.– Structure.



• (4) Fossils– Identical fossils found on continents which are

now separate

– Must have been connected at some point in the distant past

– Can determine age of separation by dating fossils


Continental Drift (cont.)

• But Wegener’s theory lacked a mechanism– What made the continents move?

• Most scientists scoffed at the theory

• It was not accepted

for more than 50 years!

http://www.ucmp.berkeley.edu/history/wegener.gifhttp://www.ucmp.berkeley.edu/history/wegener.gif


Breakup of Pangaea

Drift 1 Drift 2


Sea-Floor Spreading

• Sea-floor spreading explains how ocean basins form

• But it also provides the mechanism that Wegener was missing

• It is based upon various pieces of scientific evidence– Deep sea soundings showing the existence and structure

of midocean ridge systems– Magnetometer readings of ridge systems– Pattern of earthquakes– Distribution of volcanoes– Chemistry of the rocks– Lots of other evidence


Sea-Floor Spreading (Cont.)

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

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

• Geologically active– Volcanism, – Earthquakes – Faulting


Sea-Floor Spreading

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

form by

compression

– Ridge systems

form by tension

ForceForce

ForceForce


Sea-Floor Spreading• the crust is pulling apart at the ridge crest (tension)

• a valley forms where this happens



• When a ridge comes on land, it rifts (tears) apart the continent!

Rifts

Red Sea

Sinai

Mediterranean Sea


Sea-Floor Spreading• The ridge is visible where it goes

ashore as “mirrored” mountains.

• It looks just like this under water

ForceForce

Normal faults

Scarp



• As the sea floor spreads.– It gets older (doesn’t everything?).– It cools and thickens.– It accumulates sediment.


Global Plate Tectonics

• Plate Tectonics = Sea Floor Spreading + Continental drift

• These two concepts came from separate sets of observations

• The Plate Tectonic theory:– Combined these ideas.– Answered almost all of our questions.– Provided incredible predictive POWER!http://www.enchantedlearning.com/egifs/Earthsplates.GIFhttp://www.enchantedlearning.com/egifs/Earthsplates.GIF


Global Plate Tectonics (Cont.)

• Surface divided into 12 major plates.



• These plates move!

• As they do, they collide with each other– Some portions are destroyed by subduction– Some are crushed into mountains

• The collisions

create

earthquakeshttp://images.google.com/imgres?http://images.google.com/imgres?imgurl=http://vulcan.wr.usgs.gov/Imgs/imgurl=http://vulcan.wr.usgs.gov/Imgs/Gif/PlateTectonics/Maps/Gif/PlateTectonics/Maps/map_plate_tectonics_world_bw.gif&imgrefmap_plate_tectonics_world_bw.gif&imgrefurl=http://vulcan.wr.usgs.gov/Glossary/url=http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/Maps/PlateTectonics/Maps/map_plate_tectonics_world.html&h=540&map_plate_tectonics_world.html&h=540&w=859&sz=23&hl=en&start=1&tbnid=15vw=859&sz=23&hl=en&start=1&tbnid=15vAfuTbbcud9M:&tbnh=91&tbnw=145&preAfuTbbcud9M:&tbnh=91&tbnw=145&prev=/images%3Fq%3Dplate%2Btectonicsv=/images%3Fq%3Dplate%2Btectonics%26hl%3Den%26hl%3Den



• Plates are slabs of lithosphere

• Lithosphere = rigid– Crust and uppermost mantle

– “floats” on the asthosphere (remember isostacy?)

• Asthenosphere = plastic, flows

Limited motion

Mesosphere

Plastic flow



• Nearly all seismic and volcanic activity is associated with plate boundaries.

• Plate boundaries and seismic activity:– Co-occur.

– Exist on continuous lines.


• There are 3 ways that plates can interact:– Separate

– Converge

– Slide past



• Three flavors of plate boundaries:– Midocean ridges

• Plates move apart• “spreading centers”• Addition of new crust to trailing

plate edge

– Subduction zones• Plates collide• Destruction of old crust

– Transform faults• Plates move past one another• Crust is neither created or

destroyed

Let’s take a look at these in some detail:Let’s take a look at these in some detail:



1) Midocean Ridges• Volcanism and

faulting

• Hydrothermal circulation

CopperZinc

Leadetc.

Basalt pillows, fresh from an eruptionin the rift valley.

