Post on 13-Mar-2018
Earth in 2-D, 3-D & 4-D
We will consider the scientific tools and
techniques used to
map surface features,
reconstruct the layered structure of Earth,
and interpret Earth history, including
the origin of the ocean
http://shadow.eas.gatech.edu/~anewman/classes/geodynamics/random/Worldmap.gif
Prior to the early 20th
century, “soundings”
were the only means
to determine water
depth
weighted lines lowered
from ships
time consuming,
relatively few, doubtful
accuracy
Voyage of the H.M.S. Challenger
First dedicated oceanographic
(scientific) exploration of the
world ocean
Echo Sounders
sound source & receiver (hydrophone) on
hull of ship
high frequency sound waves travel through
the water, reflect off the seafloor, and are
recorded by the hydrophone
provide continuous depth profiles along a
ship’s track
but only 2-dimensional
Seismic Reflection Profiles
sound source and hydrophone towed by ship
2-D, continuous profile
lower frequency energy source (stronger
sound source with fewer sound waves per
second)
deeper penetration into sedimentary layers
and ocean crust
Echo Sounder
Seismic Reflection
Profiler (sound source
and hydrophone)
1/27/2014
Echo sounder record of a seamount
Seismic Profiler record
Seismic
reflection
survey
Other tools to map the seafloor:
Side Scan Sonar
like echo sounder, but it images a 60 km
swath of seafloor
overlapping swaths = complete coverage
(3-D)
Seabeam swath
producing 3-D
bathymetric
map of seafloor (a bathymetric
map is similar to a
topographic map
used to depict
relief on land)
Sidescan Sonar of Ship Wreck
This is a
bathymetric map
based on a
seabeam survey
for Deep Sea
Drilling Project
sites off northwest
Africa (contour
interval = 50 m); note the very steep
Mazagan
Escarpment dropping
off to the deep-sea
isobaths
Multibeam echo sounding survey This image shows a depiction of the beam of sound waves mapping the ocean floor. Multibeam
surveying provides incredibly detailed imagery of the seabed. On board survey ships,
instruments emit multiple beams of sound waves, which are reflected off the ocean floor. As the
sound waves bounce back with different strengths and timing, computers analyze these differences
to determine the depth and shape of the seafloor, and whether the bottom is rock, sand or mud. http://www.teara.govt.nz/EarthSeaAndSky/OceanStudyAndConservation/ChartingTheSeaFloor/4/ENZ-Resources/Standard/1/en
http://soundwaves.usgs.gov/2006/03/outreach3.html
Schematic diagram showing the various types of seafloor-mapping
systems used by the Western Coastal and Marine Geology team. Drawing
by Bruce Rogers, modified from image on a Web page posted by the USGS Woods Hole Sea-Floor Mapping Group .
http://soundwaves.usgs.gov/2006/03/outreach3.html
http://woodshole.er.usgs.gov/operations/sfmapping/images/homerec.jpg
http://www.paulillsley.com/Gulf_of_Maine/index.html
Satellites
Precise altimeters (using
microwaves) can map the
relief of the ocean surface
with centimeter-scale
resolution
Bathymetric highs and lows
on the seafloor, and
differences in rock density,
cause measureable
gravitational distortion of
the ocean surface http://sealevel.jpl.nasa.gov/education/images/sat_earth.gif
Note: seafloor features distort the ocean surface, these
very subtle distortions can be measured from space!
http://sealevel.jpl.nasa.gov/education/tutorial1.html
Right now, T/P's
measurement precision
for sea surface height is
4.3 cm (1.7 inches).
Because the satellite
flies at about 1330 km
(830 miles) above the
Earth's surface, that's
comparable to knowing
the sea surface height to
much less than the
thickness of a dime
while flying in a jet at
35,000 feet altitude.
Map of the central and North Atlantic from satellite
Map of the world ocean from space. What do you see?
Perspective view of the seafloor of the
Atlantic Ocean and the Caribbean Sea.
The Lesser Antilles are on the lower left
side of the view and Florida is on the
upper right. The purple seafloor at the
center of the view is the Puerto Rico
trench, the deepest part of the Atlantic
Ocean and the Caribbean Sea. http://woodshole.er.usgs.gov/project-
pages/caribbean/atlantic+trench_large.html
application:
Caribbean tsunami
and earthquake
hazards studies
(USGS)
Radiometric Dating
The primary method used to
determine absolute ages of geologic
and some biologic materials.
Recall the basic
structure of the atom:
a nucleus with
protons and neutrons
surrounded by shells
or orbitals of
electrons.
Protons: + charge
Electrons: - charge
Neutrons: no charge
# protons = # electrons
# neutrons are different
in different isotopes of
an element.
Unstable isotopes of certain
elements (called parents)
radioactively decay to the stable
isotopes of other elements (called
daughters).
This happens in the nucleus by
several mechanisms.→
The bottom line is that the
number of protons and neutrons
in the parent isotope changes as it
decays to the daughter.
This decay occurs at a precisely
determined rate called a half-life.
