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Obsidian

A super-chilled igneous rock, black in colour and glassy in appearance. Fractures conchoidally. Often formed due to contact with cold seawater as the silicate minerals cool almost immediately, restricting mineral growth. Can also be called volcanic glass. Due to the rapid rate of cooling no crystals form. Obsidian forms from felsic, silica-rich, magma. Over time, crystals do form in a snowflake-like pattern, so-called “snowflake obsidian.

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Olistostrome

An olistostrome is a sedimentary rock formation that comprises a chaotic assemblage of blocks of preexisting rocks, which can be of considerable size, in a finer grained matrix. Olistostromes are the result of submarine slumping or gravity sliding on an unstable shelf. A mélange is a similar rock unit but has a tectonic origin. The term olistostrome is derived from olio - a dish of many ingredients or stew; a mixture of heterogeneous elements; hodgepodge. and stroma - a bed covering (Greek equivalent of the Latin stratum).

Ooid

A spherical grain of calcium carbonate, either aragonite or calcite. See the Ooid Formation tutorial for more details of their formation and origin.

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Ordovician

The second period of the Palaeozoic. Spans 510-443. The Ordovician is famous for its graptolite and trilobite-rich deposits, particularly in the UK. The Ordovician ended in a mass extinction, probably caused by an Ice Age.

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Oxidation

A chemical reaction in which substances combine with oxygen for form an oxide. For example, the combination of iron with oxygen to form an iron oxide (rust) or copper and oxygen produce copper oxide; the green coating on old pennies. The opposite of oxidation is reduction.

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