Obsidian from the Rocce Rosse lava flow on Lipari island (Italy)
Black volcanic glass
Obsidian is a naturally occurring glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly through the glass transition temperature and freezes without sufficient time for crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where cooling of the lava is rapid. Because of the lack of cr...
Olivine is one of the earth's most common rock-forming silicate minerals, typically contained in mafic to ultrafafic magmatic rocks with low silica content (e.g. basalt, gabbro, peridotite), which make up much of the earth's upper mantle, the oceanic crust and lava flows etc from basaltic shield volcanoes on land.
Its name refers to its often olive-green color.
Name
olivine
Category
Formula
(Mg2+, Fe2+)2SiO4
Crystal system
orthorhombic
Color
Yellowish-green to olive green
Streak
None
Luster
Vitreous
Crystal habit
Massive to granular
Mohs hardness
6.5–7
Specific weight
3.2-4.5
Uses
Other
Chemically, it combines two interchangeable ions of magnesium (Mg2+) or iron (Fe2+) with one silica ion (Si4+) in a densely-packed crystal lattice. The two end-members from this compositional range are forsterite (magnesium-olivine, Mg2SiO4) and and fayalite (iron-olivine, Fe2SiO4). In rare cases, Mg/Fe can also be replaced by calcium (Ca), manganese (Mn), or nickel (Ni).
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Introduction to Plate Tectonics The Earth's upper, rigid layer is broken into several plates which are in constant motion to one another. Most earthquakes and volcanoes occur as result of these plate boundaries.
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