Microcracks and Macrocracks

Photos of fractures from small microfractures to large macrofractures.

Healed microcracks in a quartz grain in granite from the Wolf River Batholith of Wisconsin. Two sets of prominent microcracks are preserved now as trails of liquid filled bubbles and may give clues to the paleostress history of these rocks. In addition a couple of cracks healed with dark iron oxide can also be seen. The horizontal length of the photo is about 5 mm.

Another quartz grain from the granite of the Wolf River Batholith, but stress concentrations along the grain boundary in this grain have created a swarm of cracks radiating out from the triangular shaped feldspar at the top of the photo into the quartz.

A quartz (top yellow) and feldspar (bottom dark and light stripes) in this photo show the microcracks that form along the grain boundaries. The oval spots on the quartz grain are artifacts of the ion milling process used to produce the microcrack sections.

Microcracks in a sample of granite dyed with a fluorescent dye and then illuminated under UV light. The rock which to the eye appears very strong and solid has an extensive network of microcracks, many of them along grain boundaries.

Thin section of a basalt with lath shaped plagioclase feldspar crystals, dark Fe-Ti oxides, and multicolored lumpy pyroxene. A few cracks can be seen cutting across the feldspars.

Scanning electron microscope photo of an open crack in a basalt from Iceland. The oval spots again were produced by ion milling when the sample was being prepared.

A healed crack network on a slab of limy mudstone from the Green River Formation. I am not sure of the mode of formation for these cracks, but they may be formed in a similar fashion to the septarian cracks that are found in some concretions.

 Three massive ash flow tuffs in central Nevada (look carefully in the bottom center of the photo for the group of people as a scale). The middle tuff unit has well developed columnar jointing and a dark vitrophyre at the base of the flow. The top of this middle flow is also less welded and forms a slope covered with sage brush. The lower ash flow (light yellow tan) just above the group of students does not have columnar jointing but it also grades up into a softer, less welded top that forms a slope just below the black stripe of the overlying vitrophyre.

Torres del Paine National Park, Chile

Here are a couple of photos clipped from magazine articles that I used in teaching my geology classes before we had the internet.

Granite spires in Torres del Paine National Park, southern Chile. The photo was likely originally from the National Geographic.

Another shot of the spectacular scenery of Torres del Paine National Park, Chile from the same article. You can more great photos of this spectacular area on the National Geographic website here.

Spring in the Western Utah Desert

Here are a few photos of some of the spectacular rocks found in western Utah. (Remember: you can click on the photos for a full-sized view of each picture)

Notch Peak made mostly of Cambrian limestones is one of the most dramatic peaks in western Utah. Near the base of the mountain, you can see the tan colored sills of the Notch Peak granite that intruded out into these limestones.

Sawtooth Ridge, with Notch Peak at the center, all part of western Utah's House Range.

Ibex, Utah taken from Fossil Mountain. Ibex is at the southern end of the Barn Hills in Blind Valley. The area is world famous for its Ordovician rocks and is the type area for the Ibexian fauna.

Crystal Peak, composed of 35.4 million-year-old Tunnel Spring Tuff, is one of the most distinctive peaks in western Utah.

From a distance, Crystal Peak looks almost unreal. A bright white mass amid the grays of all the other ranges and mountains.

Limestone thumb in Marjum Pass, House Range, Utah.

(All photos were taken by Urthman, and may not be republished without permission)