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Introduction to Oceanography •  Lecture 2: The shape of the seafloor

Magma degassing at Halemaumau Crater, Kilauea, Hawaii�April 2008, Mila Zinkova, wikimedia, CC A S-A 3.0

Introduction to Oceanography

1. Attend Your �Lab Section!--TA’s have �

PTE#’s

Recurring Slope Lineae (flowing water?) in Newton

Crater, Mars (NASA/JPL-Caltech/U. Arizona

image; Public Domain)

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Scientific Units: The S.I. System•  SI units:

Length: meters, mTime: seconds, sMass: kilogram, kgTemperature: ºC (ºK)

Stilfehler, Wikimedia, Public Domain

Photo by 1-1111, Wikimedia, Creative Commons A. S.-A. 3.0

K20 kilogram, NIST.gov image, �Public Domain

Racklever, Wikimedia, Creative Commons A. S.-A. 3.0

SI UnitsMASS, Kilograms (kg):•  1 kg = 1000 grams•  Kilograms are a unit of mass

– Not quite the same as weight �weight = (mass x gravity)

–  1 kg of mass weighs 2.2 pounds on Earth’s surface

– Same 1 kg weighs only 0.4 pounds on the Moon’s surface

•  A cube-shaped box, 10 cm on each side (a liter) filled with seawater is ~1.02 kg

Masses of standard kilograms are drifting!kg will likely soon be based on quantum mechanical constants.

Photo: US NIST, Public Domain, �http://museum.nist.gov/object.asp?ObjID=38

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Where does Earth’s water come from?

1.5×10-5 m

5×10-4 m

Most water probably came from water-bearing minerals in accreted planetesimals and comets. Such minerals are common in meteorites found today.

Green serpentine�~Mg3Si2O5(OH)4

Murchison meteorite�U. Glasgow Earth Science Electron Microscopy lab.

Right-side images: M. Zolensky, NASA/JSC, Public Domain

Monahans meteorite fall fragment, held by Monahans, TX police officer Reggie Bailey. Photo by Mark Sterkel, Odessa American

UCLAMeteoriteGallery

LocatedGeologyBuildingRoom3697

OpeningHours:Monday–Friday:09am–04pm

Sundays:01pm–04pmwithdocentpresent

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Carbonaceousandordinarychondrites

•  CarbonaceouschondritescomprisethemostdiverseclassofchondriLcmeteorites.Therearefivemajorgroups,derivedfromfiveseparateasteroids.

•  ChondrulesareamongtheprincipalcomponentsinnearlyallchondriLcmeteorites;theyaretypicallysub-millimeter-sizeigneousspherules,thatformedasdropletsinthesolarnebula.

•  OrdinarychondritesconsLtute

74%ofobservedfalls.

IronMeteorites

•  The Clark Iron (called Canyon Diablo) is a 357-pound meteorite in the center of the room. It was derived from a 300,000-ton projectile that formed Meteor Crater (the freshest impact crater on Earth) about 50,000 years ago in northern Arizona.

•  The Camp Wood Iron is a 326-pound magmatic iron meteorite from Texas that crystallized in the molten core of a differentiated asteroid.

•  The Gibeon Iron is a 811-pound magmatic iron meteorite from Namibia that crystallized in the molten core of a differentiated asteroid. It has the second or third largest total mass among collected iron meteorites, probably in excess of 70 tons.

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Basic Structure of the Earth •  Layers of increasing

density – Thin Crust

2.5 to 3.0 gm/cm3 – Rocky mantle

3.2 to > 5 gm/cm3

– Metallic core >10 gm/cm3

Planetary Radius: 6371 km

Core

Mantle

Figure E. Schauble

6371 km

Crust 5-70 km

Compositional Layers of Earth

Hand samples: Granite (cont’l. crust), Basalt (ocean crust), peridotite (mantle), iron meteorite (core)

•  Thin crustal surface layer (almost all we see) Oceanic (basalt) ~8 km thick Continental (granite) ~35 km thick

•  Mantle (silicate rock) –  Bulk of planet’s volume

~ 2900 km thick •  Core

–  Iron-rich inner part of the planet

Core

Mantle

Figure E. Schauble

6371 km

Crust 5-70 km

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Mechanical Layers of Earth Lithosphere Top ~100km

Cold and strong

Includes crust, part of mantle

Asthenosphere Beneath lithosphere

Hot and plastic Rigid on short

timescales, flows on long

timescales

USGS image, http://pubs.usgs.gov/gip/dynamic/inside.html

Public Domain

Layering of mechanical strength

USGS, Public Domain

STRONG, SOLID Lithosphere WEAK, SOLID Asthenosphere

STRONGER, SOLID Deep mantle

WEAK, LIQUIDOuter core

?, SOLID Inner core

Lithosphere

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So the Mantle Isn’t Melted? NO! It’s solid, except in a few places (see plate

tectonics).

