seafloor spreading

seafloor bathymetry

Before World War II, people thought the seafloor was completely flat and featureless. There was no reason to think otherwise.

echo sounders

But during the war, battleships and submarines carried echo sounders. Their goal was to locate enemy submarines (Figure 6.9). Echo sounders produce sound waves that travel outward in all directions. The sound waves bounce off the nearest object, and then return to the ship. Scientists know the speed of sound in seawater. They then can calculate the distance to the object that the sound wave hit. Most of these sound waves did not hit submarines. They instead were used to map the ocean floor.

features of the seafloor

Scientists were surprised to find huge mountains and deep trenches when they mapped the seafloor. The mid-ocean ridges form majestic mountain ranges through the deep oceans (Figure 6.10). Deep sea trenches are found near chains of active volcanoes. These volcanoes can be at the edges of continents or in the oceans. Trenches are the deepest places on Earth. The deepest trench is the Mariana Trench in the southwestern Pacific Ocean. This trench plunges about 11 kilometers (35,840 feet) beneath sea level. The ocean floor does have lots of flat areas. These abyssal plains are like the scientists had predicted.

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seafloor magnetism

Warships also carried magnetometers. They were also used to search for submarines. The magnetometers also revealed a lot about the magnetic properties of the seafloor.

polar reversals

Indeed, scientists discovered something astonishing. Many times in Earths history, the magnetic poles have switched positions. North becomes south and south becomes north! When the north and south poles are aligned as they are now, geologists say it is normal polarity. When they are in the opposite position, they say that it is reversed polarity.

magnetic stripes

Scientists were also surprised to discover a pattern of stripes of normal and reversed polarity. These stripes surround the mid-ocean ridges. There is one long stripe with normal magnetism at the top of the ridge. Next to that stripe are two long stripes with reversed magnetism. One is on either side of the normal stripe. Next come two normal stripes and then two reversed stripes, and so on across the ocean floor. The magnetic stripes end abruptly at the edges of continents. Sometimes the stripes end at a deep sea trench (Figure 6.11).

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the seafloor spreading hypothesis

The seafloor spreading hypothesis brought all of these observations together in the early 1960s. Hot mantle material rises up at mid-ocean ridges. The hot magma erupts as lava. The lava cools to form new seafloor. Later, more lava erupts at the ridge. The new lava pushes the seafloor that is at the ridge horizontally away from ridge axis. The seafloor moves! In some places, the oceanic crust comes up to a continent. The moving crust pushes that continent away from the ridge axis as well. If the moving oceanic crust reaches a deep sea trench, the crust sinks into the mantle. The creation and destruction of oceanic crust is the reason that continents move. Seafloor spreading is the mechanism that Wegener was looking for!

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seafloor ages

The scientists used geologic dating techniques on seafloor rocks. They found that the youngest rocks on the seafloor were at the mid-ocean ridges. The rocks get older with distance from the ridge crest. The scientists were surprised to find that the oldest seafloor is less than 180 million years old. This may seem old, but the oldest continental crust is around 4 billion years old. Scientists also discovered that the mid-ocean ridge crest is nearly sediment free. The crust is also very thin there. With distance from the ridge crest, the sediments and crust get thicker. This also supports the idea that the youngest rocks are on the ridge axis and that the rocks get older with distance away from the ridge (Figure 6.12). Something causes the seafloor to be created at the ridge crest. The seafloor is also destroyed in a relatively short time.

instructional diagrams

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This image shows the sea floor spreading. Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere. Basaltic magma rises up the fractures and cools on the ocean floor to form new sea floor. Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone.

description_image

This diagram shows how seafloor spreading happens. Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading occurs along mid-ocean ridges large mountain ranges rising from the ocean floor. The Mid-Atlantic Ridge separates the South American plate from the African plate. As new seafloor forms and spreads apart from the mid-ocean ridge it slowly cools over time. Seafloor is the youngest near the mid-ocean ridges and gets progressively older with distance from the ridge. The age, density, and thickness of oceanic crust increases with distance from the mid-ocean ridge. Orange areas show the youngest seafloor. The oldest seafloor is near the edges of continents or deep sea trenches.

