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.

Outlines

00:00

🌏 Plate Tectonics and Earth's Layers

This paragraph introduces the concept of plate tectonics, explaining that the Earth's lithosphere is divided into major and minor plates that are constantly moving. It delves into the composition and structure of the Earth's mantle, distinguishing between the lithospheric mantle and the asthenosphere, using the analogy of a taffy bar to illustrate the behavior of materials under different temperatures. The paragraph also describes the lithospheric plates, which include both crust and the rigid part of the mantle, and how they are the site of earthquakes due to brittle rock fractures. It further explains the differences between continental and oceanic lithospheres in terms of thickness, composition, and buoyancy, leading to their distinct roles in forming continents and ocean floors. Finally, it outlines the three main types of plate boundaries: transform, divergent, and convergent, providing examples of each and their associated geological activities.

05:04

🌋 Earthquake Types and Convergent Boundaries

The second paragraph focuses on the types of earthquakes and the processes at convergent boundaries. It discusses how earthquakes of magnitude 8 can occur along long transform boundaries, especially those that cut through continental crust. The paragraph then describes divergent boundaries, where oceanic crust forms at spreading ridges due to the upwelling of mantle material, and how most earthquakes in these areas are shallow and of smaller magnitude. The main focus shifts to convergent boundaries, where the paragraph explains the process of subduction, where an oceanic plate is forced under a continental plate, leading to the world's largest earthquakes and the formation of tsunamis. It also touches on the formation of island arcs and volcanoes in the case of oceanic-oceanic convergence. The paragraph concludes by mentioning continental collision, where neither plate subducts, leading to the formation of mountains and plateaus, and the occurrence of shallow, high-magnitude earthquakes. Additionally, it acknowledges the existence of diffuse boundary zones and intraplate earthquakes, emphasizing that most tectonic activity and earthquakes are concentrated at or near plate boundaries.

Keywords

💡Lithosphere

The lithosphere is the rigid outer layer of the Earth, consisting of the crust and the uppermost part of the mantle. It is the layer where tectonic plates are found and where earthquakes and volcanic activity occur. In the video, the lithosphere is described as being broken into tectonic plates that are in constant motion, which is central to understanding the theme of plate tectonics.

💡Tectonic Plates

Tectonic plates are massive pieces of the lithosphere that move over the Earth's surface. They are the primary agents of geologic activity, including the creation of mountains and the occurrence of earthquakes. The script mentions that there are about a dozen major plates and many smaller ones, which are key to understanding the dynamic nature of the Earth's surface.

💡Mantle

The mantle is a thick layer of the Earth located between the crust and the core. It is composed mainly of high-magnesium silicate and is divided into the lithospheric mantle and the asthenosphere. The mantle's properties, including its composition and temperature variations, are crucial to the movement of tectonic plates, as explained in the video.

💡Asthenosphere

The asthenosphere is a part of the Earth's mantle that lies below the lithospheric mantle. It is hotter and more ductile, allowing tectonic plates to move over it. The video uses the asthenosphere to explain the mechanism of plate movement, highlighting its importance in the process of plate tectonics.

💡Crust

The crust is the outermost layer of the Earth, composed of less dense rocks that form the continents and ocean floor. The video distinguishes between continental crust, which is thicker and more buoyant, and oceanic crust, which is thinner and denser. The crust's composition and density are key to understanding the different behaviors of tectonic plates.

💡Transform Boundaries

Transform boundaries, also known as strike-slip boundaries, are where tectonic plates slide horizontally past each other. An example given in the script is the San Andreas Fault Zone, which is a transform boundary that connects different segments of plate boundaries and is associated with significant seismic activity.

💡Divergent Boundaries

Divergent boundaries are locations where tectonic plates move apart from each other, allowing new crust to form as the mantle melts. The video describes how oceanic crust is created at spreading ridges, which are part of divergent boundaries, and how these processes contribute to the formation of Earth's longest mountain systems.

💡Convergent Boundaries

Convergent boundaries occur where two tectonic plates collide. The video explains that when an oceanic plate subducts beneath a continental plate, it can lead to the formation of deep ocean trenches and volcanic island arcs. These boundaries are also associated with the world's largest earthquakes.

💡Subduction

Subduction is the process where one tectonic plate is forced under another, typically an oceanic plate beneath a continental plate at a convergent boundary. The script describes how this process can lead to the formation of deep trenches and significant seismic activity, including the potential for large earthquakes and tsunamis.

💡Continental Collision

Continental collision occurs when the continental parts of converging plates come together and neither can subduct. The video explains that this results in horizontal deformation and the uplift of the continental crust, leading to the formation of mountains and plateaus. This process is also associated with frequent and powerful earthquakes.

💡Intraplate Earthquakes

Intraplate earthquakes are seismic events that occur within the interior of a tectonic plate, away from the plate boundaries. Although less common than those at plate boundaries, the video mentions that intraplate earthquakes do occur and are an important aspect of understanding the full range of seismic activity on Earth.

Highlights

The lithosphere is divided into tectonic plates that are in continuous motion, with about a dozen major and many smaller plates.

Plates can be entirely oceanic or a combination of oceanic and continental lithosphere, like the North American plate.

The Earth's mantle, 2900 km thick, lies between the crust and core and is composed of high-magnesium silicate.

The lithospheric mantle is cooler and more rigid than the underlying asthenosphere, which is hotter and more ductile.

An analogy of a taffy bar illustrates the brittle and ductile behavior of rocks under different temperatures.

Lithospheric plates consist of crust and the rigid part of the mantle, migrating together as a single mechanical layer.

Earthquakes occur in the lithosphere where brittle rock fractures, unlike the ductile asthenosphere.