For example, the parent isotope 238U decays to
the daughter isotope 206Pb with a half-life (t1/2) =
4.5 x 109 years.
This means that with the passage of every 4.5 x 109 years, the
number of remaining 238U is reduced by 50%:
t1/2 = 0 238U = 100 206Pb = 0
t1/2 = 1 238U = 50 206Pb = 50
t1/2 = 2 238U = 25 206Pb = 75
The parent isotope 14C decays to the daughter
isotope 14N with a t1/2 = 5730 years.
How do we know what the internal
structure of Earth is like?
See pages 94-95 in “Investigating the
Ocean”
Seismic Waves & Earthquakes:
Earth Structure Revealed
Earthquakes release a tremendous amount of
energy (“seismic energy”)
seismic waves radiate away from their point of
origin (= focus)
3 different forms of seismic waves:
1. Rayleigh waves travel along the surface of the Earth
2. P-waves travel fast through the Earth
3. S-waves are slower and cannot travel through liquids
p. 94
Refraction of seismic waves
P-waves and S-waves bend (refract) when
they pass from a material of one density
into a material with a different density
By measuring the arrival times of P- and
S- waves around the globe from many
earthquakes, it is clear that our Earth is
layered in concentric spheres of different
composition and density
p. 94
I shown a laser pointer at the floor where it
produced a spot. Then I placed a clear plastic
block in the path of the beam. Part of the beam
reflected off the block and produced a spot on
the ceiling. Part of the beam passed through the
clear plastic, but its path was bent or refracted
causing the spot on the floor to move to a
different position.
This is the way seismic rays pass through the
Earth. When they encounter a new layer part of
the seismic ray reflects back toward the surface,
and part of it passes through the layer but is
refracted.
Earthquake-generated P-wave refraction
and reflection within the Earth.
Seismic Energy: P & S Waves This text figure is shown in color in the follow two slides.
S-waves are blocked by the liquid outer core yielding the S-wave
shadow zone. So we know the outer core is liquid and from the size of
the shadow we know the size of the outer core.
P-waves are focused by the core yielding the P-wave
shadow zone. This gives us more information about
Earth’s internal layering.
The breakup of
the supercontinent
Pangaea, the
drifting of the
continents, and
seafloor spreading
over the last 225
million years.
Now we are going to compare a simple bar
magnet to the magnetic field of the Earth. We
can use this information to demonstrate
continental drift and seafloor spreading!
Iron filings outlining the dipolar
field of a bar magnet.
Earth’s dipolar field with its magnetic lines of force is
similar to a bar magnet.
Note that the magnetic lines of force intersect the
surface of the Earth at different angles.
At the equator the lines of force parallel the ground,
but the higher the latitude the steeper the lines of
force intersect the ground. At the poles the lines of
force are vertical.
So you can determine the latitude of an area by the
dip of the lines of force in that area.
Paleomagnetism: the study of ancient magnetic fields
As a magma cools and solidifies into an igneous rock minerals
crystallize from the magma.
Among these crystallizing minerals are usually small quantities of
minerals that are magnetic and/or with magnetic susceptibilities, such
as magnetite.
As these minerals cool below their Curie temperatures, they record the
surrounding magnetic field of the Earth that exists at the time of cooling.
Sediments being deposited at the bottom of a lake or ocean may also
record the magnetic field, so the magnetic field may also be recorded in
sedimentary rocks.
This preserved magnetism is also called remnant magnetism. The latitude at which an ancient rock formed can be determined from the inclination of the remnant magnetic field.
Now these cooling lavas will record the ambient
magnetic fields for their latitudes as they cool
down below their Curie temperatures.
Remnant magnetism recorded in igneous rocks that have cooled below their Curie point. Notice how the different rocks have different fields preserved in them. Even if subsequent continental drift moves these rocks to different latitudes, they will preserve their original fields.
Real example: 200 million year old lava
flows just a few miles from campus have
remnant fields that tell us that 200 million
years ago Massachusetts was close to the
equator! Continental drift really happens!
By measuring the magnetic fields of rocks of different
ages, it has been discovered that the Earth’s magnetic
field has reversed polarity episodically many times.
The magnetic reversals recorded in ocean floor basalts
have proven valuable in demonstrating sea floor
spreading.
Polarity reversal in the Earth’s magnetic field.
Fig. 17.23
History of
magnetic reversals.
So the Earth’s
magnetic polarity
flips in an irregular
way over geologic
time.
Development of magnetic stripes on the seafloor on either side of the spreading ridge.
Lava comes up in the axial valley as the two halves of the ocean floor spread apart.
The lava cools and records either a normal or reversed polarity. As spreading
continues, the rocks move away from the axial valley of the ridge as new lava fills in
the axial valley, cools down, and records the new polarity. The magnetic stripes on the
seafloor are symmetrical about the ridge.
Another view of the magnetic stripes
on the seafloor.
Age of the ocean crust. The blue is the oldest ocean floor (about
200 million years old) and the red is the youngest floor (forming
right up to the present day). Seafloor spreading!