questions

hypothesis explaining how the ocean floor forms

a. echo sounder

b. mid-ocean ridges

c. abyssal plains

-->  d. seafloor spreading

e. polar reversal

f. magnetometer

g. trenches

Before echo sounders, scientists thought topography of the seafloor

a. was just like the topography of the continents

b. had many long linear mountain ranges, like Japan

c. had lots of small hills, but nothing else

-->  d. was completely flat

device used to map the ocean floor

-->  a. echo sounder

b. mid-ocean ridges

c. abyssal plains

d. seafloor spreading

e. polar reversal

f. magnetometer

g. trenches

In the Atlantic Ocean, the mid-ocean ridge is

a. a straight line between the Americas and Europe/Africa

-->  b. a line that mimics the coastlines of the Americas and Europe/Africa

c. not visible

d. none of these

switching of Earths magnetic poles

a. echo sounder

b. mid-ocean ridges

c. abyssal plains

d. seafloor spreading

-->  e. polar reversal

f. magnetometer

g. trenches

At a time of reversed magnetic polarity, the north and south poles are

a. aligned as they are now

b. in somewhat different locations from where they are now

-->  c. in the opposite positions from where they are now

d. none of these

New oceanic crust is created

-->  a. at mid-ocean ridges

b. at deep sea trenches

c. within abyssal plains

d. at long, linear chains of volcanoes

deepest places on the ocean floor

a. echo sounder

b. mid-ocean ridges

c. abyssal plains

d. seafloor spreading

e. polar reversal

f. magnetometer

-->  g. trenches

device used to study magnetic properties of the seafloor

a. echo sounder

b. mid-ocean ridges

c. abyssal plains

d. seafloor spreading

e. polar reversal

-->  f. magnetometer

g. trenches

Since new oceanic crust is being created

a. Earth must be getting larger

b. mountains must be rising somewhere

-->  c. old crust must be destroyed somewhere

d. none of these

flat regions of the ocean floor

a. echo sounder

b. mid-ocean ridges

-->  c. abyssal plains

d. seafloor spreading

e. polar reversal

f. magnetometer

g. trenches

mountain ranges on the ocean floor

a. echo sounder

-->  b. mid-ocean ridges

c. abyssal plains

d. seafloor spreading

e. polar reversal

f. magnetometer

g. trenches

Before World War II, people thought the seafloor

a. had huge mountain ranges.

b. contained deep trenches.

-->  c. was flat and featureless.

d. had active volcanoes.

Echo sounders were first developed to

a. map the ocean floor.

-->  b. locate enemy submarines.

c. determine the depth of the ocean.

d. find evidence for seafloor spreading.

The deepest place on Earth is

-->  a. 11 km below sea level.

b. 110 km below sea level.

c. 1100 km below sea level.

d. none of the above

Two different plates of lithosphere lie on each side of the mid-ocean ridge.

-->  a. true

b. false

Reversed polarity means that the north and south magnetic poles are

a. located in the same positions as they are right now.

-->  b. located opposite their present positions.

c. both in the same location.

d. no longer magnetic.

The mid-ocean ridge is the longest mountain range on Earth.

-->  a. true

b. false

The alternating magnetic stripes on the ocean floor show

a. how Earth first formed.

b. why the seafloor spreads.

-->  c. when polar reversals occurred.

d. where sediments were deposite

New seafloor forms at

a. deep-sea trenches.

-->  b. mid-ocean ridges.

c. continental edges.

d. two of the above

The mid-ocean ridge is only found in the Atlantic Ocean.

a. true

-->  b. false

Old seafloor sinks into the mantle at

a. deep-sea trenches.

b. mid-ocean ridges.

c. continental edges.

-->  d. two of the above

The seafloor is oldest at the mid-ocean ridges

a. true

-->  b. false

Magnetic polarity stripes end at the edges of continents.

-->  a. true

b. false

An echo sounder with just one beam can create a three-dimensional map of the ocean floor.

a. true

-->  b. false

A mid-ocean ridge runs from east to west through the center of the Atlantic Ocean.

a. true

-->  b. false

Deep-sea trenches are found near the west coast of Central and South America.

