Transcript Ocean Floor

Exploring the inner and
outer shells of earth
Chapters 2-3-4
Additional reading: MAR discovery, pdf, class web site
Learning Objectives:
Earth structure, Plate tectonics
and Ocean floor
Difference between oceanic and continental crust.
Understand the processes that are continuously changing Earth’s
surface as lithospheric plates move relative to one another.
Identify the role of oceanic ridges, transform faults and deep-sea
trenches in defining the edges of lithospheric plates.
Understand the importance of asthenospheric thermal convection
in plate tectonics and the resulting compression or tensional forces
at the plate boundaries.
Explain the distribution of magnetic anomaly stripes, seismicity,
and volcanism in terms of the concept of global plate tectonics.
Spreading rates of ocean basins.
Earth’s Structure
•
Layered system (like an onion, concentric regions)
~ differentiation of mineral material
Earth’s Structure
(cont’d.)
Classification according to chemical composition
4 concentric regions of
mineral material:
1. crust
2. mantle
3. outer core - molten
4. inner core - solid
Earth’s Structure
(cont’d.)
Classification according to chemical composition
1. Crust
Two types:
continental g granite – composed of
silicates rich in Na, K & Al
ocean g basalt – composed of
silicates rich in Ca, Mg & Fe
• represents 0.4% of Earth’s mass
• extends down to 75 km
Earth’s Structure
(cont’d.)
Classification according to chemical composition
2. Mantle
Three parts:
uppermost/middle/innermost
• Composed of Mg-Fe silicates
• represents 68% of Earth’s mass
• extends down from base of
crust to ~2,900 km
Earth’s Structure
(cont’d.)
Classification according to chemical composition
3. Core
Two parts:
Outer
Inner
• Composed of Fe & Ni
• Represents 28% of Earth’s mass
• Extends down from base of
mantle ~ 6400km
Earth’s Structure
(cont’d.)
Classification according to physical properties
(factor in temperature and pressure)
4 concentric regions:
1. lithosphere - rigid outer shell (crust & uppermost mantle)
• 100 - 150km thick
• does not change shape
Earth’s Structure
(cont’d.)
2. Asthenosphere - soft, flows over geologic time under the weight of
the lithosphere (small fraction of middle mantle)
• lithosphere ‘floats on top’
• zone where magma formed
• 200 – 350km thick
• easily deformed, can be pushed down by overlying lithosphere –
“plastic” – tar or asphalt
Earth’s Structure
(cont’d.)
Classification according to physical properties
3. Mesosphere - rigid but not as hard as lithosphere
• higher temp than asthenosphere, but not molten because of
compression pressure
• 4950km thick
Earth’s Structure
(cont’d.)
Classification according to physical properties
4. Core - outer is molten, inner is solid
Earth consists of a
series of concentric
layers or spheres
which differ in
chemistry and
physical properties.
Chemical
Layers
Physical
Layers
Physical state is determined
by the combined effects of
pressure and temperature.
• Increasing pressure raises the melting
point of a material.
• Increasing temperature provides additional
energy to the atoms and molecules of
matter allowing them to move farther
apart, eventually causing the material to
melt.
• Both pressure and temperature increase
toward the center of the Earth, but at
variable rates.
The Oceans of the World
How do we learn about the inner
structure of the planet?
Seismology is the study of elastic waves
that travel through the earth
Two main wave types:
Compression waves (Pwaves): travel by
squeezing and
expanding medium they
travel through. They can
travel through both
solids and liquids (e.g.,
sound waves);
Shear waves (S-waves):
travel by shearing
medium they pass
through. S-waves can
travel only through solids
since particles need to be
bonded to each other to
propagate wave;
Earthquake are an incredible
source of seismic waves
Seismic waves travel
along the quickest
route, generally
through the planetary
interior to the seismic
stations, changing
speed every time
material properties
change.
Reconstructing the internal
structure of the planet
•
Combining information
from many seismic
waves we can get a
detailed internal
structure of the earth.
•
Crust & upper mantle:
large increases in
seismic velocities and
density
•
Mantle: Gradual velocity
and density increase
•
Outer core: dramatic
density increase and no
S-waves
•
Inner core: Jump in
density and P-wave
velocity, S-waves return
Other planets?
