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.