Impact Breccias and Terrestrial Look-Alike Rocks

Lunar Impact Breccias

During the impact of a meteorite, old rocks are broken apart and new rocks are formed. Most lunar meteorites, many HED meteorites (howardites, eucrites, and diogenites), and one martian meteorite are breccias – rocks composed of fragments of older rocks that have been broken apart and relithified (“glued” back together), often many times, by impacts of meteorites. Impact brecciation results in rock that consists of clasts (rock fragments) of a range of sizes imbedded in a matrix of smaller clasts and perhaps glass.

Photo of a thin section of Apollo 17 polymict (mixture of many rock types) breccia sample 72275, about 3.8 cm wide. The breccia consists of fragments of a variety older rocks, many of which are smaller breccias, embedded in a matrix of still finer fragments.

In school, we learn that there are three kinds of rocks – sedimentary, metamorphic, and igneous. An impact breccia can be any of the three but the distinction among them is sometimes difficult to determine. Some impact breccias are sedimentary rocks in that they are fine-grained, near-surface material (regolith) that was consolidated (“glued together”) into a coherent rock by the heat and shock pressure of an impact. NWA 12604 (below) is such a rock, a regolith breccia. A few lunar breccias are metamorphic in they they formed from fragmental material, perhaps several kilometers deep in the Moon. A large impact occurred that was not close enough (laterally or vertically) to melt the material but was close enough to “cook” and recrystallize the minerals (thermal metamorphism). NWA 3163 and NWA 8687 (below) are metamorphic breccias, which are usually termed granulitic breccias because of their sugar-crystal like texture. Finally, meteorite impacts will melt target material below the point of impact. Some of the liquid will pool in the crater while the rest is ejected to form an ejecta deposit. The chaotic turbulence during and after the impact causes hot impact melt to mix with cold rock clasts. The clasts cool the melt and the melt heats the clasts sometimes to the point where the exteriors melt. This is the only mechanism of which I’m aware that can lead to rare, rounded clasts in lunar impact breccias. Liquid impact melt that forms in small craters cools quickly to form glassy-matrix breccias. Crystalline-matrix breccias form in large craters and basins where the melt cools slow enough to crystallize. Both kinds of breccias are igneous in that they cooled and solidified from a liquid. Dhofar 1085, Dhofar 1443, and Shişr 166 (below) are clast-laden, impact-melt breccias. Veins of liquid impact melt injected into the space between rock fragments during an impact, which cool quickly to glass, are common in lunar breccias as are vesicles (gas bubbles) that get trapped in impact glass before it solidifies.

Sawn faces of 6 lunar meteorite breccias up close. Dhofar 1085 and Shişr 166 (the only lunar meteorite to have been found at night with a flashlight) are from Oman and rusty red from terrestrial hematite staining. Shişr 166 has some vesicles in the dark grayish veins of impact melt. Several of these vesicles are amygdules – vesicles now filled with whitish secondary (terrestrial) minerals such as zeolites, calcite, quartz, or chalcedony. In Oman, barite and celestine also occur in fractures and amygdules. The pink clast – a spinel grain – on the lower left side of NWA 8687 is the largest reddish thing that I’ve ever seen in a lunar rock. The color of the rock is a bit greenish because it contains 15-25% olivine, a greenish mineral olivine (think olives). In Dhofar 1443, the large, light-colored clast at the top is rounded. Rounded clasts are uncommon in lunar breccias because there is no wind and water to cause abrasion. NWA 8687 and NWA 3163 are granulitic breccias. The processes that lead to this metamorphic texture tends to erase the distinction between clasts and finer-grained matrix. There are many veins from fracturing during impacts while on the Moon in most of these meteorites but notice that none of the are linear. The matrix of Dhofar 1443 and NWA 12604 is dark because it is mostly glassy impact melt. Image credits: Randy Korotev

Nonlunar Meteorite Breccias

NWA 7496 (polymict eucrite). Some clasts are reddish from terrestrial weathering. This rock has a fusion crust. Image credit: Randy Korotev

NWA 6072 (granulitic eucrite). This rock has a fusion crust. Image credit: Randy Korotev

NWA 6073 (howardite). The clasts are eucrites and diogenites. This rock has a fusion crust. Image credit: Randy Korotev

NWA 6074 (polymict diogenite). Some areas are reddish from terrestrial weathering and there are fractures full of terrestrial secondary minerals. This rock has a fusion crust. Image credit: Randy Korotev

NWA 7475, one of the several NWA 7034 pairs. At this time (June, 2021), this meteorite is the only brecciated meteorite from Mars. It is also the only meteorite of which I am aware that has large “round things” in it. Some of the clasts have rims. Image credit: Luc Labenne

Impact Breccias Are Fractal

Impact breccias are fractal objects – they look the same regardless of the scale that you look at them. The clasts have a large range in size. On the left, below, is a photograph of a sawn face of lunar meteorite NWA 5000, about 12 cm high and 8 cm wide. In the middle is an enlarged image of the portion within the yellow rectangle on the left. Similarly, on the right is an enlargement of the area within the yellow rectangle of the middle. This fractal characteristic is important because in some terrestrial sedimentary rocks clasts sizes have been sorted.

