1. Describe the landscape of the ocean floor.
2. (a) What is the Mid-Atlantic Ridge?
(b) What is the relationship between an ocean
ridge and an ocean trench?
3. What new technologies allowed scientists to
confirm that the sea floors are spreading?
4. Hess hypothesized that new sea floor was
produced at ocean ridges.
(a) Why are deep ocean trenches important to
his hypothesis?
(b) What would happen to the dimensions of
Earth if the sea floor did not descend into
the mantle at ocean trenches?
5. Which layers of Earth are involved in sea-floor
spreading?
A. crust and core
B. core and mantle
C. crust and asthenosphere
D. lithosphere and asthenosphere
6. (a) List the sources of heat within Earth.
(b) How does this heat contribute to mantle
convection?
7. How do convection currents in the mantle
contribute to sea-floor spreading?
8. Why does the sea floor bulge up at a spreading
ridge?
A. Rising mantle pushes up on the crust.
B. Magma piles up as it rises from the mantle.
C. Heated crust is less dense, so it floats higher.
D. Friction curls up the edges of the crust as it
moves.
9. Explain how the magnetic orientation of lava
layers demonstrates reversals of Earth’s
magnetic field.
10. Describe how each of the following was used to
argue that the sea floor is spreading.
(a) magnetic reversals
(b) age of rocks
(c) mid-ocean ridges
(d) deep ocean trenches
11. What evidence shows that the rock further from
ocean ridges is older than rock closer to the
ridges?
12. The colour coding in Figure 4 shows the age of
the sea floor around the Juan de a spreading
ridge.
(a) Which colour represents the current
location of the spreading ridge?
(b) The white regions indicate rocks with
reversed magnetic polarity. Explain how the
age and polarity of the rock suggests the
ocean floor on either side of the ridge has
moved.
(c) In what direction is the sea floor spreading?
How do you know?
All are wrong
1. The landscape of the ocean floor is characterized by various features such as underwater mountains, known as seamounts, and ridges, deep ocean trenches, and underwater volcanic activity. It also includes flat plains called abyssal plains and features like hydrothermal vents and coral reefs.
2. (a) The Mid-Atlantic Ridge is a long underwater mountain range that runs through the middle of the Atlantic Ocean from north to south. It is known as a divergent boundary where tectonic plates are moving away from each other.
(b) The relationship between an ocean ridge and an ocean trench is that they both occur at plate boundaries. Ocean ridges are formed at divergent boundaries where tectonic plates move apart, whereas ocean trenches are formed at convergent boundaries where plates collide and one is forced beneath the other.
3. New technologies such as sonar mapping and magnetometers allowed scientists to confirm sea floor spreading. Sonar mapping allowed for detailed mapping of the seafloor using sound waves, while magnetometers helped measure the magnetic properties of the rocks on the ocean floor.
4. (a) Deep ocean trenches are important to Hess's hypothesis because they were observed to be sites where the ocean floor was descending into the mantle. This was consistent with the idea of sea-floor spreading, where new crust is created at ocean ridges and eventually destroyed at trenches, completing a recycling process.
(b) If the sea floor did not descend into the mantle at ocean trenches, there would not be a mechanism for the recycling of crust and the formation of new crust at ocean ridges. Over time, this would lead to a build-up of crust and changes in the dimensions of the Earth's surface.
5. The layers of Earth involved in sea-floor spreading are the crust and the lithosphere (which includes the uppermost part of the mantle) and the asthenosphere (a partially molten zone below the lithosphere).
6. (a) The sources of heat within Earth include radioactive decay of elements, residual heat from the planet's formation, and heat from the core.
(b) This heat contributes to mantle convection by causing the heated material to become less dense and rise, while the cooler, denser material sinks. This creates a cycle of circulation that drives the movement of tectonic plates.
7. Convection currents in the mantle contribute to sea-floor spreading by creating the motion of the plates. As the heated mantle material rises at the mid-ocean ridges, it pushes the tectonic plates apart, causing them to move away from each other.
