File:Gray obsidian 27.jpg

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English: Igneous rocks form by the cooling & crystallization of hot, molten rock (magma & lava). If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks. If this happens deep underground, they are intrusive igneous rocks. Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.

Obsidian is readily identifiable. It is a glassy-textured, extrusive igneous rock. Obsidian is natural glass - it lacks crystals, and therefore lacks minerals. Obsidian is typically black in color, but most obsidians have a felsic to intermediate chemistry. Felsic igneous rocks are generally light-colored, so a felsic obsidian seems a paradox. Mafic obsidians are scarce, but they are also black and glassy.

Obsidian is an uncommon rock, but can be examined at several famous localities in America, such as Obsidian Cliff at the Yellowstone Hotspot (northwestern Wyoming, USA) and Big Obsidian Flow at the Newberry Volcano (central Oregon, USA).

Obsidian is moderately hard and has a conchoidal fracture (smooth and curved fracture surface), with sharp broken edges. Freshly-broken obsidian has the sharpest edges of any material known, natural or man-made (as seen under a scanning electron microscope).

Obsidian forms two ways: 1) very rapid cooling of lava, which prevents the formation of crystals; 2) cooling of high-viscosity lava, which prevents easy movement of atoms to form crystals. An example of obsidian that formed the first way is along the margins of basaltic lava flows at Kilaeua Volcano (Hawaii Hotspot, central Pacific Ocean). Most obsidian formed the second way.

Seen here is flow-banded gray obsidian - an unusual color.

Locality: unrecorded
Date
Source https://www.flickr.com/photos/47445767@N05/53184119354/
Author James St. John

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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/53184119354. It was reviewed on 16 September 2023 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

16 September 2023

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11 September 2023

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current21:53, 16 September 2023Thumbnail for version as of 21:53, 16 September 20233,486 × 2,072 (3.46 MB)Ser Amantio di NicolaoUploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/53184119354/ with UploadWizard

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