Plate Tectonics—What Drives the Plates? Overview of processes (Educational)

Summary

This script delves into the dynamics of plate tectonics, explaining how early theories of mantle convection driving plate motion have evolved. It highlights the role of gravity in plate movement, with 'ridge push' and 'slab pull' forces shaping the Earth's crust. The script illustrates how oceanic plates form at spreading ridges, cool and thicken with age, and ultimately descend into subduction zones. It emphasizes the planetary scale of thermal convection systems fueled by Earth's internal heat, which powers the tectonic processes leading to earthquakes and the formation of ocean basins.

Takeaways

• 🌍 Mantle convection is a key driver of plate tectonics, with current models emphasizing gravity-driven convection influencing plate movement.
• 🔥 Young, hot lithospheric plates are pushed away from spreading ridges, while old, cold plates are pulled into subduction zones.
• 🌊 Oceanic crust is formed at spreading ridges through the upwelling and partial melting of asthenosphere, creating a 7-km-thick layer.
• 📏 The seafloor's age correlates with its depth, with older plates having deeper ocean basins due to the cooling and thickening process.
• 🌡 The lithospheric plate cools and thickens as it moves away from the spreading ridge, transitioning from 'young, hot, and high' to 'old, cold, and low'.
• 📉 Mathematical models show that most of the ocean plate's cooling occurs within the first 80 million years, reaching about 100 km in thickness.
• 🌡️ The temperature gradient within the plate is significant, with the bottom at about 1300°C and the upper part cooling to below 600°C where it becomes brittle and prone to earthquakes.
• 🏔 Ridge push is a force that acts on the slope of the undersea mountain ranges, pushing the plate away from the ridge as it cools and thickens.
• 🌐 Slab pull is a gravitational force that acts on subducting plates, pulling them deeper into the mantle due to their cooler and denser nature compared to the surrounding asthenosphere.
• 🌳 The subducting plate's temperature decreases with depth, with significant temperature differences between the plate and the asthenosphere at the same depth.
• 🌏 Plate tectonics is powered by Earth's internal heat and driven by 'ridge push' and 'slab pull' forces, illustrating the interconnectedness of the Earth's thermal convection system.

Q & A

What was the early understanding of how tectonic plates move?

Early textbooks suggested that mantle convection cells, similar to those in a beaker of hot liquid on a Bunsen burner, were responsible for pushing tectonic plates along from below.

How do current dynamic models explain the movement of tectonic plates?

Current dynamic models propose that plates move as part of a gravity-driven convection system, where young, hot plates are pushed away from spreading ridges and old, cold plates are pulled down into subduction zones.

What is the composition of a lithospheric plate?

A lithospheric plate, also known as a tectonic plate, consists of a layer of crust on top of the lithospheric mantle, which is the outermost rigid part of the mantle. These move as a single unit.

What is the role of the asthenosphere in plate tectonics?

The asthenosphere is a hotter, less-rigid layer of mantle rock beneath the plates that can slowly flow, allowing the lithospheric plates to move over it.

How does the age of the oceanic plate affect the depth of the ocean?

The ocean depth increases with the age of the underlying plate. For example, at a spreading ridge, the ocean is only about 3000 meters deep, but where the plate is over 80 million years old, the ocean can be as deep as 5500 meters.

What process forms the oceanic crust at the spreading ridges?

As hot mantle rock rises to lower pressure, a portion of the upwelling asthenosphere melts to form magma, which builds the 7-km-thick oceanic crust at the edges of two diverging plates along the ridge axis.

How does the lithospheric plate change as it moves away from the spreading ridge?

As the plate moves away from the ridge, it cools by conducting heat through the crust to the cold ocean water above, and the underlying asthenosphere cools and adds to the bottom of the lithospheric plate, causing it to thicken and cool, thus creating deeper ocean basins.

What is the concept of 'ridge push' in plate tectonics?

Ridge push is the force exerted by the spreading ridge on the ocean plate, which is zero at the ridge but increases with distance and age, pushing the cooling and thickening plate away from the ridge.

What is 'slab pull' and how does it affect the movement of ocean plates?