Seismic Velocity and Density
solid
solid,
more dense
Liquid Iron
solid Iron
Earthquakes produce waves in the
ocean
Sea level:
RISE
FALL
Courtesy: K. Satake, unpublished
World Seismicity
(1898-2003)
Mw ≥ 6.0
Mw ≥ 7.7
Many
large earthquakes
along subduction
zonesevents
Most “Great”
earthquakesoccur
are subduction
mega-thrust
The Physiography of the North Atlantic Ocean Floor
continental
margins
deep ocean
basins
midoceanic
ridges
Mid Atlantic Ridge – new findings:
• http://www.noc.soton.ac.uk/gg/classroom
@sea/JC007/about.html
• http://www.sciencedaily.com/releases/200
7/03/070301103112.htm
http://www.noc.soton.ac.uk/gg/classroom@s
ea/JC007/background.html
MAR discovery - http://alpha.es.umb.edu/faculty/af/intro_ocean.htm#Unit1
Type of continental margins
Continental margins are the submerged edges of the
continents and consist of massive wedges of sediment
eroded from the land and deposited along the continental
edge. The Continental Margin can be divided into three
parts: the Continental shelf, the Continental slope, and the
Continental rise.
Passive
Continental Margin
• Midoceanic Ridge Province consists of a
continuous submarine mountain range
that covers about one third of the ocean
floor and extends for about 60,000 km
around the Earth.
Midocean Ridge
• Deep Ocean Province is between the
continental margins and the midoceanic
ridge and includes a variety of features
from mountainous to flat plains: Abyssal
plains, Abyssal hills, Seamounts, and
Deep sea trenches.
Deep Ocean Basin
Hydrothermal Vents
03_13a
This three-dimensional rendition of a bathymetric map shows Patton
Seamount, a Gulf of Alaska seamount we visited in 1999, with two
smaller seamounts in the foreground. Deep areas are blue, and shallow
areas are red.
WHY DO LAND AND OCEAN EXIST?
Geologic Differences between
2-3
Continents and Ocean Basins
Continents and ocean basins differ in
composition, elevation and physiographic
features.
• Elevation of Earth’s surface displays a bimodal
distribution with about 29% above sea level and
much of the remainder at a depth of 4 to 5 kilometers
below sea level.
• Continental crust is mainly composed of granite, a
light colored, lower density, igneous rock rich in
aluminum, silicon and oxygen.
• Oceanic crust is composed of basalt, a dark colored,
higher density, volcanic rock rich in silicon, oxygen
and magnesium.
Geologic Differences between
Continents and Ocean Basins
WHY DO LAND AND OCEAN EXIST?
OCEANIC CRUST = THIN AND DENSER
CONTINENTAL CRUST = THICK AND LEIGTHER
• Continents are thick (30 to 40 km), have
low density and rise high above the
supporting mantle rocks.
• Sea floor is thin (4 to 10 km), has greater
density and does not rise as high above the
mantle.
Oceanic Crust Versus Continental Crust
Isostacy
Principle that dictates how different parts
of the lithosphere stand in relation to each
other in the vertical direction
•
Continental crust less dense (granitic) therefore rises higher
relative to ocean crust (basaltic)
•
Continents move up and down depending on weight on top (i.e.
from glaciers - ‘isostatic rebound’)
~
Continents pop up after glaciers melt
~
Canada and Scandinavia rising at a rate of 1m/100yrs
because the glaciers are receding
Isostasy refers to the balance of an
object “floating” upon a fluid
medium. Height of the mass above
and below the surface of the medium
is controlled by the thickness of the
mass and its density (similar to ice
floating in water).
http://atlas.geo.cornell.edu/education/student/isostasy.html
http://woodshole.er.usgs.gov/operations/modeling/movies/fli/stellrise.flc
altimetry
Satellites in orbit around the planet use radar altimetry to measure
the height of the sea level (accuracy of 2 cm).
http://www.ecco-group.org/animations_iter21/TP_ps21.mpeg
Model of the shape of the Earth
geoid: The equipotential surface of the Earth's gravity field which best fits, in a
least squares sense, global mean sea level (MSL)
http://www.esri.com/news/arcuser/0703/geoid1of3.html
Why ocean bathymetry?
Ocean Circulation and Climate
Deep ocean mixing and pathways
Social impacts:
Tsunami
Social impacts:
Gas and oil extraction