Overview
This lecture explains what volcanoes are, how and where they form, the types of volcanoes, and the processes resulting in volcanic eruptions.
What Is a Volcano?
- A volcano is a rupture in Earth's crust through which molten rock, gases, and ash escape.
- Earth's crust and upper mantle form the lithosphere, which is split into tectonic plates.
Tectonic Plates and Volcano Locations
- Most volcanoes form along the boundaries (margins) of tectonic plates.
- Destructive (convergent) margins: An oceanic plate sinks under a continental plate, melts, and causes eruptions.
- Constructive (divergent) margins: Plates move apart, magma rises through the gap and can erupt.
- 75% of active volcanoes are found along the Pacific Plate, called the "Ring of Fire".
- 90% of the most powerful eruptions since 1850 have happened in the Ring of Fire.
Volcanoes Away from Plate Boundaries: Hot Spots
- Hot spot volcanoes form where rising magma breaks through the middle of a tectonic plate.
- The Hawaiian Islands are an example of hot spot volcanoes.
Types of Volcanoes
- Composite (strata) volcanoes: Cone-shaped, found on destructive margins, made of layers of ash and lava, and erupt explosively.
- Shield volcanoes: Gentle sloping sides, found at constructive margins or hot spots, have runny lava and less violent eruptions.
Structure and Eruption of Composite Volcanoes
- Magma collects in a magma chamber before erupting through a main vent and crater.
- Eruptions release lava, volcanic bombs (rock pieces), ash, and gases.
- Layers of lava and ash create the composite volcano's structure.
Key Terms & Definitions
- Volcano — A rupture in Earth's crust where magma, gas, and ash escape.
- Lithosphere — The rigid outer layer of Earth, including the crust and upper mantle.
- Tectonic plates — Large, moving slabs of lithosphere that make up Earth's surface.
- Destructive plate margin — Where plates move towards each other and one sinks.
- Constructive plate margin — Where plates move apart, creating new crust.
- Ring of Fire — A zone around the Pacific Plate with many active volcanoes.
- Hot spot — An area where magma rises through a plate, forming volcanoes.
- Composite volcano (strata volcano) — A steep, layered volcano formed by explosive eruptions.
- Shield volcano — A wide, gently-sloping volcano formed by fluid lava flows.
- Magma chamber — Underground reservoir where magma collects before an eruption.
Action Items / Next Steps
- Review volcano types and structures for upcoming quiz.
- Map locations of major tectonic plates and the Ring of Fire for homework.