Black dikes
Even the most casual visitor to the Teton Range notices the remarkable black band that extends down the east face of Mount Moran (figs. [27] and [28]) from the summit and disappears into the trees north of Leigh Lake. This is the outcropping edge of a steeply inclined dike composed of diabase, a nearly black igneous rock very similar to basalt. Thinner diabase dikes are visible on the east face of Middle Teton, on the south side of the Grand Teton, and in several other places in the range (see [geologic map] inside back cover).
Figure 25. Garnet crystal in pegmatite. The crystal is about 6 inches in diameter. Other minerals are feldspar (white) and clusters of white mica flakes. The mica crystals appear dark in the photograph because they are wet.
The diabase is a heavy dark-greenish-gray to black rock that turns rust brown on faces that have been exposed to the weather. It is studded with small lath-shaped crystals of feldspar that are greenish gray in the fresh rock and milk white on weathered surfaces.
The black dikes formed from molten rock that welled up into nearly vertical fissures in the older Precambrian rocks. Toward the edges of the dikes the feldspar laths in the diabase become smaller and smaller ([fig. 29]), indicating that the wall rocks were relatively cool when the magma or melted rock was intruded. Rapid chilling at the edges prevented growth of large crystals. In many places hot solutions from the dike permeated the wall rocks, staining them rosy red.
Figure 26. A small dike of pegmatite and granite cutting through folded layered gneiss in Death Canyon. Coarse-grained pegmatite forms most of the dike, but fine-grained granite is found near the center. Small offshoots of the dike penetrate into the wall rocks. The dike cuts straight across folds in the enclosing gneisses and must therefore have been intruded after development of the folds. The white ruler is about 6 inches long.
The black dike on Mount Moran is about 150 feet thick near the summit of the peak. This dike has been traced westward for more than 7 miles. Where it passes out of the park south of Green Lakes Mountain it is 100 feet thick. The amount of molten material needed to form the exposed segment of this single dike could fill Jenny Lake three times over. The other dikes are thinner and not as long: the dike on Middle Teton is 20 to 40 feet thick, and the dike on Grand Teton is 40 to 60 feet thick.
Figure 27. Air oblique view of the east face of Mt. Moran, showing the great black dike. Main mass of the mountain is layered gneiss and streaky granite gneiss. White lines are dikes of granite and pegmatite; light-gray mound on the summit is about 50 feet of Cambrian sedimentary rock (Flathead Sandstone). Notice that the black dike cuts across the dikes of granite and pegmatite but that its upper edge is covered by the much younger layer of sandstone. Falling Ice Glacier is in the left center; Skillet Glacier is in the lower right center. Photo by A. S. Post. University of Washington, August 19, 1963.
Figure 28. The great black dike on the east face of Mt. Moran. The dike is about 150 feet thick and its vertical extent in the picture is about 3,000 feet. The fractures in the dike perpendicular to its walls are cracks formed as the liquified rock cooled and crystallized. Falling Ice Glacier is in the center. National Park Service photo by H. D. Pownall.
Figure 29. Closeup view of the edge of the Middle Teton black dike exposed on the north wall of Garnet Canyon near the west end of the trail. Dike rock (diabase) is on the right; wall rock (gneiss) is on the left. Match shows scale.
The black dikes must be the youngest of the Precambrian units because they cut across all other Precambrian rocks. The dikes must have been intruded before the beginning of Cambrian deposition inasmuch as they do not cut the oldest Cambrian beds. Gneiss adjacent to the dike on Mount Moran contains biotite that was heated and altered about 1.3 billion years ago according to Professor Giletti. The alteration is believed to have occurred when the dike was emplaced; therefore this and similar dikes elsewhere in the range are probably about 1.3 billion years old.