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!