Continental lithosphere, with continental crust, is typically 150 to 200 km thick and more buoyant due to its composition.

Oceanic lithosphere, formed at spreading ridges, is 50–140 km thick with denser oceanic crust.

Relative plate motion can be categorized into Transform, Divergent, and Convergent boundaries.

Transform boundaries involve horizontal plate movement with little creation or destruction of lithosphere.

Divergent boundaries are where plates move apart, forming oceanic crust at spreading ridges.

Convergent boundaries are characterized by one plate being forced beneath another, often causing large earthquakes.

Subduction at convergent boundaries can lead to the formation of tsunamis and a broad zone of shallow earthquakes.

When oceanic plates subduct beneath another oceanic plate, an island arc and ocean trench are formed.

Convergent boundaries without subduction can result in continental collision, leading to mountain formation.

Diffuse boundary zones and intraplate earthquakes occur within plates, though less commonly than at plate boundaries.

Tectonic activity and earthquakes are primarily concentrated at or near plate boundaries where geological features like volcanoes and mountains form.

Transcripts

The cool, rigid, outer layer of the earth, the lithosphere, is broken into massive plates.

There are about a dozen major plates and many smaller plates in continuous motion as they

collide with, slide under, or move past each other in a process known as plate tectonics.

A plate may be entirely oceanic lithosphere, like the Pacific Plate, or,

like the NorthAmerican plate can be part oceanic and part continental lithosphere.

We will return to lithosphere types in a minute, but first, let’s look inside the EArth to

clarify the layers. The mantle is a 2900 km-thick rock layer between the crust and core. The

composition is high-magnesium silicate. The uppermost part, the lithospheric mantle, is

cooler and more rigid than the deeper mantle. It lies above a hotter and more-ductile layer

of similar composition called the asthenosphere. As an analogy to how a rock can be either

brittle or ductile, consider a taffy bar, like the Big Hunk. When force is applied it

is brittle when cold, and ductile when warm. Zooming in to a tectonic cross section we

see that the lithospheric plates are composed of crust on top of the outermost rigid part

of the mantle. Although the mantle has a different composition and higher density than the crust,

these two layers migrate as a single mechanical layer called a lithospheric, or tectonic plate.

This is where earthquakes occur as brittle rock fractures and breaks. The hotter and

more-ductile asthenospheric rock does not fracture to produce earthquakes.

Continental lithosphere, with continental crust above lithospheric mantle, typically

ranges from 150 to 200 km thick. Continental crust stands above sea level because it has

an average thickness of 40 km and is mostly more buoyant silica-rich low-density granitic,

sedimentary and metamorphic rocks which form the continents.

OCEANIC lithosphere, formed at spreading ridges, is typically 50–140 km thick. Oceanic crust

is only around eight kilometers thick and is denser than continental crust because it

contains less low-density silica, and more high-density iron and magnesium. Thus, it

forms the ocean floor with its top surface below sea level.

Relative motion between plates can be broadly grouped into three main categories: Transform,

Divergent, and Convergent. RETURNING to the world map view, we show.

1) Transform, or strike-slip, boundaries: where plates move horizontally against each other,

2) Divergent, or constructive, boundaries: where plates move apart from one another, and

3) Places were plates press into one another are called convergent, or destructive, boundaries.

Lithosphere is neither created nor destroyed along transform boundaries which connect segments

of spreading oceanic ridges and other plate boundaries. Transform boundaries can also

cut across continents as the San Andreas Fault Zone does in California where it connects the East

Pacific Rise  to the Cascadia subduction zone. Shallow earthquakes on long transform

boundaries that cut continental crust, can approach magnitude 8 whereas those on oceanic

transform boundaries tend to be smaller. At divergent boundaries, oceanic crust forms

at spreading ridges where plates pull away from each other. A small volume of the mantle

melts to create the crust. The hot, buoyant upwelling mantle supports the oceanic ridges

that forms Earth’s longest mountain systems. Because temperature increases rapidly with

depth at divergent boundaries, there is only a thin layer of brittle rock to fracture in

earthquakes. Most earthquakes occur within the upper ten kilometers and have magnitudes

that are generally less than six. More than 75% of all earthquakes occur on

or near convergent boundaries. Here, an oceanic plate is forced beneath a continental

in a process called subduction. Indeed, the world’s largest earthquakes occur near the

shallow edge of the boundary where magnitude 9’s have been recorded.. At this location

stress builds up over 10’s to 100’s of years until it releases like a spring. and

can produce tsunamis. A broad zone of shallow earthquakes occurs within the overriding plate

due to compressive forces near the convergent boundary. earthquakes can reach depths of

700 km within the subducting plates because the oceanic plate can remain cold and brittle

as it dives into the deeper mantle. Similar processes occur when an oceanic plate

subducts beneath another oceanic plate. Here, an ocean trench marks the location where the

plate is pushed down into the mantle. In this case, the line of volcanoes that grows on

the upper oceanic plate is an island arc. Not all convergent boundaries involve subduction.

When the continental parts of converging plates come together, neither can subduct. Instead

the two continents collide, producing horizontal deformation and uplift of continental crust

to build mountains and plateaus. Frequent shallow earthquakes in continental collision

zones can exceed magnitude 8 and are generally less than 40 km deep.

In addition to these three boundaries, there are also diffuse boundary zones in which deformation

occurs over a wide region. Although less common than earthquakes along the plate boundaries,

earthquakes in the interior part of plates, called intraplate earthquakes, do occur.

Nevertheless, tectonic activity and earthquakes are chiefly concentrated at, or near plate

boundaries where many geological features including volcanoes, mountains, trenches occur.