Solid lithosphere

Solid asthenosphere

Partly-melted mantle

Mid-ocean ridge volcanoes

~ 100 km

USGS, Presumed Public Domain

Convection in the Mantle

•  Mantle convection removes heat from Earth’s interior

•  Occurs on 100 million year timescales •  How do we know this? Seismic tomography

Simulated mantle convection, by Rene Gassmöller, Colo. State U http://

www.math.colostate.edu/~gassmoel/

This simulation is sped up a lot: 1 sec à 3 million years!

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QUESTIONS?

http://www.gps.caltech.edu/~gurnis/Movies/movies-more.html

Simulation and animation: G.B. Wright, N. Flyer, and D.A. Yuen. A hybrid radial

basis function - pseudospectral method for thermal convection in a 3D spherical shell. Geochem.

Geophys. Geosyst., 11 (2010), Q07003.

http://www.youtube.com/watch?v=-kDb0HlDsIM

The Big Picture: Continents vs. Ocean Basins

Plumbago, wikimedia commons, C C A S-A 3.0

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The Big Picture: Bimodal Distribution

-10000

-8000

-6000

-4000

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0

2000

4000

6000

8000

10000

0 0.01 0.02 0.03 0.04 0.05

Ele

vatio

n (m

eter

s)

Fraction of Earth's surface area

Histogram of elevations on Earth

Ocean

Land

Figure by E. Schauble based on ETOPO5 data (NOAA), as sampled by S.L. Goldstein and S. Hemming, Columbia U. Bin heights are 100m.

Continents vs. Oceans •  Why do the continents tend to lie a little

above sea level? •  Why is the

ocean mostly 2-6 km below sea level?

•  How do we measure these elevations, anyway?

-10000

-8000

-6000

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0

2000

4000

6000

8000

10000

0 0.01 0.02 0.03 0.04 0.05

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vatio

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eter

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Fraction of Earth's surface area

Histogram of elevations on Earth

Ocean

Land

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•  Multibeam Sonar Sound waves are blasted from the

ship, and echoes are recorded. The distance to the seafloor is

determined from the time between making the sound and the echo:

d ≈ (sound speed)•(time delay)/2 High spatial resolution, mapping a

small area (under ship)

Explorer ridge west of Vancouver Island, NOAA, www.photolib.noaa.gov/ bigs/expl1571.jpg, Public

Domain.

USNS Bowditch, http://www.navy.mil/view_single.asp?id=2767, Public Domain

Satellite radar mapping (gravity) Satellites (like TOPEX-Poseidon and Jason-1&2) can measure their distance from the sea surface.

Knowing this distance and the orbit of the satellite, we can determine the topography (shape) of the ocean surface.

Any extra mass on the seafloor will exert extra gravity on the ocean, causing a “hump” in the sea surface.

Thus it is possible to� extract the seafloor�

topography

Great spatial coverage,�lower resolution &�precision (far away).

Painting of JASON-2, http://sealevel.jpl.nasa.gov/mission/images/OSTM-06.jpg, Public Domain

Geoid Geoid Modified by E. Schauble from original by MesserWoland, Wikimedia Commons, http://en.wikipedia.org/wiki/File:Geoida.svg, CC A S-A 3.0

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Oceanic vs. Continental Crust •  Bimodal elevation distribution due to

– Continental Crust Vs.