-->  a. true

b. false

The only mountains on the ocean floor are part of mid-ocean ridges.

a. true

-->  b. false

Magnetometers were first used on ships to search for submarines.

-->  a. true

b. false

Polar reversals have occurred only twice in Earths history.

a. true

-->  b. false

Magnetic stripes on the ocean floor end abruptly at the edges of continents.

-->  a. true

b. false

The rocks currently found at mid-ocean ridges have reversed polarity.

a. true

-->  b. false

The seafloor is older than the continents.

a. true

-->  b. false

The seafloor is spreading away from mid-ocean ridges.

-->  a. true

b. false

diagram questions

question_image

Which layer lies immediately below the lithosphere?

-->  a. Asthenosphere

b. Mantle

c. Crust

d. Ocean ridge

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What happens when asthenosphere forces increase?

a. All of the above

b. mid ocean ridge contracts

c. Nothing happens

-->  d. Mid ocean ridge expands

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Identify the part of the diagram where hot mantle material rises out of the sea floor

a. Continental Crust

b. Oceanic Crust

c. Subduction Zone

-->  d. Mid-oceanic ridge

question_image

How many times the continental crust appears?

-->  a. 2

b. 3

c. 4

d. 1

question_image

What do the mid-oceanic ridges help in the formation of?

a. oceanic crust

-->  b. mountain ranges

c. continental crust

d. subduction zone

question_image

What is the space between the Continental crust and Asthenosphere?

a. Subduction zone

b. Oceanic crust

c. Mid-oceanic ridge

-->  d. Lithosphere

question_image

Which part of the diagram consists of molten rock?

a. Trench

b. Mid-ocean ridge

c. Continent

-->  d. Mantle

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What among these is on the right side of the Mid-ocean ridge axis?

a. Trench

b. Continental drift

-->  c. Mantle

d. Continent

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What is seen when there is sea-floor spreading?

a. trench

b. mantle

-->  c. rising magma

d. continent

question_image

How many of the events in the diagram result in earthquakes?

a. 3

b. 4

c. 1

-->  d. 2

question_image

According to the Hess/Dietz concept, what causes sea-floor spreading?

a. greenhouse gases

-->  b. rising magma

c. continental drift

d. sinking old ocean floor

question_image

What happens when an earthquake occurs?

a. Old ocean floor rises into the mantle.

-->  b. Old ocean floor sinks into the mantle.

c. Sea-floor contracts.

d. Trenches close.

question_image

What lies in between plate 1 and plate 2?

a. convection current

-->  b. magma

c. lava

d. oceanic ridge

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How many kinds of currents are involved in the sea floor spreading process, shown in the diagram?

a. 3

b. 1

c. 4

-->  d. 2

question_image

From the diagram, what is between plate 1 and plate 2?

a. mud

b. soil

-->  c. magma

d. ocean

question_image

How many plates form this mid-oceanic ridge?

a. 4

-->  b. 2

c. 3

d. 1

question_image

What happens during an eruption?

a. Oceanic crust move toward the center

-->  b. Seafloor spreads

c. Faults close

d. Seafloor contracts

question_image

What layer sits underneath the subducting oceanic plate?

-->  a. asthenosphere

b. Oceanic Trench

c. Lithosphere

d. Oceanic Crust

question_image

What layer does rising magma come from?

a. oceanic crust

-->  b. asthenosphere

c. seafloor

d. oceanic plate

question_image

What causes the oceanic crust to part?

-->  a. rising magma

b. ocean currents

c. continent moving

d. ocean waves

question_image

Material from what forms new crust?

a. Oceanic Crust

b. Magma

-->  c. Upper mantle

d. Continent

question_image

What is between oceanic crust and magma?

-->  a. Upper mantle

b. Lower Mantle

c. Continent

d. Ocean

question_image

How many continents are shown?

a. 3

b. 1

c. 4

-->  d. 2

question_image

How many parts are there to this diagram?

a. 7

b. 3

c. 6

-->  d. 4

question_image

What is the break in the crust?

a. continental crust

-->  b. mid oceanic ridge

c. lithosphere

d. subduction zone