Volcanic Activity on the Earth's Surface

Key Terms

Term	Definition
Pyroclast/Pyroclastic Material	Any material ejected from a volcano including ash, dust and rocks
Subduction	When one plate is brought underneath the other
Volcanologist	Studies volcanoes
Plutons	Granite structures
Caldera	Cauldron like volcano
Silica	The name given to a group of minerals composed of silicon and oxygen, the two most abundant elements in the earth's crust

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Introduction to Volcanoes

• A volcano forms when molten magma forces its way to the Earth's surface through a narrow tube called a vent and reaches the surface through an opening called a crater.
• Once magma reaches the surface it is called lava.
• Magma is stored underground in a magma chamber – it can reach 1,000°C here so the rock is in a molten state.
• During an eruption hot ash, volcanic rock bombs, dust and lava erupt – collectively known as pyroclastic material.

Location of Volcanoes

1. Most Famous Example: Pacific Ring of Fire

• Contains 75% of the world's volcanoes, making it the most volcanic region globally.

2. Divergent Plate Boundaries

What happens?

• Two tectonic plates pull apart, creating a fissure in the Earth's crust.

• Magma rises through the fissure to the surface. Characteristics:

• Gentle eruptions, as magma has a larger opening to escape. Example: Mid-Atlantic Ridge – a prominent divergent boundary.

3. Convergent Plate Boundaries

What happens?

• Two tectonic plates collide, and one plate is subducted beneath the other.

• Magma forms due to intense pressure and reaches the surface through a narrow opening. Characteristics:

• Violent eruptions, as magma escapes under extreme pressure.

• Example: The Nazca Plate subducting beneath the South American Plate, forming the Andes with 2,000 active volcanoes.

4. Hotspots

What happens?

• Weaknesses in the middle of tectonic plates allow magma to rise due to superheated plumes.

• Hotspots are stationary, but as plates move over them, new volcanic islands form. Characteristics:

• Volcanoes at hotspots eventually become less active and extinct, leaving behind a trail of volcanic islands. Examples: The Hawaiian Islands.

5. Silica Content and Eruption Type

• Silica content in magma determines the type of eruption:
• High silica content: Thick magma traps gases, causing explosive eruptions.
• Low silica content: Magma flows more easily, leading to gentler eruptions.