8. The correct answer is B. Magma piles up as it rises from the mantle. The sea floor bulges up at a spreading ridge because the upwelling of magma creates a build-up of new crust, causing the sea floor to rise.
9. The magnetic orientation of lava layers demonstrates reversals of Earth's magnetic field because as lava cools and solidifies, it preserves the magnetic signature of the Earth's magnetic field at the time of its formation. By studying the magnetic orientation of lava layers in rocks on the ocean floor, scientists can determine the past magnetic field orientations and identify periods of magnetic reversals.
10. (a) Magnetic reversals were used to argue that the sea floor is spreading because the patterns of magnetic orientation in rocks on the ocean floor showed a symmetrical pattern around the mid-ocean ridges, indicating that new crust was continuously being formed and adding to the sea floor.
(b) The age of rocks was used to argue sea-floor spreading by comparing the age of rocks at different distances from the mid-ocean ridges. It was found that rocks closer to the ridges were younger, while rocks further away were older, supporting the idea that new crust was being formed at the ridges and moving away as it aged.
(c) Mid-ocean ridges were used to argue sea-floor spreading because they were observed to be sites of volcanic activity and the creation of new crust. The consistent presence of mid-ocean ridges across the world's oceans suggested that new crust was continuously being formed.
(d) Deep ocean trenches were used to argue sea-floor spreading because they were observed to be sites where the ocean floor descends into the mantle, completing a cycle of crust creation and destruction. The presence of deep ocean trenches provided evidence for the recycling of crust at subduction zones.
11. The evidence that shows that rock further from ocean ridges is older than rock closer to the ridges is the age dating of the rocks themselves. By analyzing the rocks' composition and using radiometric dating techniques, scientists have determined that rocks closer to the ridges are younger, while rocks further away are older.
12. (a) The current location of the spreading ridge would be represented by the color that corresponds to the youngest rocks, which would be the color closest to the ridge.
(b) The white regions, which indicate rocks with reversed magnetic polarity, suggest that the ocean floor on either side of the ridge has moved. This is because the reversed polarity indicates a past magnetic field orientation that is opposite to the Earth's current magnetic field. As new crust is formed at the ridge and spreads outward, it records the changing magnetic polarity over time.
(c) The sea floor is spreading in opposite directions on either side of the spreading ridge. This can be inferred from the symmetrical pattern of magnetic reversals on either side of the ridge, indicating that new crust has been continuously added and moved away from the ridge in opposite directions.
1. The landscape of the ocean floor is diverse and includes features such as underwater mountains (seamounts), plateaus, trenches, and ridges. It also has underwater volcanoes, known as seafloor spreading centers, where new oceanic crust is formed. The ocean floor is covered by sediment, which varies in thickness depending on the location.
2. (a) The Mid-Atlantic Ridge is a major underwater mountain range that runs down the center of the Atlantic Ocean, extending from the Arctic Ocean to the Southern Ocean. It is the boundary between two tectonic plates, the North American Plate and the Eurasian Plate (in the northern part) and the South American Plate and the African Plate (in the southern part).
(b) An ocean ridge and an ocean trench are both related to plate tectonics. Ocean ridges, like the Mid-Atlantic Ridge, are formed by the divergence of tectonic plates, where new oceanic crust is created through volcanic activity. On the other hand, ocean trenches are formed by the convergence of plates, where one plate subducts (or sinks) beneath another plate, creating a deep trench.
3. Scientists were able to confirm sea-floor spreading using several new technologies. One of the key technologies was the development of sonar, which allowed scientists to map the ocean floor in detail. They were able to identify the presence of the mid-ocean ridges and other features of the sea floor. Additionally, the discovery and study of magnetic anomalies on the sea floor provided further evidence for sea-floor spreading.
