Transcript SOund Navigation And Ranging

Sea Floor Mapping
with Sonar and Magnetometers
Sonar
SOund Navigation And Ranging
Sound waves are
directed to the bottom of
the ocean and reflect
back to the source.
The time it takes the
sound wave to return is
used to calculate depth.
Seafloor Mapping
Sonar was first used in World
War I. Seafloor mapping began
in the 1920s.
During World War II, advances
in sonar and electronics led to
much improved systems.
These systems were used to
construct the first detailed maps
of important features, such as
deep-sea trenches and midocean ridges.
Seafloor Mapping
Mid-Atlantic Ridge − World’s Largest Mountain Range
It is under water!
Earth’s Magnetic Field
Earth has a magnetic field
A compass needle will line up to the
magnetic field
The north end of a compass needle
points to Earth’s north magnetic pole
which is different from Earth’s
geographic north pole.
(Note: Earth’s north magnetic pole has
the same polarity as the south pole of a
bar magnet)
Earth’s Magnetic Field
Earth’s magnetic field
changes over time as
evidenced by the
changing location of
the magnetic pole.
Magnetism
Most iron-bearing minerals are at least weakly magnetic
Each magnetic mineral has a Curie temperature, the
temperature below which it remains magnetic
Above the Curie temperature the mineral is not magnetic
The Curie temperature varies from mineral to mineral, but it
is always below the melting temperature of the mineral
Magnetism
Hot magma is not magnetic
As a magma cools and solidifies,
the iron-bearing minerals
crystallize
As the minerals cool below the
Curie temperature, the ironbearing minerals become
magnetic
These magnetic minerals align
themselves parallel to Earth’s
magnetic field
Photograph by J.D. Griggs on
March 28, 1984, USGS
Magnetic Reversals
In 1905, French geophysicist Bernard
Brunhes found that rocks in an ancient
lava flow in France were magnetized in
a direction nearly opposite to that of
Earth’s current magnetic field.
From this, he deduced that when that
magma solidified our current magnetic
North Pole was close to the
geographical South Pole.
This could only have happened if the
magnetic field of Earth was reversed at
some point in the past.
Magnetic Reversals
By examining rocks in many locations on Earth,
geologists have confirmed Brunhes’ finding’s and
found evidence that magnetic reversals have
occurred though out Earth’s history.
They occur on an irregular basis ranging in time
from tens of thousands of years to millions of years
Seafloor Mapping
In the 1950s, scientists began using an instrument called a
magnetometer (developed during World War II to detect
submarines) to study the ocean floor.
From the
USGS.
They found that the ocean floor showed a zebra-like pattern of
alternating stripes of magnetically different rock.
Magnetism on the Sea Floor
An important discovery was made when they mapped the magnetic
profile of the sea floor around the Mid-Atlantic Ridge.
The maps showed parallel
magnetic ‘stripes’ of
alternating magnetic polarity
that were symmetrical across
the ridge axis.
How could this magnetic
striping pattern form?
Magnetic “Stripes” on Seafloor
Why are the stripes symmetrical around the crests of the midocean ridges?
Seafloor Spreading
By 1962, Harry Hess at Princeton University (and a Naval Reserve
Rear Admiral), and Robert S. Dietz had coined the term “seafloor
spreading”
In 1963, the team of F. J. Vine and D. H. Matthews (and
independently L. W. Morley) proposed that seafloor spreading could
explain the observed magnetic reversal striping on the seafloor.
Generation of Sea Floor Magnetic Stripes
Map of Sea Floor Magnetic Stripes
Map of Sea Floor Magnetic Stripes
Sometimes it’s fun to pretend that our record of the
seafloor’s magnetic stripes is complete
Seafloor Drilling
In 1968, a research vessel named the Glomar Challenger
embarked on a year-long scientific expedition, criss-crossing the
Mid-Atlantic Ridge between South America and Africa and
drilling core samples at specific locations.
Seafloor Drilling
When the ages of the samples were calculated, they found that at
or near the crest of the ridge, the rocks are very young, and they
become progressively older away from the ridge crest.
Seafloor Spreading
From this and other evidence geologists think that mid-ocean
ridges are structurally weak zones where the ocean floor is being
pulled apart lengthwise along the ridge crest. New magma from
deep within the Earth rises from below and eventually erupts
along the crest of the ridges to create new oceanic crust. This
process operating over many millions of years has built the long
system of mid-ocean ridges.
Black smoker at a mid-ocean ridge hydrothermal vent.
(Credit: OAR/National Undersea Research Program (NURP); NOAA)
Seafloor Spreading
Mid-oceanic ridges and seafloor spreading was found
to occur in all the oceans on Earth
Seafloor Spreading
Note that ALL of the ocean sea floors are younger
than 180 million years old
Question
Because Earth’s size has not changed, expansion of
the crust in one area requires destruction of the crust
elsewhere.
Where and how is crust being destroyed?