Lecture Quiz #6
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Geology
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Apr 3, 2024
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Lecture Quiz Week 6 – Ocean Basins
1.
Describe for me the difference between an active (west coast of North and South
America) and passive continental margin?
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An Active Continental Margin is a boundary between a continent and the ocean where there is significant tectonic activity, for example earthquakes, volcanoes, and mountain building. It is characterized by subduction, where one tectonic plate is being forced beneath another, resulting in the
formation of deep-sea trenches, volcanic arcs, and mountain ranges. -
A Passive Continental Margin is a boundary where the tectonic activity is minimal, and the continent is gradually subsiding or sinking. It is characterized by the absence of subduction and the presence of sedimentary basins, where sediments accumulate over time. Passive Margins are typically formed when a continent breaks apart and moves away from a mid-ocean ridge, resulting in the formation of a passive margin on the edge of the continent. 2.
Name for me a fast-spreading ocean ridge and a slow-spreading ocean ridge? What is the
key topographic difference between them?
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A fast-spreading ocean ridge is the East Pacific Rise (approximately 15-20 cm/year). The key topographic difference is they have a wider and shallower profile, with a broader axial valley and gentler slopes. They also have more prominent seamounts and hydrothermal activity. -
A slow-spreading ocean ridge is the Mid-Atlantic Ridge (approximately 2-3 cm/year). The key topographic difference is they have a narrower and deeper profile, with a narrower axial valley and steeper slopes. 3.
Why are the abyssal plains so much deeper than the ocean ridge spreading centers?
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Abyssal plains are deeper than ocean ridge spreading centers because they are further away from the mid-ocean ridge and have had more time to accumulate sediments and subsidence. As the oceanic crust moves away from the ridge, it cools and becomes denser, causing it to sink or subside, resulting in a deeper basin. Additionally, abyssal plains receive sediments from surrounding areas, which adds to their depth over time. In contrast, ocean ridge spreading centers are areas of active volcanic
activity, where new crust is being created, and the sedimentation rate is much lower.
4.
With few exceptions, the ocean floor is covered by what? From where is that material
sourced?
The ocean floor is covered by sediments:
Pelagic sediments: fine-grained particles like clay, silt, and sand that settle from the water column.
Terrigenous sediments: sediments derived from land, like sand gravel, and rocks, that are transported by rivers and glaciers.
Biogenic sediments: shells, coral, and other organic matter that accumulate on the seafloor.
Hydrothermal sediments: minerals and metals deposited from hydrothermal vents.
The sediments on the ocean floor are sourced from a variety of locations:
Terrigenous sources: sediments derived from land, like rivers, glaciers, and wind-blown dust.
Pelagic sources: sediments formed in the water column like shells, and coral.
Biogenic sources: sediments formed by living organisms, like any calcium carbonate structures.
Hydrothermal sources: sediments formed from minerals and metals deposited from hydrothermal vents.
Oceanic crust sources: sediments formed from the alteration and weathering of the oceanic crust itself.
5.
The Angle of plate subduction depends on its? Given this, where might you expect to find the deepest trench in the world? Why?
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Density, and The Mariana Trench is the deepest trench in the world. This is because of its unique location and geological features. It is situated at the boundary between two tectonic plates, the Pacific
Plate, and the Mariana Plate, where the older and cooler Pacific Plate is being subducted beneath the Mariana Plate. This process results in the formation of a deep trench, with the Challenger Deep being the lowest point, reaching a depth of approximately 36,000 feet (10,973 meters). Additionally, the trench is also influenced by the movement of the Philippine Sea Plate, which further contributes to its extreme depth.
6.
A new ocean begins with the development… Where is one of these active today? How do we know?
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Of a new mid-ocean ridge
.
Where magma rises from the Earth's mantle and solidifies into new oceanic crust. As the magma cools, it splits apart and creates a new ocean basin, marking the beginning of a new ocean. This process is known as seafloor spreading and is the driving force behind
plate tectonics. Over time, the new ocean will expand as more magma rises and solidifies, eventually becoming a full-fledged ocean.
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In the South west Indian Ridge in the Atlantic and Indian Oceans, and the Gakkel Ridge beneath the Arctic ice cap. This discovery has changed the previously held belief that sea floor spreading creates a uniform layer of volcanic rock and highlights the importance of hot springs at these ridges, which produce deposits rich in copper, zinc, and other metals. 7.
Once developed, does a rift necessarily act to form a new ocean? What does the
Midcontinent Rift in North America represent?
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No, not all rifts will necessarily form a new ocean. Rifts can develop into different types of features, depending on the underlying tectonic forces and the rate of spreading. Some possibilities include Continental rifts: These can develop into large rivers, lakes, or basins, but may not necessarily lead to ocean formation. Incipient oceans: These are early stages of ocean formation, where the rift has begun
to spread, but the crust has not yet fully broken apart. Failed rifts: These are rifts that have stopped spreading and may eventually close up. Aulacogens: These are abandoned rifts that have cooled and solidified, leaving a scar on the Earth's surface.
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The midcontinent Rift in North America represents a 2,000km long geological rift that formed 1.1 billion years ago during the Mesoproterozoic era, which attempted to split the continent but failed, leaving behind a scar with thick layers of igneous rock. Today, the rift is buried beneath sedimentary rocks, but its presence can be inferred through geological data and exposed rocks in the Lake Superior region. The rift represents a unique combination of rifting and flood basalt volcanism, which
is not observed in other rifts or large igneous provinces. 8.
What is meant by an “Accretionary Prism”? Why are they important?
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An Accretionary Prism is a type of geological structure that forms when sediments and rocks are scraped off from one tectonic plate and added to another plate. This process occurs at a convergent plate boundary, where one plate is being pushed beneath another. The scraped-off material accumulates in a wedge-shaped formation, called an accretionary prism, which grows as more material is added to it. -
Accretionary prisms are important for understanding plate tectonics, earthquake generation, and the formation of natural resources like minerals and hydrocarbons.
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