4. (a) Deep ocean trenches are important to Hess's hypothesis because they are the areas where oceanic crust is consumed or subducted back into the mantle. This process is essential for the recycling of crust and the continuous formation of new crust at ocean ridges.
(b) If the sea floor did not descend into the mantle at ocean trenches, it would lead to a buildup of oceanic crust over time. This would result in the Earth's surface having significantly more crust, leading to an increase in the overall dimensions of the planet.
5. The layers of Earth involved in sea-floor spreading are the crust and the asthenosphere. The crust refers to the outermost layer of the Earth, which includes both the continental crust and the oceanic crust. The asthenosphere is a layer within the upper mantle that is partially molten and behaves like a plastic material.
6. (a) The sources of heat within Earth include residual heat from its formation (primordial heat), heat generated by the decay of radioactive isotopes (radiogenic heat), and frictional heating due to the movement of tectonic plates.
(b) This heat contributes to mantle convection by causing the material in the mantle to become less dense and rise towards the surface. As the heated material rises, it cools down, becomes denser, and sinks back down. This movement of material creates convection currents within the mantle.
7. Convection currents in the mantle play a vital role in sea-floor spreading. As the heated material in the mantle rises due to convection, it carries with it the overlying crust. This movement of the mantle material causes the plates on the surface of the Earth to move apart, leading to the formation of new crust at the mid-ocean ridges.
8. A. Rising mantle pushes up on the crust. The rising mantle material, driven by convection currents, exerts pressure on the overlying crust, causing it to bulge up at a spreading ridge.
9. The magnetic orientation of lava layers, which solidify as they erupt onto the sea floor, demonstrates reversals of Earth's magnetic field. The Earth's magnetic field periodically changes its polarity, switching between normal and reversed. As the lava cools and solidifies, it preserves the magnetic orientation of the Earth's field at that time. By comparing the direction of magnetization in different layers of lava, scientists can determine the timing and occurrence of magnetic field reversals.
10. (a) Magnetic reversals were used as evidence for sea-floor spreading. By studying the orientation of magnetic minerals in rocks on the sea floor, scientists found symmetrical patterns of normal and reversed polarity, which provided support for the idea of sea-floor spreading.
(b) The age of rocks was another piece of evidence for sea-floor spreading. By analyzing samples of volcanic rock from the ocean floor, scientists discovered that the youngest rocks were closest to the mid-ocean ridges, and the rocks became progressively older as they moved away from the ridges. This age progression supported the concept of new crust being formed at the ridges and spreading away.
(c) Mid-ocean ridges themselves served as evidence for sea-floor spreading. The presence and mapping of these underwater mountain ranges provided a clear indication that the Earth's crust was actively spreading apart at these locations.
(d) Deep ocean trenches were important in arguing for sea-floor spreading as they were the areas where old oceanic crust was being consumed through subduction, providing a mechanism for the continuous creation of new crust at mid-ocean ridges.
11. The age of rocks on the sea floor indicates that rock further from the ocean ridges is older than rock closer to the ridges. This age difference is observed through the study of volcanic rock samples obtained from various locations on the ocean floor. By determining the age of these samples, scientists have established that new crust is formed at the ridges and moves away, creating a pattern of progressively older rocks as one moves further from the ridge.
12. (a) The current location of the spreading ridge is represented by the youngest color in the age-coded map. It could be represented by the color that corresponds to the most recent time period.
(b) The presence of rocks with reversed magnetic polarity (white regions) indicates that the magnetic field has undergone a reversal at some point. The age of these rocks, combined with their magnetic polarity, suggests that these rocks were formed before the most recent reversal. The age progression and the presence of reversed polarity on both sides of the ridge indicate that the ocean floor on each side has moved away from the ridge over time.
(c) The sea floor is spreading in opposite directions on each side of the ridge. This can be determined by looking at the age of the rocks. As mentioned before, the rocks become progressively older as one moves away from the ridge. By studying the age patterns, one can determine the direction of sea-floor spreading.