- VOLCANOES are places where lava (molten rock, often as hot as 1200o C) exits to the surface and spreads out.
Lava flowing from a volcano over time generally builds up to form the volcano into a low, cone-like shape.
Lava flow from a volcano can be gentle and smooth, but it can also come in the form of a large, explosive eruption.
When lava stops flowing, the top surface of the underground lava supply may harden and crust over, halting the flow
until enough pressure builds up to force the next eruption.
During an eruption, many pyroclastic materials (literally, "broken up by fire") are thrown upwards with tremendous force.
- Lava flows:
Most lava cools slowly, forming a smooth surface with gentle ripples, known in Hawaii as pahoehoe.
Lava that cools fast or contains bubbles may form a very sharp surface with many broken bubbles of sharp, glassy matrerial called Aa.
Lava generally solidifies into basalt, a generally black and very fine-grained igneous rock.
Magma (molten rock that remains below the surface) may solidify into gabbro, a black but coarse-grained igneous rock similar to basalt in composition.
Many volcanoes contain small fissures or vents along the sides; these are generall the sources of more gentle lava flows.
- Pyroclastics consist of material forcibly ejected from a volcano. In general, the chemical composition of pyroclastics is more acidic, rich in silica (SiO2).
Large chunks of pyroclastic material are called bombs is they are teardrop shaped, or blocks if they are more angular.
Medium-sized and small balls (often fist-sized) of glassy material may form a material called obsidian (volcanic glass).
Large pieces of obsidian look black, but thin pieces look dark green, and very thin pieces are transparent.
Like glass, obsidian breaks with conchoidal fracture to form very sharp surfaces with ripples.
Because of its sharpness, this material was often used to make arrowheads.
Volcanic eruptions are accompanied by large amounts of gas, including CO2, sulfur dioxide (SO2), and hydrogen sulfide (H2S).
Volcanic gasses often form bubbles inside molten rocks. Rocks that solidify with bubbles inside are called either scoria or pumice.
Scoria is usually darker and similar to basalt in chemical composition.
Pumice is usually lighter in color, more silica-rich, with tinier and more numerous bubbles. Pumice often has so many bubbles that it can float!
- Volcanoes are considered active if they have erupted within the past hundred years and are likely to erupt again.
Volcanoes that are no longer active are considered dormant of extinct.
- Calderas:
If a volcano has ejected lots of lava and pyroclastics, an empty chamber may result underneath.
The collapse of overlying rock into such an empty chember can form a wide crater, called a Caldera.
|