– Oceanic Crust

Adapted from USGS image, Public Domain

Lithosphere 0 km

100 km

Continental Crust: Granitic •  Continents typically

made up of the rock granite

•  Density: 2.7-2.8 gm/cm3

–  density = mass/volume

•  Light in color, coarse in texture –  Yosemite/

Mt. Whitney rock

Granite hand specimenGranite, Yosemite N.P.(?), David Monniaux, �Wikimedia Commons, Creative Commons Att. S-A 3.0

Approx. 1 cm

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Oceanic Crust: Basaltic •  Ocean crust made up

of basaltic rock •  Density: 2.9-3.0 gm/

cm3

•  Basalts are typically about 0.2 gm/cm3

denser than granites – about 7% denser Mid-ocean ridge basalt, East Pacific Rise, �

photo by E. Schauble, �Sample courtesy A. Schmitt

Basalt hand specimen

Oceanic vs. Continental Crust •  Continental Crust

– 30-40 km thickness •  Oceanic Crust

– 5-10 km (thinner) & denser than CC

•  Crust underlain by denser mantle ~ 3.3 gm/cm3

~ 15% denser than crustal materials Can flow at depths below ~100 km, i.e. in the

asthenosphere.

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What is Buoyancy?

•  Archimedes’ Principle: A solid will sink into a fluid until the displaced fluid’s mass is equal to the mass of the solid.

Figure E. Schauble. Profile of Emma Maersk by Delphine Ménard, Wikimedia Commons, Creative Commons Share-alike -2.0-fr

What is Buoyancy? •  Any object in a fluid will be

pushed up as the fluid tries to fill in the space taken up by the object.

•  At rest, a buoyant object will sit so that the mass of fluid it displaces equals the mass of the object.

•  The downward force of gravity on the object is balanced by the restoring force from gravity pushing fluid into the displaced volume.

1)  Low density materials don’t have to displace as much fluid to match their mass, so they float higher

2)  Thicker pieces of material have more volume left over after displacing their mass, so they float higher

Figure by Christophe Finot, Wikimedia Commons, Public Domain

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Isostatic Balance •  Blocks of lithosphere (crust + uppermost mantle, ca. 100 km thick)

float atop the plastic asthenosphere

Lithosphere Lithosphere

Asthenosphere Asthenosphere

Which is more like continental lithosphere? Which is more like oceanic lithosphere?

Figure by Kurgus, Wikimedia Commons, Public Domain

Elevation of Continents vs. Oceans

Continental Crust:Thicker & Lighter

Oceanic Crust:Thinner & Denser

Adapted from USGS image, Public Domain

Lithosphere 0 km

100 km

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Oceans vs. Continents OCEANS CONTINENTS

Average Elevation

–3800m +840m

Surface Area 71% 29%

Crustal Distribution

59% 41% (Margins)

Crustal Thickness

5 - 10 km 30 - 70 km

Density 3.0 gm/cm3 2.7 gm/cm3

QUESTIONS?

Supercontinent breakup simulation by Gurnis et al., Caltech, http://www.gps.caltech.edu/~gurnis/Movies/Science_Captions/aggdisp.html

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Morphology of the Oceans Continental Margins

AbyssalPlains Mid-Ocean

Ridges

Deep-sea Trenches

Continent

Ocean Basin

Image by Plumbago, Wikimedia Commons, Creative Commons A S-A 3.0

•  Flattest regions on Earth’s surface –  Large-scale slopes ~ 0.1o

•  ~1m drop per km

•  Sediments covering old oceanic crust •  Depths: 3 - 5 km •  Widths:

~1000-3000 km

Abyssal Plains

Abyssal Plains

Bathymetry from GEBCO world map, http://www.gebco.net, educational use expressly allowed.

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Abyssal Hills Old volcanic domes Usually covered in sediments < 1000m high

Seamounts > 1000 m high but

below sea level Often found in

clusters & chains

Abyssal Hills & Seamounts

Seamount Chain

Bathymetry from GEBCO world map, http://www.gebco.net, educational use expressly allowed.

Continental Margins

•  Two Types – Atlantic style “passive” margins

•  Broad flat shelves •  Examples are Florida, Virginia

– Pacific style “active” margins •  Narrow shelf adjacent to a deep-sea trench •  Examples are Chile, Japan

Distances and depths of margin features are variable. Active margin features are narrower and extend deeper than on

passive margins.

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Broad continental shelf, gradual transition to deep ocean.

Passive Margins (Atlantic-style)

Bathymetry from GEBCO world

map, http://www.gebco.net, educational use

expressly allowed.

