Sea Floor Mapping

with Sonar and Magnetometers

SonarSOund 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.

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 mid-ocean ridges.

Seafloor Mapping

Seafloor MappingMid-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 changes over time as evidenced by the changing location of the magnetic pole.

Earth’s Magnetic Field

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 iron-bearing minerals become magnetic

These magnetic minerals align themselves parallel to Earth’s magnetic field

Photograph by J.D. Griggs on March 28, 1984, USGS

Magnetism

 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

Magnetic Reversals

They found that the ocean floor showed a zebra-like pattern of alternating stripes of magnetically different rock.

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.

Seafloor Mapping

Magnetism on the Sea FloorAn 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 mid-

ocean ridges?

Seafloor SpreadingBy 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

Sometimes it’s fun to pretend that our record of the seafloor’s magnetic stripes is complete

Map of Sea Floor Magnetic Stripes

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.

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.

Seafloor Spreading

Black smoker at a mid-ocean ridge hydrothermal vent. (Credit: OAR/National Undersea Research Program (NURP); NOAA)

Seafloor SpreadingMid-oceanic ridges and seafloor spreading was found to occur in all the oceans on Earth

Seafloor SpreadingNote that ALL of the ocean sea floors are younger than 180 million years old

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?

Question