Fire from Below: Volcanism
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
Magma Properties and Eruption Style
The behavior of a volcano is largely determined by the properties of its magma, especially its viscosity.
Viscosity: A measure of a fluid's resistance to flow. High viscosity means thick and sticky (like honey); low viscosity means thin and runny (like water).
Low-Viscosity Magma (Mafic): Low in silica, hot, and allows gases to escape easily. This leads to effusive eruptions, where lava flows gently out of the volcano.
High-Viscosity Magma (Felsic): High in silica, cooler, and traps gases. The trapped gases build up immense pressure, leading to explosive eruptions that produce ash clouds and pyroclastic flows.
Types of Volcanoes
1.Shield Volcano:
Formed by low-viscosity, fluid lava flows.
Has a very broad, gently sloping profile, resembling a warrior's shield.
Eruptions are generally effusive.
Example: Mauna Loa, Hawaii.
2.Composite Volcano (or Stratovolcano):
Formed from alternating layers of viscous lava flows and pyroclastic deposits (ash, pumice).
Has a steep, conical shape.
Eruptions are often highly explosive and dangerous.
Example: Mount St. Helens, Mount Fuji.
3.Cinder Cone:
The simplest type of volcano.
Built from blobs of congealed lava (cinders or scoria) ejected from a single vent.
Has a steep conical shape but is much smaller than a composite volcano.
Intrusive Igneous Features (Plutons)
When magma cools and solidifies beneath the surface, it forms intrusive igneous bodies called plutons. When erosion exposes them, we see features like:
Dike: A sheet-like intrusion that cuts across existing rock layers.
Sill: A sheet-like intrusion that is injected between existing rock layers.
Batholith: A very large, irregularly shaped mass of intrusive igneous rock (e.g., the core of a mountain range like the Sierra Nevada).