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Questions and Answers
What is the primary process that leads to the partial melting of peridotite at mid-ocean ridges?
What is the primary process that leads to the partial melting of peridotite at mid-ocean ridges?
- Decrease in temperature caused by the upwelling of asthenospheric material.
- Adiabatic decompression as the asthenosphere rises. (correct)
- Introduction of water into the mantle reducing the melting point.
- Increase in pressure due to the weight of the overlying rocks.
Which of the following best describes how the crystallization of magma at mid-ocean ridges contributes to the formation of the oceanic crust?
Which of the following best describes how the crystallization of magma at mid-ocean ridges contributes to the formation of the oceanic crust?
- Rapid crystallization at depth leads to the formation of granite, while slow crystallization at the surface forms basalt.
- Slow crystallization at depth leads to the formation of basalt, while rapid crystallization at the surface forms gabbro.
- Slow crystallization at depth leads to the formation of gabbro, while rapid crystallization at the surface forms basalt. (correct)
- Slow crystallization at the surface leads to the formation of gabbro, while rapid crystallization at depth forms basalt.
What is the significance of the 1300°C isotherm in the context of the oceanic lithosphere?
What is the significance of the 1300°C isotherm in the context of the oceanic lithosphere?
- It represents the temperature at which basalt melts to form magma.
- It indicates the depth at which hydrothermal vents are commonly found.
- It marks the boundary between the oceanic crust and the upper mantle.
- It defines the thermal boundary between the lithosphere and the asthenosphere. (correct)
As oceanic lithosphere ages and moves away from the mid-ocean ridge, what happens to its density and thickness?
As oceanic lithosphere ages and moves away from the mid-ocean ridge, what happens to its density and thickness?
What is the main difference between slow-spreading and fast-spreading mid-ocean ridges?
What is the main difference between slow-spreading and fast-spreading mid-ocean ridges?
What is meant by 'oceanic accretion'?
What is meant by 'oceanic accretion'?
What characterizes the volcanism at a slow-spreading ridge?
What characterizes the volcanism at a slow-spreading ridge?
Which process describes the chemical alteration of oceanic crust minerals by seawater?
Which process describes the chemical alteration of oceanic crust minerals by seawater?
How does the density of the lithosphere change as it moves away from the axis of the ridge?
How does the density of the lithosphere change as it moves away from the axis of the ridge?
What is the approximate spreading rate observed at slow-spreading ridges such as the Mid-Atlantic Ridge?
What is the approximate spreading rate observed at slow-spreading ridges such as the Mid-Atlantic Ridge?
Flashcards
Oceanic Ridges (Dorsales)
Oceanic Ridges (Dorsales)
Zones of extension in oceanic areas, characterized by high relief, geothermal flux, and volcanic activity.
Mantle Upwelling at Ridges
Mantle Upwelling at Ridges
The upward movement of asthenospheric mantle due to oceanic extension, leading to decompression.
Adiabatic Decompression
Adiabatic Decompression
Partial melting of peridotite due to reduced pressure, occurring without a significant temperature drop.
Oceanic Accretion
Oceanic Accretion
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Slow-spreading Ridges
Slow-spreading Ridges
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Fast-spreading Ridges
Fast-spreading Ridges
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Lithospheric Cooling
Lithospheric Cooling
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Hydrothermal Metamorphism
Hydrothermal Metamorphism
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Study Notes
- Oceanic spreading zones are called ridges.
- Ridges are characterized by higher relief, high geothermal flux, and strong volcanic activity, indicating internal Earth activity.
Oceanic Divergence Zone Consequences
- Geothermal flow at ridges indicates magmatic activity.
- Seismic tomography shows mantle rock rising from 200-300 km depth, indicating upwelling of the asthenospheric mantle due to oceanic extension.
- Upwelling asthenosphere brings peridotite, causing decompression and rapid adiabatic decompression without significant temperature drop.
- Partial melting of peridotite occurs under pressure and temperature conditions corresponding to the solidus.
- Formed magma rises and accumulates in a magma chamber 2-7km beneath the ridge.
- Magma crystallization forms oceanic crust: slow crystallization at depth creates gabbro, rapid crystallization near the surface creates basalt.
- Partially melted peridotite, gabbro, and basalt form new oceanic lithosphere, known as oceanic accretion.
- Oceanic accretion rate matches the combined divergence rate of the two plates.
Ridge Types
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GPS data determines plate movement speeds at ridge zones, distinguishing between slow and fast spreading ridges.
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Slow spreading ridge (e.g., Atlantic): 2-4 cm/year spreading rate, weak volcanism.
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Fast spreading ridge (e.g., Pacific): 10-12 cm/year spreading rate, significant volcanism.
Rock Evolution at the Ridge Axis
Moving Away From the Ridge
- As oceanic lithosphere moves away from the ridge, it ages and cools. The lithosphere-asthenosphere boundary, defined by the 1300°C isotherm, deepens, thickening the lithosphere.
- Lithospheric mantle (denser than oceanic crust) proportion increases, raising the overall average density.
Metamorphism
- Rocks constituting the lithosphere undergo significant hydration (hydrothermal metamorphism).
- Seawater cools and hydrates rocks, altering mineral composition, especially in gabbro and mantle peridotites.
- Gabbro transforms into greenschist facies metagabbro.
- Peridotite transforms into serpentinized peridotite or serpentinite.
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Description
Explore the creation of oceanic ridges at divergence zones. These ridges feature high relief, geothermal flux, and volcanic activity due to internal Earth processes. Upwelling asthenosphere leads to peridotite melting, magma formation, and crystallization into gabbro and basalt, forming new oceanic lithosphere.
