Earth's Fiery Vents
A volcano is an opening in the Earth's crust through which molten rock, hot gases, and ash can escape to the surface. Volcanic eruptions can be gentle, flowing events or incredibly violent explosions.
Why Do Volcanoes Form?
Most volcanoes are found at the boundaries of tectonic plates.
Convergent Boundaries (Subduction Zones): When a dense oceanic plate sinks (subducts) beneath another plate, it melts as it enters the hot mantle. This molten rock, called magma, is less dense than the surrounding rock, so it rises to the surface and can erupt to form a volcano. The 'Ring of Fire' around the Pacific Ocean is a chain of subduction zone volcanoes.
Divergent Boundaries: When two plates pull apart, magma from the mantle rises up to fill the gap, creating new crust. This can form volcanoes along mid-ocean ridges or in rift valleys.
Hot Spots: Some volcanoes form in the middle of a tectonic plate, far from any boundary. These are caused by hot spots, which are exceptionally hot plumes of magma rising from deep within the mantle. As the tectonic plate moves over the stationary hot spot, it can create a chain of volcanic islands, like the Hawaiian Islands.
Magma vs. Lava
Magma is molten rock that is beneath the Earth's surface.
Lava is molten rock that has erupted onto the Earth's surface.
Types of Volcanoes
The shape of a volcano and the way it erupts depends on the properties of its magma, especially its viscosity. Viscosity is a measure of a fluid's resistance to flow (how thick or runny it is).
1.Shield Volcano:
Magma: Low-viscosity (runny) magma that flows easily.
Shape: Built from many layers of fluid lava flows, creating a very broad, gently sloping volcano that looks like a warrior's shield.
Eruption: Generally gentle, effusive eruptions where lava flows out smoothly.
Example: Mauna Loa in Hawaii.
2.Cinder Cone:
Magma: Thick magma with a lot of trapped gas.
Shape: Built from explosive eruptions of blobs of lava (cinders) that are thrown into the air and fall back down around the vent, creating a small, steep-sided cone.
Eruption: Explosive but relatively small.
3.Composite Volcano (or Stratovolcano):
Magma: High-viscosity (thick and sticky) magma that traps a lot of gas.
Shape: Built from alternating layers of sticky lava flows and explosive ash deposits. They are large, steep-sided, and cone-shaped—the classic volcano shape.
Eruption: Can be extremely explosive and dangerous, producing huge ash clouds and fast-moving pyroclastic flows.
Example: Mount St. Helens, Mount Fuji.