21. Dikes Dan Barker March, 2009 Dikes, Big Bend National Park

Dikes are the simplest of intrusive igneous bodies. They are planar, with two roughly parallel large cooling surfaces. Dikes are efficient conduits of magma toward the uppermost crust, and serve as feeders for other igneous bodies, intrusive and extrusive. When they are complicated, it is because more than one magma batch follows the same upward path. Many dikes take advantage of older fractures in their movement, and they pre-heat the wall rocks. As a result, a later magma batch follows the same fracture, especially if the interior of the first dike is still hot and weak. Composite dikes, containing more than one rock type, are the result. Dikes need not travel with a purely vertical flow; many show signs of nearly horizontal intrusion.

This basaltic dike, cutting a Pleistocene alluvial fan deposit, baked and reddened its walls. Panamint Springs, CA. J Conel for scale.

This small granite dike near Augusta ME forced its walls apart, as indicated by some matching irregularities on both sides and by the rotated splinter of metamorphic rock bridging the dike.

This dike cut through Ordovician limestone (left) at Cote de Neiges, Montreal, Que. Heat from the magma recrystallized a thin margin of limestone into white marble. Beside the pen is a fragment of limestone removed from the wall to our left.

The dark dike, in the same roadcut as the previous image, nearly succeeded in wedging out a block of limestone.

Another dike, in the Christmas Mountains, Texas, has partly peeled off some Cretaceous limestone (arrow).

This dike, also in the Christmas Mountains, took an irregular path through the limestone.

Cerro Castollon, Big Bend National Park, is capped by a rhyolite lava dome that was fed by a dike (arrow).

Dike (white arrow) feeding small lava flow (red arrow). S coast of Pantelleria.

This dike fed a small lava dome. S coast, Pantelleria

Dike cutting hyaloclastite, Ryugazaki, Hokkaido. Note the "fingers".

Feeder dike and overlying spatter cone, sliced by canyon wall, Hafragilsfoss, Iceland.

Feeder dike for the 1886 Tarawera eruption, North Island, NZ

A composite dike with at least three rock types (the red left and right edges may be parts of one dike), cutting the Needles Range pyroclastic flow deposit, UT.

Composite dike, Moeraki Point, South Island, NZ. Successively younger units are nearer the center of the dike.

Composite dike, Paisano Pass, W of Alpine TX. Note the bilateral symmetry.

Some dikes "stand proud" of their surroundings. Here rhyolite dikes cut the soft tuffs and sandstone of the Chisos Formation, Big Bend National Park.

A "proud" dike, Cat's Nick, Salisbury Crags, Edinburgh

Other dikes are "recessive", weathering and eroding more rapidly than their wall rock. Central Tugtutoq Island, SW Greenland.

Recessive dikes (arrows) E Tugtutoq, SW Greenland

Dike gridlock, Mont Royal, Quebec

Rhyolite dikes in schist, Homestake gold mine, Lead, SD. The dikes are thought to be the sources for the gold mineralization.

The following examples show that some dikes are shaped like knife blades, pinch out at their top and bottom edges, and are intruded with a largely horizontal component of motion. In addition, seismic signals from dikes splitting their way through shallow rocks in Iceland and Hawaii show that magma can travel horizontally at meters per second through fractures of its own making.

The dike in this scoria cone at Steffelnkopf, Germany, terminates above and below.

This dike also pinches out at the top and (in rubble) the bottom. Crater Elegante, Pinacate volcanic field, Sonora, Mexico.

This dike ends downward in a "pigtail curl" to our left. Hamblin-Cleopatra volcano, NV.

The fissure/dike from which the great 1783 Laki eruption, Iceland, issued ends abruptly on this lower slope. Chemical evidence shows that the magma came from a volcanic center 35 km to the NE.

Flow lines on the contact surface of this Paisano Pass, TX, dike show horizontal and vertical components.

A closeup of the previous dike surface.

Dike swarms, sets of parallel dikes or dikes radiating from a single point, are common. Spanish Peaks, CO.

Andesite dike with thin black glassy margins, Kasube Cape, Hokkaido

Closeup of margin of dike, Kasube Cape. The margin is very similar to those on pillow lavas. dike hyaloclastite

Thin dike in marble, Ledmore quarry, Loch Borralan, Scotland

Finally, just to show that not all dikes are small, here is a map showing a swarm of giant dikes, SW Greenland. Note scale. (after BGJ Upton)

Most of the large dikes on Tugtutoq erode more rapidly than the surrounding granite, making linear valleys.

This entire valley, 1 km wide, is floored in a nepheline syenite dike. Central Tugtutoq.

Koolau dike swarm, Pali Highway, Oahu. GPL Walker for scale.