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Introduction to Oceanography• Lecture 3: The shape of the seafloor

Map of the North Atlantic, 1957 by Marie Tharp and Bruce HeezenGeological Society of America

Introduction to Oceanography

1. Attend Your Lab Section

1977 map of the North Atlantic.

Berann, Tharp and Heezen. National

Geographic Society

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What Planetary Bodies have Oceans?

• Definitely Earth!

NASA Image, Public Domain

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 meteoriteU. 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

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UCLAMeteoriteGallery

LocatedGeologyBuildingRoom3697

OpeningHours:Monday– Friday:09am– 04pm

Sundays:01pm– 04pmwithdocentpresent

Carbonaceousandordinarychondrites

• Carbonaceouschondritescomprisethemostdiverseclassofchondriticmeteorites.Therearefivemajorgroups,derivedfromfiveseparateasteroids.

• Chondrulesareamongthe

principalcomponentsinnearlyallchondriticmeteorites;theyaretypicallysub-millimeter-sizeigneousspherules,thatformedasdropletsinthesolarnebula.

• Ordinarychondritesconstitute74%ofobservedfalls.

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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,

Basic Structure of the Earth• Layers of increasing

density– Thin Crust

2.5 to 3.0 gm/cm3

– Rocky mantle3.2 to > 5 gm/cm3

– Metallic core >10 gm/cm3

Planetary Radius:6371 km

Core

Figure E. Schauble

Crust5-70 km

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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 thickContinental (granite) ~35 km thick

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

~ 2900 km thick

• Core– Iron-rich inner part of the planet

Core

Figure E. Schauble

Crust5-70 km

Mechanical Layers of EarthLithosphereTop ~100km

Cold and strong

Includes crust, part of mantle

AsthenosphereBeneath lithosphere

Hot and plasticRigid on short

timescales,flows on long

timescales

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

Public Domain

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Layering of mechanical strength

USGS, Public Domain

STRONG, SOLID Lithosphere

WEAK, SOLID Asthenosphere

STRONGER, SOLIDDeep mantle

WEAK, LIQUIDOuter core

?, SOLID Inner core

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

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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!

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

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The Big Picture:Continents vs. Ocean Basins

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

The Big Picture: Bimodal Distribution

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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.

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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?

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• Multibeam SonarSound 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)/2High 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

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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 toextract the seafloortopography

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

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

Public Domain

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

QUESTIONS?

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

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Oceanic vs. Continental Crust• Bimodal elevation distribution due to

– Continental CrustVs.

– Oceanic Crust

Adapted from USGS image, Public Domain

Lithosphere0 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 materialsCan flow at depths below ~100 km, i.e. in the

asthenosphere.

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QUESTIONS?

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

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

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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

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

float atop the plastic asthenosphere

Lithosphere Lithosphere

AsthenosphereAsthenosphere

Which is more like continental lithosphere?

Which is more like oceanic lithosphere?

Figure by Kurgus, Wikimedia Commons, Public Domain

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Elevation of Continents vs. Oceans

Continental Crust:Thicker & Lighter

Oceanic Crust:Thinner & Denser

Adapted from USGS image, Public Domain

Lithosphere0 km

100 km

Oceans vs. ContinentsOCEANS 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