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.