Crystal Cove Columnar Joints EarthCache

Columnar jointing is a geological structure where sets of intersecting closely spaced fractures, referred to as joints, result in the formation of a regular array of polygonal prisms, or columns. Columnar jointing occurs in many types of volcanic rocks and forms as the rock cools and contracts. Columnar jointing can occur in cooling lava flows and ashflow tuffs (ignimbrites), as well as in some shallow intrusions.

The columns can vary from 3 meters to a few centimeters in diameter, and can be as much as 30 meters tall. They are typically parallel and straight, but can also be curved and vary in diameter. Whereas an array of regular and straight columns is called a colonnade, an irregular array is termed an entablature. The number of sides of the individual columns can vary from 3 to 8, with 6 sides being the most common.

At the southern-most point of Crystal Cove you will find cliffs of andesite (a fine-grained igneous rock which cooled from lava at or near Earth’s surface). Geologists term this kind of rock volcanic as opposed to coarse-grained plutonic rock which cooled deep in Earth’s crust. Here the lava cooled quickly and produced well-defined prismatic columns. Two world famous locations visited by tourists for these columnar joints are Devils Post Pile near Mammoth Mountain, California, and the Giant’s Causeway in Northern Ireland.

Some would say these rocks here are Basalt, others diabase, still others would say gabbro. The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt. Rocks with a phaneritic (coarser) groundmass are generally referred to as diabase (also called dolerite). Rcoks with more coarse-grained (crystals over 2 mm across) are referred to as gabbro.

When the lava cools, it contracts. This is because hot things generally take up more space than cool things. Think about hot steam, for instance. When you open the lid of a simmering pot or a tea kettle, that hot steam wants to escape and expand into the air. Or think about those balloons from your last birthday celebration. Have you ever notice how balloons tend to droop overnight? Partly, that may be because the helium in the balloons is escaping, but it’s also often because the gas inside the balloons cools down and contracts with the cooler nighttime temperatures. Sometimes, if you prop those drooping birthday balloons in the sun the next morning, they’ll pop back up again as the gas inside them warms up and expands.

When objects contract, they often crack or fracture. When contraction occurs at centers which are equally spaced (see the above diagram), then a hexagonal fracture pattern will develop. If the contraction is not evenly spaced, then other geometries of fractures, such as 5-sided or 7-sided fractures, may occur. Contraction may not be equally spaced if, for example, the thickness or composition of the lava flow varies. The fracture pattern that forms at the cooling surface will tend to be propagated down the lava as it cools, forming long, geometric columns. Thus, as lava cools, it may crack in a hexagonal (or other) shape and form columns. These columns form in a variety of sizes– some are fairly small, and some are wider and much taller than people!

http://www.everytrail.com/guide/crystal-cove-state-park-beach-geology

https://en.wikipedia.org/wiki/Columnar_jointing

http://blogs.agu.org/georneys/2012/11/18/geology-word-of-the-week-c-is-for-columnar-jointing//

http://maps.unomaha.edu/maher/geo330/julia1.html

https://www.saddleback.edu/faculty/mhill/Geo3/Powerpoints/Peninsular%20Ranges.pdf/

https://en.wikipedia.org/wiki/Basalt/