Slab pull is the gravitational force that pulls denser, cooler rocks in a subducting plate down into the asthenosphere beneath a continental plate. This force is usually larger than ridge push and contributes significantly to the movement of ocean plates.

How does the temperature of the subducting plate affect the occurrence of earthquakes?

The cooler temperatures within the subducting plate increase its density, enhancing the gravitational force and leading to the occurrence of earthquakes, including megathrust earthquakes, on the boundary between converging plates and within the shallow parts of both plates.

What is the energy source for plate tectonics and what forces move the plates?

The energy source for plate tectonics is Earth's internal heat. The forces that move the plates are the 'ridge push' and 'slab pull' gravity forces, which are part of a planetary scale thermal convection system.

Outlines

00:00

🌋 Plate Tectonics and Mantle Convection

This paragraph discusses the evolution of our understanding of plate tectonics, moving from the early concept of mantle convection cells to the current model of a gravity-driven convection system. It explains how young, hot plates are pushed away from spreading ridges and how old, cold plates are pulled into subduction zones. The paragraph also describes the structure of lithospheric plates, the role of the asthenosphere, and the process of seafloor spreading, where new oceanic crust is formed at the ridge axis and how it cools and thickens with age, affecting the elevation of the seafloor. The forces of ridge push and slab pull are introduced as the primary movers of the plates, with a focus on the mechanics of plate cooling and the formation of ocean basins.

05:03

🌊 Forces Behind Plate Tectonics

The second paragraph delves into the forces that drive plate tectonics, emphasizing the role of gravity in both ridge push and slab pull. It describes how the gravitational force on denser, cooler rocks in a subducting plate creates slab pull, which is often the dominant force in plate movement. The paragraph also discusses the occurrence of earthquakes, particularly megathrust earthquakes, and how they are related to the temperature and density of the subducting plate. The summary highlights the importance of the thermal convection system in moving lithospheric plates, with the energy source being Earth's internal heat. It concludes by pointing out that fast-moving plates, such as the Pacific Plate, are influenced by both ridge push and slab pull forces, illustrating the dynamic interplay of these forces in the context of the Earth's tectonic activity.

Keywords

💡Mantle Convection

Mantle convection refers to the slow movement of the semi-solid mantle material due to heat transfer from the Earth's core. In the context of the video, it was initially thought to be the driving force behind plate tectonics. However, current models suggest that plate movement is more complex and involves gravity-driven convection. The script mentions that 'mantle convection below the tectonic plates could drive plate motions' and later explains the role of mantle convection in the formation of oceanic crust at spreading ridges.

💡Tectonic Plates

Tectonic plates, also known as lithospheric plates, are the rigid outermost layer of the Earth composed of the crust and the uppermost mantle. They are central to the video's theme, as they move and interact to cause various geological phenomena. The script describes how these plates 'move as a single unit' and are involved in the process of seafloor spreading and subduction.

💡Asthenosphere

The asthenosphere is a part of the upper mantle that is solid but can slowly flow due to its partially molten state. It is crucial in the video's narrative as it allows the tectonic plates to move over it. The script explains that 'the hotter asthenosphere beneath the plates is solid but less-rigid mantle rock that can slowly flow', illustrating its role in plate tectonics.

💡Spreading Ridges

Spreading ridges are underwater mountain ranges where new oceanic crust is formed due to upwelling magma from the mantle. They are a key concept in the video as they are the sites of seafloor spreading. The script describes how 'spreading ridges stand 2500 meters higher than deep ocean basins' and are the location where 'new ocean plate is forming'.

💡Subduction Zones

Subduction zones are areas where one tectonic plate moves under another and is forced into the mantle due to gravity. They are significant in the video's theme as they explain the recycling of the Earth's crust. The script mentions that 'old cold plates down into subduction zones' and discusses the process of 'slab pull' which is related to subduction.

💡Seafloor Spreading

Seafloor spreading is the process by which new oceanic crust is formed at mid-ocean ridges and then moves away from the ridge as more material is added. It is a central process in the video's explanation of plate tectonics. The script details how 'as hot mantle rock rises to lower pressure, a small portion of this upwelling asthenosphere melts to form magma', contributing to seafloor spreading.