Terrestrial Rocks That Are Not Impact Breccias

Breccias occur on Earth, too, and some rare ones were produced by the impact of meteorites. Most terrestrial breccias, as well as rocks that are not breccias but look like breccias, were produced by other processes such as faulting, sedimentation, and volcanism. All of the photos below were sent to me by people who wanted to know if the rocks were meteorites. None of them have self-evident fusion crusts, so I think, but do not know for certain, that all are terrestrial rocks. Several are pyroclastic (volcaniclastic) rocks or porphyritic basalts. I have been sent photos of breccia-like “rocks” that I am rather certain were just broken pieces of concrete. 

On the Moon, rocks are not colorful; They are never dark red. Nearly all brecciated lunar rocks are polymict breccias – breccias that are mixtures (-mict) of many different kinds of rocks (poly-). In two of these rocks, the clasts all seem to be of the same kind of rock, i.e., the rocks are not polymict. Clasts in meteorites rarely have rims. Quartz does not occur as clasts in meteorites. Some kinds of minerals that crystalize from terrestrial magmas have geometric shapes with flat sides. Clasts of rock fragments in impact breccias do not have geometric shapes. Large geometric-shaped mineral grains are known as phenocrysts, not clasts, because the rock is igneous (crystallized from a magma), not a breccia. Fracturing of rock by meteorite impacts only rarely leads to fragments with aspect ratios (length/width) greater than 3/1. If it has layers, other stratification, or preferred orientation of the clasts, it is not a meteorite. The annotations in the photos below indicate why I suspect that the rocks are not meteorites.

Preferred Orientation

Because of the Moon’s low gravity, clasts in lunar breccias do not display preferred orientationThis is a terrestrial sedimentary rock showing preferred orientation of the clasts. All of the elongated clasts are oriented in the same direction because they settled from water with the elongated axis horizontal.

I have only seen 2 lunar meteorite breccias with even a hint of preferred orientation of the clasts, and both may be artifacts of sawing or some optical illusion. If it is real, it likely results from flow of clasts in an ejecta deposit. Image credits: David Gregory and Mirko Graul.

Other Sedimentary and Volcanic Rocks

Pebble Conglomerates

One type of sedimentary rock that is mistaken for meteorites are pebble conglomerates. Such rocks usually start as deposits of rounded pebbles and fine grained silt and clay.

Terlingua Creek in Big Bend National Park. All the pebbles have washed downstream along with fine-grained sediment following rain storms. The pebbles are rounded from abrasion against other pebbles. Most pebble conglomerates start in this way. If this sediment layer were to be buried for “a long time,” the sediment might lithify into a solid rock – a pebble conglomerate. Walking stick for scale. Image credit: Randy Korotev

Here’s a Story

Below are photos of a rock that an experienced and successful meteorite hunter asked me to analyze in 2006 because he thought that it was a lunar meteorite. He found it in Oman, where he has found other lunar meteorites. After sawing it in two (right), it looked pretty good then to me, too. It certainly looks like a breccia. Examining the photo now, however, I see my error. Most of the light-colored clasts appear to be the same rock type (= not polymict).  The composition was definitely terrestrial and, of course, the rock has no fusion crust (which is not unusual for lunar meteorites from Oman). The rock is sedimentary, with high, compared to the Moon, concentrations of chalcophile elements.

Here’s a Better Story

In 2014 geology professor Nigel Brush of Ashland University in Ashland Ohio sent me a photo of a rock that he had found in a local creek bed while on a field trip. He wanted to know if the rock (below) could be a lunar breccia.

I thought the rock looked enough like a lunar breccia to pursue it, so we had a petrographic thin section made.

Left. Photomicrograph of a thin section of Prof. Brush’s breccia from Ashland, Ohio. Right. Photomicrograph of a thin section of Dr. Wittmann’s breccia from Sudbury, Ontario.

Ashland, Ohio, is ~640 km south of the Sudbury impact structure in Ontario. Image credit: Google Earth

At the time this occurred, we happened to have one of the world’s experts on the terrestrial impact craters, Dr. Axel Wittmann, working with us. He took one short look at the thin section on the upper left, said “wait a minute,” walked to his office, and came back with the thin section on the right. He had collected the rock on a field trip to the Sudbury impact structure (euphemism for a highly deformed and eroded impact crater) in Ontario, Canada. The impact occurred 1.85 billion years ago. The rock was from the Onaping formation, a deposit of ejecta from the crater. The Onaping breccia has a distinct appearance and almost certainly Prof. Brush’s breccia also originates from the Onaping formation, 640 km north of Ashland. The rock was transported south to Ohio by glaciers, likely during the period of Pleistocene glaciation. Probably most of the rocks in the creek bed where Prof. Brush found the breccia were emplaced in the area during the Pleistocene glaciation. Glacial rocks are typically rounded from abrasion against other rocks carried south by the glaciers.

See also: Lunar Regolith Breccias and Fragmental Breccias