Lecture 2.3: When Continents Collide

The surface of our planet is constantly being created and destroyed at the edges of the great tectonic plates.

Today’s Essential Questions

• What are the three main types of plate boundaries?
• What geological features are created at a convergent boundary?
• What geological features are created at a divergent boundary?

Connecting to Our Last Investigation

In the seismic data lab, you saw a profound pattern: earthquakes and volcanoes are not scattered randomly across the globe. They occur in distinct, narrow lines. Today, we will see that those lines are the boundaries between the Earth’s tectonic plates. They are the active zones of creation and destruction where the drama of geology unfolds.

Divergent Boundaries: Worlds Pulling Apart

At a divergent boundary, two tectonic plates are moving away from each other.

• As the plates separate, magma from the mantle rises to fill the gap, cooling and solidifying to create new crust.
• On land, this forms rift valleys, like the Afar Triangle in Ethiopia which anchored this unit.
• Under the ocean, it forms mid-ocean ridges, the largest mountain ranges on Earth.
• This is where new planetary surface is born.

Convergent Boundaries: When Worlds Collide

At a convergent boundary, two plates are moving toward each other. The outcome depends on the types of crust involved:

1. Oceanic-Continental: The denser oceanic plate subducts, or dives beneath, the continental plate. This creates a deep ocean trench and a line of volcanoes on the continent.
2. Oceanic-Oceanic: One oceanic plate subducts beneath another, creating a trench and a chain of volcanic islands.
3. Continental-Continental: The two continents crumple and fold, pushing up enormous mountain ranges, like the Himalayas.

Transform Boundaries: Grinding Past

At a transform boundary, two plates slide horizontally past one another.

• Crust is neither created nor destroyed.
• The immense friction between the plates causes them to stick and then slip, generating massive and frequent earthquakes.
• The San Andreas Fault in California is a famous example of a transform boundary.

Thinking Lens: Patterns

The theory of plate tectonics is powerful because it reveals a simple, underlying pattern that explains a huge range of seemingly unrelated geological features.

Question: How does recognizing the pattern of plate boundaries help us explain the global distribution of mountains, volcanoes, and earthquakes as interconnected phenomena rather than isolated events?

Preparing for Our Next Task

You now have a classification system for what happens at the edges of the plates. The models for convergent, divergent, and transform boundaries are the exact tools you will use to analyze the different geological scenarios in the ‘Plate Boundary Interactions’ lab.

Summary: Answering Our Questions

• What are the three main types of plate boundaries? Divergent (pulling apart), Convergent (coming together), and Transform (sliding past).

• What features are created at a convergent boundary? Volcanoes, island arcs, deep ocean trenches, and massive mountain ranges.

• What features are created at a divergent boundary? Rift valleys and mid-ocean ridges, where new crust is formed.

Prompt: In 2-3 sentences, explain how the same process—plate tectonics—can be responsible for creating both new crust (at divergent boundaries) and destroying old crust (at convergent boundaries).