💡Oceanic Crust

Oceanic crust is the outermost layer of the ocean floor, composed of basaltic rocks formed by the cooling and solidification of magma. It is a key component in the video's discussion of the formation and aging of ocean plates. The script explains that 'magma that builds the 7-km-thick oceanic crust at the edges of two diverging plates along the ridge axis'.

💡Ridge Push

Ridge push is the force exerted by the rise of new material at the spreading ridge that pushes the older, cooler lithosphere away from the ridge. It is a critical force discussed in the video's explanation of plate movement. The script states that 'the force of ridge push is zero at the ridge but increases quickly with distance and age', indicating its role in plate tectonics.

💡Slab Pull

Slab pull is the gravitational force that pulls denser, cooler lithospheric material into the mantle at subduction zones. It is a key concept in the video's narrative as it explains one of the primary forces driving plate tectonics. The script describes 'slab pull gravity force' as being due to the enhanced gravitational pull on 'cooler and denser rocks in the subducting plate'.

💡Mega-thrust Earthquakes

Mega-thrust earthquakes are very large earthquakes that occur at subduction zones, where one tectonic plate is being forced under another. They are relevant to the video's theme as they are a result of the forces acting on tectonic plates. The script mentions that 'earthquakes, including great megathrust earthquakes, occur on the shallow part of the boundary between the converging plates'.

💡Planetary Scale Thermal Convection

Planetary scale thermal convection refers to the large-scale movement of material within the Earth driven by heat from the core. It is the overarching concept in the video that explains the energy source for plate tectonics. The script concludes by stating that 'lithospheric plates are part of a planetary scale thermal convection system' and that 'the energy source for plate tectonics is Earth’s internal heat'.

Highlights

Mantle convection below tectonic plates was previously thought to drive plate motions, but current models show plates moving as part of a gravity-driven convection system.

Lithospheric plates, also known as tectonic plates, consist of a layer of crust on top of lithospheric mantle and move as a single unit.

The asthenosphere beneath the plates is a solid but less-rigid mantle rock that can slowly flow.

Seafloor age increases with distance away from spreading ridges, where new ocean plate is formed.

Spreading ridges are 2500 meters higher than deep ocean basins, with ocean depth increasing with the age of the underlying plate.

Hot mantle rock rising to lower pressure melts to form magma, building the 7-km-thick oceanic crust at the edges of diverging plates along the ridge axis.

The lithospheric mantle beneath the crust at the spreading ridge is unusually hot and thin, supporting the elevated ridge.

As the plate moves away from the ridge, it cools and thickens, creating ocean basins exceeding five kilometers in depth.

The plate is 'young, hot, and high' at the spreading ridge and becomes 'old, cold, and low' during the aging and cooling process.

Mathematical modeling illustrates the ocean plate becoming cooler and thicker with age, with most cooling occurring between ages zero and 80 million years.

The force of ridge push, driven by gravity, increases with distance and age, pushing the cooling and thickening ocean plate away from the ridge.

Slab pull is the enhanced gravitational force on cooler and denser rocks in the subducting plate, causing it to descend into the asthenosphere.

Earthquakes, including megathrust earthquakes, occur on the boundary between converging plates and within the shallow parts of both plates.

Observations suggest that the slab pull force is usually larger than the ridge push force in moving tectonic plates.

Fast-moving plates like the Pacific Plate have a fast spreading ridge pushing on one side and a subduction zone pulling on the other.

Lithospheric plates are part of a planetary-scale thermal convection system, with Earth's internal heat as the energy source and 'ridge push' and 'slab pull' as the driving forces.

Transcripts

We once thought that mantle convection below the tectonic plates could drive plate motions.

Early textbooks showed mantle convection cells, like in a beaker of hot liquid on a Bunson

burner, pushing plates along from below. Current dynamic models have plates moving as part

of a gravity-driven convection system that pushes young hot plates away from spreading

ridges and pulls old cold plates down into subduction zones.

