Tectonic Plates—What are the lithospheric plates? (Educational)

Summary

The Earth's lithosphere is divided into tectonic plates, major and minor, that interact through processes like collision, subduction, and sliding in plate tectonics. The mantle, composed of high-magnesium silicate, includes the rigid lithospheric mantle above the ductile asthenosphere. Earthquakes primarily occur in the brittle crust and lithospheric mantle. Plate interactions are categorized into transform, divergent, and convergent boundaries, with the majority of earthquakes happening at convergent boundaries where one plate subducts beneath another, potentially causing large earthquakes and tsunamis. Tectonic activity is mainly concentrated at plate boundaries, shaping geological features like volcanoes and mountains.

Takeaways

• 🌏 The lithosphere is the Earth's rigid outer layer, consisting of about a dozen major and many smaller plates that are in constant motion.
• 🔄 Plates can be entirely oceanic or a combination of oceanic and continental lithosphere, and they interact through processes like collision, subduction, or sliding past each other.
• 🌌 The Earth's mantle, 2900 km thick, lies beneath the crust and is composed of high-magnesium silicate, with the lithospheric mantle being cooler and more rigid than the asthenosphere below.
• 📍 The asthenosphere is a ductile layer of the mantle that allows for the movement of the rigid lithospheric plates above.
• 🏔 The lithospheric plates include the crust and the uppermost rigid part of the mantle, and they are the site of most earthquakes due to brittle rock fractures.
• 🌊 Continental lithosphere is thicker, with continental crust that is buoyant and forms the continents, while oceanic lithosphere is thinner and denser, forming the ocean floor.
• 🔗 Plate tectonics involve three main types of boundaries: Transform (horizontal movement), Divergent (plates moving apart), and Convergent (plates pressing into each other).
• 🌀 Transform boundaries do not create or destroy lithosphere and can be associated with large earthquakes, especially on continental crust.
• 🌋 Divergent boundaries are characterized by the formation of oceanic crust at spreading ridges, with most earthquakes occurring in the upper ten kilometers and generally being of smaller magnitude.
• 🌎 Convergent boundaries, where an oceanic plate subducts beneath a continental plate, are the sites of the world's largest earthquakes and can produce tsunamis.
• ⛰️ When continental plates collide at convergent boundaries, they uplift to form mountains and plateaus, with frequent shallow earthquakes that can be highly magnitude.

Q & A

What is the lithosphere and how is it divided?

The lithosphere is the cool, rigid outer layer of the Earth, which is broken into massive plates. There are about a dozen major plates and many smaller ones that are in continuous motion.

What are the two types of lithosphere mentioned in the script?

The two types of lithosphere mentioned are oceanic lithosphere, like the Pacific Plate, and continental lithosphere, like the North American plate, which can be part oceanic and part continental.

What is the mantle and what is its composition?

The mantle is a 2900 km-thick rock layer between the crust and core, composed mainly of high-magnesium silicate.

How does the lithospheric mantle differ from the asthenosphere?

The lithospheric mantle is cooler and more rigid than the deeper mantle, while the asthenosphere, which lies beneath it, is hotter and more ductile but of similar composition.

What is the analogy used to explain the difference between brittle and ductile behavior?

The analogy of a taffy bar, like the Big Hunk, is used to explain the difference. It is brittle when cold and ductile when warm, similar to the behavior of the lithospheric and asthenospheric rocks.

What is the relationship between the crust and the mantle in terms of their migration?

Although the mantle has a different composition and higher density, it migrates with the crust as a single mechanical layer known as a lithospheric or tectonic plate.

Why do earthquakes occur in the lithosphere but not in the asthenosphere?

Earthquakes occur in the lithosphere because it is brittle and can fracture and break. The asthenospheric rock, being hotter and more ductile, does not fracture to produce earthquakes.

What are the typical thicknesses of continental and oceanic lithospheres?

Continental lithosphere typically ranges from 150 to 200 km thick, while oceanic lithosphere is typically 50–140 km thick.

What causes the oceanic crust to be denser than the continental crust?

Oceanic crust is denser than continental crust because it contains less low-density silica and more high-density iron and magnesium.

What are the three main types of plate boundaries?

The three main types of plate boundaries are transform (or strike-slip), divergent (or constructive), and convergent (or destructive).

Why are most earthquakes concentrated near plate boundaries?

Most earthquakes are concentrated near plate boundaries because this is where tectonic activity is most pronounced, involving the movement and interaction of tectonic plates.

How do earthquakes at divergent boundaries differ from those at convergent boundaries?

Earthquakes at divergent boundaries are generally shallower and of smaller magnitude, usually less than six, due to the thin layer of brittle rock. In contrast, earthquakes at convergent boundaries can be much deeper and larger, with magnitudes reaching up to nine.

What happens when two continental plates collide at a convergent boundary?

When two continental plates collide at a convergent boundary, they cannot subduct due to their similar composition. Instead, they produce horizontal deformation and uplift of the continental crust, leading to the formation of mountains and plateaus.

What is an intraplate earthquake and where do they occur?

An intraplate earthquake is an earthquake that occurs within the interior part of a tectonic plate, away from the plate boundaries. Although less common, they can still happen and contribute to tectonic activity.