Faultat axial fissure caused

by extensional force in therift valley.

Normal faults

Rift valley of a midocean ridge.

Hydrothermal vent


2) Subduction Zones• Subduction zones are regions where lithosphere with

oceanic crust is being destroyed– Subducted lithosphere melts into the mantle.

– Earthquakes from the surface to 660 km.

– Magma rises to the surface and erupt at volcanoes.

– Ocean sediments are partially scraped off at the trench.


Global Plate Tectonics• Lithosphere with

ocean crust is destroyed at subduction zones.

– So, amount of seafloor built at spreading centers equals the amount recycled into the mantle in subduction zones.

http://whyfiles.org/031volcano/images/subduction.gifhttp://whyfiles.org/031volcano/images/subduction.gif


Subduction

• Explosive volcanic activity.

• Ash falls.

• Lava flows.

• Pyroclastic flows.

• Mud flows.

• Gases.

Mt. St. Helens, May 18, 1980

Stromboli, Italy



• Subduction is ocurring under:– Japan– Western South America– Central America– Alaska– New Zealand– Caribbean– Northwest USA!

Mt. St. Helens

Mt. Ranier



• The Pacific Ocean is surrounded by a “Ring of Fire”– The Ring of Fire includes both volcanoes and earthquakes.

– All are subduction-related


3) Transform Faults

• “Transform”= side by side motion

• Offset the ridge axis• Relative movement is

horizontal• Plates are moving in

opposite directions• This wreaks havoc!

Transform faults

Rift valley

Rift valley

Pla

te A

Pla

te B

Movement


Plate Tectonics (cont.)

• On our West Coast:– Subduction motion has

been transformed into lateral motion in central and southern California.

– This causes the Pacific Plate to move past the North American Plate.



• This transform fault is the infamous San Andreas of California.

• The fault joins the Juan de Fuca Ridge and EPR.

• The strike-slip fault causes massive earthquakes.

San Andreas Fault

San Andreas,Corrizo Plain


Transform faults• The same topography

you see near San Francisco exists in many, many places on the sea floor

http://www.oceanexplorer.noaa.gov/explorations/05lostcity/logs/july23/media/karson1atf_600.jpghttp://www.oceanexplorer.noaa.gov/explorations/05lostcity/logs/july23/media/karson1atf_600.jpg



• These earthquakes:– Are certain to continue

– Vary in strength according to built-up stress

San Francisco, Marina District

Nimitz Freeway, Oakland

Farm field in Santa Cruz


New Zealand Quake 2-22-2011



What causes the plates to move?

• Thermal convection– Produces slow moving currents in the

underlying asthenosphere which exerts drag on the lithospheric plates setting them in motion.


Consequences of PT: Hot Spots• Deep sources of extra-hot magma

• rise up through mantle

• are cause of intra-plate volcanism YellowstoneYellowstone


Consequences of PT: Hot Spots

• As plate passes over source of volcanism, a chain of islands is formed


Island Chains• For

example, the Hawaiian Islands

• Note ages• Ancient

Hawaiians noticed this– referred to

it as Pele moving from island to island


Hawaiian / Emperor Seamount Chain

• All produced by the same hot spot

• age progresses uniformly out the chain

• plate motion changed to produce the bend in the chain


Hot Spot Traces

• The Hawaiian Islands are only one example

• each hot spot under the Pacific plate produces an island chain


Consequences of PT: Atolls and Guyots

- Off-axis volcanism creates- Off-axis volcanism createsVolcanoesVolcanoes- as the plate settles, so do - as the plate settles, so do

the volcanoes the volcanoes


Formation of an Atoll or Guyot

• Starts off with volcanic seamount

• as plate and seamount subside, coral grows upward

• if coral keeps up, an atoll is formed

• if not, a guyot• Coral requires

warm, clear water


Atoll / Guyot Animation


Atoll Structure

Diego Garcia, Chagos Islands, Indian Ocean


Future – Headed to “Pangaea Ultima”????


Summary

• Features of Earth’s surface are determined by the interaction of lithospheric plates.

• Seismic activity is most intense at plate boundaries.

• Crustal material is created at ridges and destroyed at subduction zones.

• The movement of lithospheric plates disrupts continents and leads to the expansion or contraction of ocean basins.

©1996, West Publishing Company (Modified by Asper, 2006) Slide 1 Lesson #4 How did the Oceans get to be the way they are?

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