The entangling tendency of the silicon-oxygen bond in silicic magmas leads to notable differences in the behavior of their volcanic eruptions, compared to those of mafic magmas.
Mafic minerals melt at high temperatures, and the magmas containing them are hot. Mafic magmas are relatively low in silica, which reduces their internal stickiness. Since they rise from the Earth's dry mantle, they are generally poor in superheated water. Hot and slippery, mafic magmas easily penetrate the crust. Their eruptions are fountain-like. Their flows are fluid and spread freely.
These lavas cool into a rock called basalt. Clots of dark-colored scoria pile up around their fountaining vents.
Silicic magmas are cooler than mafic ones. They are over-saturated in silica, which makes them sticky and resistant to flow. Some have the silica content of glass! Since these melts are born in the Earth's crust, they have the opportunity to take up water.
Silicic magmas find it difficult to erupt and usually remain trapped in the crust. Those that do make it out cool into pudgy domes of dacite or rhyolite. Pockets of effervescent lava collapse and freeze into a natural volcanic glass called obsidian. Lava squirted into the air foams into clots of pumice.
Silicic magmas are often rich in dissolved water. The combination of extreme stickiness and trapped steam in decompressing high silica melts can be a fatal one. The exploding steam shatters the magma into hot, sticky shards of glass and fragments of pumice. In large eruptions, the volume of hot matter increases by orders of magnitude in just seconds, often penetrating the stratosphere, only to collapse back into ground-hugging flows of superheated gas and ash.
The difference can perhaps be illustrated by comparing that ubiquitous 5th grade science fair project, the sedate vinegar and baking soda volcano, representing the fluid eruptions of mafic magma:
Mafic minerals melt at high temperatures, and the magmas containing them are hot. Mafic magmas are relatively low in silica, which reduces their internal stickiness. Since they rise from the Earth's dry mantle, they are generally poor in superheated water. Hot and slippery, mafic magmas easily penetrate the crust. Their eruptions are fountain-like. Their flows are fluid and spread freely.
These lavas cool into a rock called basalt. Clots of dark-colored scoria pile up around their fountaining vents.
Silicic magmas are cooler than mafic ones. They are over-saturated in silica, which makes them sticky and resistant to flow. Some have the silica content of glass! Since these melts are born in the Earth's crust, they have the opportunity to take up water.
Silicic magmas find it difficult to erupt and usually remain trapped in the crust. Those that do make it out cool into pudgy domes of dacite or rhyolite. Pockets of effervescent lava collapse and freeze into a natural volcanic glass called obsidian. Lava squirted into the air foams into clots of pumice.
Silicic magmas are often rich in dissolved water. The combination of extreme stickiness and trapped steam in decompressing high silica melts can be a fatal one. The exploding steam shatters the magma into hot, sticky shards of glass and fragments of pumice. In large eruptions, the volume of hot matter increases by orders of magnitude in just seconds, often penetrating the stratosphere, only to collapse back into ground-hugging flows of superheated gas and ash.
The difference can perhaps be illustrated by comparing that ubiquitous 5th grade science fair project, the sedate vinegar and baking soda volcano, representing the fluid eruptions of mafic magma:
with the "elephant toothpaste" eruptions of hydrogen peroxide, a catalyst to trigger gas release, and a detergent to polymerize and entangle the silicic "magma":
This would not look good in the High School gymnasium. . .
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