geologia: Rhyolite

Evidence: 100 • authority: 72

Rhyolite ( RY-ə-lyte) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral assemblage is predominantly quartz, sanidine, and plagioclase. It is the extrusive equivalent of granite. Its high silica content makes rhyolitic magma extremely viscous. This favors explosive eruptions over effusive eruptions, so this type of magma is more often erupted as pyroclastic rock than as lava flows. Rhyolitic ash-flow tuffs are among the most voluminous of continental igneous rock formations. Rhyolitic tuff has been used extensive…

Evidence: 68 • authority: 72

Obsidian (, əb-SID-ee-ən ob-) is a naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is rich in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows. These flows have a high content of silica, giving them a high viscosity. The high viscosity inhibits the diffusion of atoms through the lava, which inhibits the first step (nucleation) in the formation of mineral crystals. Together with rapid cooling, this results in a natural glass forming from the lava.…

Evidence: 43 • authority: 88

Nomad lives: from Prehistoric Times to the Present Day. This book illustrates fragments of the lives of past and present “nomadic” peoples, from Africa, Asia, North and South America and Europe, from different angles (habitats, material production, economic and territorial organisation, social organisation, rites and beliefs, art). This way of life prevailed for millions of years before another sedentary way of life gradually replaced it from 9,000 years ago onwards. However, nomads did not disappear. Today they are a minority and are often marginalised, but they are still an integral part of our humanity. This is the point of view we have adopted in Nomad Lives and we invite our readers to…

Evidence: 43 • authority: 80

Medicine has always been “Modern” and “Scientific” from ancient times to the present day