Compare and contrast fissure eruptions and pyroclastic sheet deposits
During both fissure eruptions and pyroclastic sheet deposits, eruptions issue from long cracks or fissures, cover vast areas, and volcanoes fail to develop.
During fissure eruptions, the lava is so fluid (has such low viscosity) that it spreads out and covers vast areas.
In contrast, it seems that pyroclastic sheet deposits originated as pyroclastic flows—hence the name. The pyroclastic materials of many of these flows were so hot that they fused together to form welded tuff. Geologists now think that major pyroclastic flows issue from fissures formed during the origin of calderas.
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