La dynamique des zones de divergence - 1ère spé SVT - Madame SVT

Summary

This video delves into the geology of divergent zones, specifically oceanic ridges, explaining how tectonic plates move apart and create new lithosphere through magma production. The video highlights seismic techniques like tomography and reflection seismology used to study the Earth's interior. It also contrasts slow and fast spreading ridges, explaining the formation of basalt pillow structures and the role of mantle convection. The video emphasizes the creation of oceanic lithosphere and its eventual subduction, as well as hydrothermal circulation and the formation of unique minerals in the oceanic crust. It concludes with practical tips for reviewing course material on the subject.

Takeaways

• 😀 Divergent zones, specifically oceanic ridges, are areas where two lithospheric plates are moving apart, leading to the formation of new lithosphere.
• 😀 The process of divergence results in the rise of magma from the Earth's mantle, which cools quickly when it comes into contact with cold seawater, forming basalt.
• 😀 Seismic tomography and reflection are two key geophysical techniques used to study the Earth's interior, providing data on the physical properties of the mantle and crust.
• 😀 Oceanic ridges are the boundaries between tectonic plates, where the lithosphere is being created through the upwelling of mantle material.
• 😀 The magma that rises at oceanic ridges originates from the mantle, where a decrease in pressure allows for partial melting of materials like peridotite.
• 😀 Slow and fast spreading ridges differ in their expansion rates, with slow ridges spreading 3-5 cm per year and fast ridges spreading 6-16 cm per year.
• 😀 The topography of slow and fast spreading ridges varies, with slow ridges forming valleys (rifts) and fast ridges forming raised features (horsts).
• 😀 The creation of oceanic lithosphere at ridges is part of the process that drives the expansion of oceans and the movement of tectonic plates.
• 😀 Hydrothermal circulation occurs at oceanic ridges, where seawater circulates through fractures, leading to mineral changes and the formation of altered rocks like hydrated gabbros.
• 😀 As the oceanic lithosphere ages, it becomes denser and sinks back into the mantle, a process facilitated by cooling and thickening of the lithosphere over time.

Q & A

What is the significance of oceanic ridges in the process of lithosphere formation?

Oceanic ridges are considered the birthplaces of the oceanic lithosphere. They are zones of divergence where tectonic plates pull apart, allowing magma from the mantle to rise, cool, and solidify into new crust.

How do seismic tomography and seismic reflection contribute to our understanding of Earth's interior?

Seismic tomography uses the travel times of seismic waves to reveal variations in material properties within the Earth, indicating areas of ductile or solid rock. Seismic reflection, on the other hand, measures how seismic waves bounce off different layers, helping us study the structure of Earth's layers.

What role does magma play at oceanic ridges?

Magma rises from the mantle at oceanic ridges, cooling rapidly upon contact with cold seawater. This process forms basalt, which solidifies into new oceanic crust, contributing to the creation of the lithosphere.

What causes the partial melting of mantle peridotite at divergent plate boundaries?

The partial melting of mantle peridotite occurs due to a decrease in pressure at the ridge axis. As pressure decreases, the temperature increases, causing the peridotite to partially melt and form magma.

How do slow and fast-spreading ridges differ in terms of magma activity and topography?

Slow-spreading ridges have a slower rate of expansion (3-5 cm per year), with less intense magmatic activity and a valley-like topography (rift or graben). Fast-spreading ridges, on the other hand, expand more rapidly (6-16 cm per year), leading to more intense magmatic activity and a more dome-like topography (horst).

What happens to the oceanic lithosphere once it is formed at divergent boundaries?

Once the oceanic lithosphere is created at divergent boundaries, it moves away from the ridge axis, carrying the ocean water with it and causing the expansion of the ocean basins.

What is hydrothermal circulation, and how does it affect the rocks at oceanic ridges?

Hydrothermal circulation involves seawater infiltrating fractures in the rocks at oceanic ridges, where it undergoes chemical changes. This process leads to the hydration of minerals, causing changes in the composition of rocks, such as the formation of serpentine in the mantle.

What are 'black smokers,' and how are they related to hydrothermal activity at ridges?

Black smokers are underwater vents where hot, mineral-rich water is expelled, resulting from hydrothermal circulation. These vents are associated with high-temperature water, which causes the formation of unique minerals as the water cools.

Why does the oceanic lithosphere become denser as it moves away from the ridge?

As the oceanic lithosphere moves away from the ridge, it cools and thickens, increasing in density. The lithosphere's increased density causes it to sink back into the mantle at subduction zones.

What minerals are characteristic of oceanic lithosphere formed at divergent plate boundaries?

The oceanic lithosphere formed at divergent plate boundaries is primarily composed of basalt and gabbro, with minerals like serpentine found deeper in the mantle due to the hydration processes during hydrothermal activity.