Fix the mistake: The Formation of the Earth

Volcanoes and Igneous Rocks

Volcanic activity is the phenomenon in which magma, formed when rocks melt because of heat inside Earth, erupts onto the surface. The shape of a volcano and the appearance of its eruption vary greatly depending on the properties of the magma.

Basic Knowledge

Magma that reaches Earth's surface is called lava. The higher the proportion of silicon dioxide(SiO₂) in magma, the greater its viscosity, and the more explosive the eruption becomes. In contrast, magma with little SiO₂ and low viscosity flows out gently.
Volcanoes are classified into three types: shield volcanoes(low viscosity, such as Mauna Loa), stratovolcanoes(moderate viscosity, such as Mount Fuji and Sakurajima), and lava domes(high viscosity, such as Showa-Shinzan and Mount Unzen-Fugen).
Rocks formed when magma cools and solidifies are called igneous rocks. They are divided into volcanic rocks, which cool rapidly near the surface, and plutonic rocks, which cool slowly deep underground.

Key Terms
Magma(A high-temperature fluid formed when rocks melt underground. Approximately 900〜1200℃)
Volcanic rocks(Rapidly cooled magma near the surface. Porphyritic texture. Rhyolite, andesite, and basalt)
Plutonic rocks(Magma that cools slowly underground. Granular texture. Granite, diorite, and gabbro)
Porphyritic texture(A texture in which large phenocrysts are scattered in a fine-grained groundmass. A characteristic of volcanic rocks)
Granular texture(A texture in which crystals of about the same size are densely arranged. A characteristic of plutonic rocks)
Colorless minerals and colored minerals(Quartz and feldspar are colorless / biotite, hornblende, pyroxene, and olivine are colored)

In Depth (Background and Significance)

Japan has about 111 active volcanoes, and about 7% of the world's active volcanoes are concentrated there. This is because the Japanese archipelago lies at the boundary of four plates, where magma is easily generated from subducting plates.
Volcanoes cause disasters, but they also provide benefits such as hot springs, geothermal power generation, fertile soil, and tourism resources. Pumice and volcanic ash, which contain much volcanic glass, can be carried long distances and are used as key beds to correlate strata. It is important to consider the balance between disaster prevention and the benefits of volcanoes, including the 2014 eruption of Mount Ontake and past caldera eruptions(Aso and Aira).

Key Points
  • Magma viscosity is determined by the amount of SiO₂
  • High viscosity → dome-shaped and explosive / low viscosity → shield-shaped and gentle
  • Volcanic rocks = rapid cooling = porphyritic texture / plutonic rocks = slow cooling = granular texture
  • Many colorless minerals → light-colored rhyolite and granite
  • Many colored minerals → dark-colored basalt and gabbro
  • About 7% of the world's active volcanoes are concentrated in Japan
  • Volcanic ash is useful as a key bed for correlating strata

Points to Note (Common Confusions)

① Do not reverse volcanic rocks(rapid cooling and porphyritic texture) and plutonic rocks(slow cooling and granular texture). ② The pairs “rhyolite↔granite,” “andesite↔diorite,” and “basalt↔gabbro” correspond to volcanic and plutonic rocks. ③ Viscosity and temperature are different concepts; magma with low viscosity(basaltic magma) has the higher temperature. ④ Lava is magma that has reached the surface, whereas magma is underground.

Practice

  1. What is the main component that determines the viscosity of magma?
  2. What is the name of the texture of volcanic rocks?
  3. Is granite a volcanic rock or a plutonic rock?

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