Passive Continental Margins •  “Drowned” continental sediments pile up adjacent to

the continents •  Comprised of:

–  Continental Shelf –  Shelf Break –  Continental Slope –  Continental Rise

Shelf

Break

Slope

Rise

Cont’l Crust Oceanic Crust

Sediment

Modified from figure by Cidnye, Wikimedia Commons, Public Domain

(Not to scale)

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Continental Shelf •  Shelf: terraces of sediment

•  Width: variable,

~10 km (active) ~100’s km (passive).

•  Slope ≤ 0.5o

Very flat

•  Ends at the Shelf Break Occurs at average water depth

approx. 140 m (variable)

BREAKSHELF

Florida

Georgia

S. Carolina

N. Carolina

Figure from NOAA Ocean Explorer, http://oceanexplorer.noaa.gov/technology/tools/

mapping/media/GulfofMexico.jpg Public Domain

Continental Slope

•  Beyond Shelf Break is the Continental Slope

•  Much steeper, ~4o •  Depths: to ~3-4 km •  Typical width ~20 km

SLOPE

SLOPE

Florida

Georgia

S. Carolina

N. Carolina

Figure from NOAA Ocean Explorer, http://oceanexplorer.noaa.gov/technology/tools/

mapping/media/GulfofMexico.jpg Public Domain

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Continental Rise •  At the base of the

continental slope •  Slope lessens

•  Depths: from ~2km - 5km

•  Width: ~ 100-1000 km

•  Sedimentary “apron” or “fan”

RISE

Bathymetry from GEBCO world map, http://www.gebco.net,

educational use expressly allowed.

Submarine Canyons Image from Divins, D.L., and D. Metzger, NGDC Coastal Relief Model,

http://www.ngdc.noaa.gov/mgg/coastal/coastal.html. Public Domain

Monterey

Santa Cruz

Monterey Bay

Erosional incisions through shelf and slope

Transport sediments from the rise out onto abyssal plains –  Turbidity

currents •  Transport

sediments onto Abyssal Plains

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Monterey

Santa Cruz

Monterey Bay

Movie by Gary Parker, St. Anthony Falls Hydraulic Laboratory,

University of Minnesota

Submarine Canyons, carved by debris

flows and turbidity currents, not rivers

Image from Divins, D.L., and D. Metzger, NGDC Coastal Relief Model,

http://www.ngdc.noaa.gov/mgg/coastal/coastal.html. Public Domain

Active Margins •  A steeper, narrow

margin, usually bordered by a deep sea trench.

•  Particularly common around the Pacific Ocean.

Active Margin

Passive Margin

Bathymetry from GEBCO world map, http://www.gebco.net, educational use expressly allowed.

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Deep-Sea Trenches Depths: 5 - 11 km Widths: 30 - 100 km Associated with volcanism and island arcs

–  i.e., the Andes and the Aleutians, respectively

Also associated with the strongest and deepest earthquakes on the planet

Including last week’s Chile earthquake

Active Margin

Passive Margin

Same image credit as prevous slide.

Deep-Sea Trenches The Ring of Fire – Trenches, earthquakes and volcanoes concentrated along the Pacific, including active margins.

Figure by Gringer, Wikimedia Commons, Public Domain, http://en.wikipedia.org/wiki/File:Pacific_Ring_of_Fire.svg

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LA

OC

Catalina

Santa Barbara

SD

Southern California Margin •  Southern CA has an unusual margin

Map Courtesy C. Goldfinger and J. Chaytor, OSU, from USCD Earthguide online classroom

~30 km

Southern Californian Borderland •  Pervasive active faulting and tectonics

–  No broad flat shelf region –  Instead, fault bounded ridges and basins –  Ridges can form islands (i.e., Catalina) –  Basins can be

1 - 2 km deep –  Continental slope

~80-100 km west Los Angeles sits on

a silted up basin!

LA

OC

Catalina

Santa Barbara

SD

Map Courtesy C. Goldfinger and J. Chaytor, OSU, from USCD Earthguide

online classroom

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Deep Ocean Basins What about in the very center of the ocean basins?

Mid-ocean ridge

Image by Plumbago, Wikimedia Commons,

Creative Commons A S-A 3.0

Mid-Ocean Ridges •  Earth’s longest continuous mountain chain

~ 60,000 km long, ~1/3 of ocean floor area Relief: ~ 2-3 km above abyssal plains

NOAA global relief map, Public Domain