Dikes cross-cutting lava flows, near Kalalau, Kaua‘i

The axis of one of the Wai‘anae Volcano rift zones is exposed in the cliff at Kāneana. Farrington Hwy. Makua cave

Dike rock is usually more resistant to erosion than the lava flows that the dikes are cutting through. They end up standing above the more- eroded flows to form narrow blade-like ridges. Dikes exposed in the eroding wall of Haleakalā crater, E. Maui

DIKE DIMENSIONS 10s of km Dike Length: the distance from the magma chamber to the eruption site (can be km, or more) Dike Height: the distance from the deepest to the shallowest rock- breaking earthquakes during a dike-propagation event (usually 1-3 km) 1-3 km Dike Width: measured in old, eroded volcanoes (usually ~1 m) ~1 m

Dikes often show columnar jointing, and because dikes are essentially vertical (and therefore have ~vertical cooling surfaces), their columnar joints are ~horizontal. It is these pre-broken fragments of dense dike rock that were often made into adzes by Polynesians. Dikes in Waihānau valley, E. Moloka‘i

A dike cutting diagonally across lava flows on Kaua‘i Notice that even though the dike margins are at a strange angle, the cooling joints are still perpendicular to the margins

Hundreds of dikes exposed in Kapa‘a Quarry, O‘ahu

Dikes cutting through weathered flows, H-3 roadcut, Kāne‘ohe

Mud cracks illustrate, in 2-d, the process of volume contraction. As the mud dries, its clay minerals contract, eventually pulling away from one another when the contraction stress exceeds the mud strength.

In a lava flow, extend the mud crack process into the third dimension, and you have columnar joints

Here, on the S. flank of E. Maui volcano, the tops of the columns are exposed in a gully. The surface is not horizontal because the flow itself was emplaced on a slope; the columns are almost always perpendicular to the cooling surface.

Kepuni Gulch, East Maui

Giant’s Causeway, Ireland

Devil’s tower (Wyoming), a remnant of a thick lava flow or dome, is probably the most famous example of columnar jointed rock in the USA. Photos by Steve Mattox, from: