Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
|Glassy materials (like the obsidian shown here) break with a distinctive curving fracture. Curved ridges are concentric round the point where the fracture begins, and fine radial ridges or grooves also point away from the point of initiation.|
Some minerals grow as radiating bundles of fine, fibrous crystals.
|Minerals that grow as parallel bundles of fine, fibrous crystals give the rock a fibrous look when it breaks. Gypsum, shown at right, often fractures this way. This variety is called satin spar.|
Lava flows, and occasionally thin sheetlike intrusions like dikes and sills, shrink as they cool and break into polygonal columns.
|Sheets of magma shrink as they cool, developing polygonal fractures called columnar jointing. Here we see columnar jointing in a dike in Scotland.|
|Columnar joints can be almost perfectly regular but they are more often rather irregular like these joints in a lava flow in southern Idaho.|
Most joints display a fan-like ribbed texture that forms during the fracturing process
Rare but very significant conical fracture surfaces that occur at meteor impact sites.
|Joints are planar cracks found in almost every rock due to small near-surface stresses in the crust.
An aerial view of jointed sandstone in Southern Utah.
|These joints along the Eau Claire Dells in Marathon County, Wisconsin break the rocks into roughly rectangular blocks|
|Along Baird Creek, Green Bay, Wisconsin. The lower part of the stream bank is scoured smooth by water, but the upper part retreats as blocks break away along joints. Note also the curving trees, evidence of soil creep.|
|The right place at the right time. A light snowfall near Little Sturgeon , Wisconsin, melts where water accumulates along joints, revealing hidden joints in fine detail.|
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Created 22 April, 2005, Last Update