Come to this joint often?

No, we aren't talking about funny cigarettes or seedy bars. Geologic joints are the most common of brittle fracture systems in rocks. They're produced by a variety of causes: shrinkage due to loss of water, cooling of magma, relaxation features resulting from the removal of stress (we could all use a little of that!) or tectonic stress (pushing or pulling on brittle rocks).

Columnar joints of the Table Rocks are shrinkage features that form vertical pillars when lava cools and contracts. Dewatering when mud dries after a rainstorm produces polygonal shrinkage cracks.

Relaxation joints form when pressure releases like a couch cushion rising once a person stands. Rocks, being less elastic than that comfortable cushion, tend to crack perpendicular to the direction of pressure release. A river carving a valley releases pressure on the valley's sides, forming joints roughly parallel to the valley's slope.

When miners drill tunnels through rock, the release of pressure might result in "rock bursts" into the tunnel, a common danger when mining deep into the earth's crust.

Tectonic pressures where Earth's crust is compressed, as along Oregon's coastal plate margin, or is extended, as near Klamath Falls, result in joint formation. Old rocks such as Mount Ashland's can show more than one joint generation. Cooling of the granite produced three joint sets at roughly 90-degree angles. Tectonic compression of crustal material about 150 million years ago produced additional sets.

Joints serve as conduits for percolation of groundwater, preferentially control chemical weathering and erosion, and can influence the direction of stream flow. Joint sets in hard, brittle rock of the Klamath Mountains localized injection of precious metal-bearing veins later exploited by rock rats (miners). Highly jointed rocks in lava are responsible for the groundwater aquifers of Big Butte Springs, Medford's major water supply. Study of joint sets in the landscape can assist in locating groundwater wells.

A topographic map of our area will show straight stretches or abrupt changes in directions of streams; many result from erosion and water flow along joints. Indeed, where would we be without joints?

Joints are found wherever rocks are exposed, as in the sandstone along I-5 near the south Medford interchange. Joints perpendicular to layering in sandstone of Monument Valley, Utah, and subsequent collapse along them create those bold vertical cliffs.

Using your powers of observation, find joints in rock outcrops, noting the enhanced weathering (discoloration and crumbling) and perhaps water seepage. Think of how, in their own modest but inevitable way, they've contributed to forming the landscape around us.

Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at

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