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

The axis of one of the Wai‘anaeVolcano rift zones is exposed inthe cliff at Kāneana.

Farrington Hwy.

Makua cave

Dike rock is usually more resistant to erosion than the lava flows thatthe 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 30-40 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 columnarjointing, and because dikesare essentially vertical(and therefore have ~verticalcooling surfaces), theircolumnar joints are ~horizontal.

It is these pre-broken fragments of dense dike rockthat were often made intoadzes 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, thecooling 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 themud dries, its clay minerals contract, eventually pulling away from oneanother 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 areexposed in a gully. The surface is not horizontal because the flow itselfwas emplaced on a slope; the columns are almost always perpendicularto the cooling surface.

Kepuni Gulch, East Maui

Giant’s Causeway, Ireland

http://www.gaschurman.com/world/europe/northern%20ireland/giant%20causeway/slides/giant%20causeway%2013.jpg

Devil’s tower (Wyoming), a remnant of athick lava flow or dome, is probably the mostfamous example of columnar jointed rockin the USA.

Photos by Steve Mattox, from: http://volcano.und.edu/vwdocs/volc_images/north_america/devils_tower.html