Formation of a Stratovolcano
Most of Indonesia's volcanoes are stratovolcanoes, also known as composite volcanoes. As shown in Figure 9.11, these volcanoes are typically highly explosive due to the high silica content of the magma. Magma with a high silica content allows gases to build up and forms lava that moves slowly. Lava is magma that has reached the Earth's surface.
The pyroclastic material ejected from stratovolcanoes is far more dangerous than the lava. Pyroclastic is derived from the Greek words for "fire" and "broken into pieces". Geologists refer to fragments or pieces of pre-existing rocks and minerals as "clasts". Pyroclastic flows contain searing hot ash, pumice, and gas and can move up to 110 km per hour.
Other types of volcanoes, such as the shield volcanoes of Hawaii, have magma with a low silica content, so the lava flows faster. The reason for the difference in silica content has to do with the composition of the tectonic plate. Continental plates have a higher silica content than oceanic plates.
Figure 9.11 Formation of a stratovolcano at a subduction zone. (Wade Greenberg-Brand/Paleontological Research Institution, 2014. CC BY-SA 4.0).
Source: Wade Greenberg-Brand/Paleontological Research Institution, https://commons.wikimedia.org/wiki/File:Subduction_Zone_Stratovolcano.tif
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