Remember that lithospheric plates, also called tectonic plates, have a layer of crust on

top of lithospheric mantle, the outermost rigid part of the mantle. These move as a

single unit. The hotter asthenosphere beneath the plates is solid but less-rigid mantle

rock that can slowly flow. Now let’s look at the broader picture.

This map shows how the seafloor increases in age with distance away from spreading ridges,

such as the East Pacific Rise or the Mid-Atlantic Ridge, where new ocean plate is forming Spreading

ridges stand 2500 meters higher than deep ocean basins. At a spreading ridge, the ocean

depth is only about 3000 meters . Ocean depth increases with age of the underlying plate,

so that where the plate is more than 80 million years old, the overlying ocean increases to

5500 meters deep. Let’s examine the formation of new ocean

plate and how that plate cools with age. As hot mantle rock rises to lower pressure,

a small portion of this upwelling asthenosphere melts to form magma that builds the 7-km-thick

oceanic crust at the edges of two diverging plates along the ridge axis. Beneath the crust

at the spreading ridge, there is only a thin layer of lithospheric mantle because it is

unusually hot in the upwelling zone. This hot, and therefore lower-density, mantle rock,

supports the 2500-meter elevation of the spreading ridge As the plate slowly moves away from

the ridge, it cools by conducting heat through the crust to the cold ocean water above. At

the same time, the underlying asthenosphere cools and adds to the bottom of the lithospheric

plate. Thus, although the crust maintains its thickness during migration away from the

ridge, the lithospheric plate thickens and cools to create ocean basins that exceed five

kilometers in depth. A simple way to think about elevation is that the plate is “young,

hot, and high” at the spreading ridge and becomes “old, cold, and low” during the

aging and cooling process. Mathematical modeling of this cooling process illustrates the ocean

plate becoming cooler and thicker with age. The temperature at the bottom of the plate

is about 1300° centigrade. Notice that most of the cooling process occurs between age

zero at the ridge and about 80 million years when the ocean plate has grown to about 100

km thick. The upper plate is less than 600°, thus is the only part of the plate cold and

brittle enough to fracture and produce earthquakes. Though still rigid, the lower plate is warmer

and can deform in a ductile or plastic fashion. So what force pushes ocean plates away from

spreading ridges? Any mass on a slope is affected by gravity, seen most dramatically with avalanches

and landslides. Spreading ridges are broad undersea mountain ranges and, although the

flanks of a spreading ridge are a relatively gentle slope, the mass on that slope is humongous.

The force of ridge push is zero at the ridge but increases quickly with distance and age,

pushing the cooling and thickening ocean plate away from the ridge.

Now, what about “slab pull”?. We’ll consider a 30-million-year-old ocean plate

subducting at 5 centimeters per year into hotter asthenospheric mantle beneath a continental

plate. As the ocean plate, subducts, the warming process takes many millions of years as the

slab descends. The deeper part it is continuously replaced, in a conveyor-belt fashion, by cooler

plate from above. Mathematical modeling again illustrates the temperatures within

the subducting ocean plate. In this example, lithospheric mantle rock

in the subducting plate at 150 km depth is 1000° cooler than the asthenospheric mantle

at the same depth. The cooler temperatures mean that the density of rocks in the subducting

slab is greater than the density of the hotter asthenosphere. While gravity pulls down on

all rocks, it pulls down harder on more-dense rocks. This enhanced gravitational force

on the cooler and denser rocks in the subducting plate is the slab pull gravity force.

Earthquakes, including great megathrust earthquakes, occur on the shallow part of the boundary

between the converging plates and within the shallow parts of both plates near that boundary.

In addition, there is a zone of rock cooler than 600°C within the subducting plate that

remains brittle and within which earthquakes can occur to depths of hundreds of kilometers.

Observations of intraplate earthquakes and other indications of stress with tectonic

plates suggest that the slab pull force is usually larger than the ridge push force.

It is noteworthy that fast-moving plates, like the Pacific Plate, generally have a fast

spreading ridge pushing on one side while a subduction zone pulls on the other side.

In this big picture view, we see that lithospheric plates are part of a planetary scale thermal

convection system. The energy source for plate tectonics is Earth’s internal heat while

the forces moving the plates are the “ridge push” and “slab pull” gravity forces.