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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
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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?
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4000
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Ele
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