Grade 11

Advanced

7 min

Lesson 2 of 12

Mechanisms of Plate Tectonics

Not Started

Analyzes the forces that move lithospheric plates, including mantle convection and slab pull.

Imagine a conveyor belt made of solid rock, thousands of kilometers long, powered by a furnace hotter than the surface of the Sun. How does the Earth move its massive crustal plates without snapping them like crackers?

Learning Objectives

1.Explain the process of mantle convection and its drag effect on the lithosphere
2.Analyze the mechanical differences between ridge push and slab pull
3.Evaluate how paleomagnetic 'stripes' provide a chronological record of seafloor spreading

The Thermal Engine: Mantle Convection

The primary driver of plate tectonics is mantle convection. Heat from the Earth's core and radioactive decay creates a thermal gradient. As mantle material heats up, its density

\rho

decreases, causing it to rise toward the crust. Upon reaching the cooler asthenosphere, it moves horizontally, losing heat and increasing in density until it sinks again. This creates a convection cell. This horizontal movement exerts a 'basal drag' on the overlying lithospheric plates, much like a tablecloth being pulled across a table. The relationship between temperature and density is expressed as:

\rho \propto \frac{1}{T}

where an increase in temperature

T

leads to a decrease in density

\rho

Quick Check

What physical change causes mantle material to begin sinking back toward the core?

Think about the relationship between temperature and density.

Answer

An increase in density caused by cooling as it moves away from the heat source.

Gravity at Work: Ridge Push and Slab Pull

While convection provides the 'drag,' gravity provides the 'push' and 'pull.' Ridge push occurs at mid-ocean ridges. Because the ridge is thermally elevated, gravity causes the lithosphere to slide 'downhill' away from the ridge axis. However, the most powerful force is slab pull. As an oceanic plate ages, it cools and becomes denser than the underlying asthenosphere. At subduction zones, this dense leading edge sinks into the mantle, dragging the rest of the plate behind it. Research suggests slab pull is significantly stronger than ridge push, acting as the primary locomotive for plate movement.

Consider two plates. Plate A is attached to a large subducting slab (strong slab pull), while Plate B is only being moved by ridge push. 1. Identify the forces: Plate A has $F_{total} = F_{push} + F_{pull}$ . 2. Plate B has $F_{total} = F_{push}$ . 3. Because $F_{pull} \gg F_{push}$ , Plate A will move significantly faster (e.g., the Nazca Plate) than Plate B (e.g., the African Plate).

Quick Check

Which force is generally considered the dominant driver of plate motion: Ridge Push or Slab Pull?

Think about which force involves the weight of the entire plate sinking into the mantle.

Answer

Slab Pull

The Magnetic Tape Recorder: Paleomagnetism

How do we prove the seafloor is actually moving? The answer lies in paleomagnetism. As basaltic magma cools at mid-ocean ridges, iron-rich minerals like magnetite align with Earth's magnetic field. Once the rock cools below the Curie Point (approx.

580^{\circ}C

), this alignment is locked in. Earth's magnetic field periodically undergoes magnetic reversals. This creates a symmetrical pattern of 'magnetic stripes' on either side of the ridge. By measuring the width of these stripes and knowing the timing of reversals, we can calculate the spreading rate

v

v = \frac{d}{t}

where

d

is the distance from the ridge and

t

is the time since the reversal.

A magnetic anomaly of 'Normal Polarity' is found

100

km from a ridge axis. Radiometric dating shows this reversal occurred

2

million years ago. 1. Convert distance to cm:

100\text{ km} = 10,000,000\text{ cm}

. 2. Use the formula:

v = \frac{10,000,000\text{ cm}}{2,000,000\text{ years}}

3. Result: The spreading rate is

5\text{ cm/year}

Key Takeaways

1Mantle convection cells transfer thermal energy into mechanical energy, dragging plates via basal drag.
2Slab pull is the most powerful tectonic force, driven by the negative buoyancy of cold, dense subducting plates.
3Ridge push is a gravitational force where plates slide away from the topographically high mid-ocean ridges.
4Paleomagnetic stripes provide a symmetrical 'map' of seafloor spreading and magnetic field history.

Quick Check

Multiple Choice

What is the primary reason the mantle undergoes convection?

Multiple Choice

In the context of paleomagnetism, what happens when magma cools below the Curie Point?

True/False

Ridge push is a stronger force than slab pull in driving plate tectonics.

What's Next?

Review Tomorrow

In 24 hours, try to sketch a diagram from memory showing the three main forces: Mantle Convection, Ridge Push, and Slab Pull.

Practice Activity

Look at a map of the world's tectonic plates. Identify which plates are likely moving fastest based on whether they have subduction zones (slab pull) or just mid-ocean ridges.

Browse

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Mechanisms of Plate Tectonics

Learning Objectives

The Thermal Engine: Mantle Convection

Gravity at Work: Ridge Push and Slab Pull

The Magnetic Tape Recorder: Paleomagnetism

Key Takeaways

Quick Check

What's Next?

Mechanisms of Plate Tectonics

Learning Objectives

The Thermal Engine: Mantle Convection

Gravity at Work: Ridge Push and Slab Pull

The Magnetic Tape Recorder: Paleomagnetism

Key Takeaways

Quick Check

What's Next?