Lightweight impact rocks having a vesicular to frothy texture (= having holes that were originally gas bubbles) are called impactites. The solid portions of such rocks are glassy or microcrystalline. They may or may not have macroscopic angular debris mixed in.
Monturaqui Impactite (2.7 cm across) from northern Chile. The Monturaqui Impactite represents lithified debris & frothy glass that fell back to Earth after an impact at Monturaqui Crater. This relatively small crater is located in the Atacama Desert of northern Chile, South America. The exact date of impact is undetermined, but available information indicates it was less than 1 million years ago. The impacting body is thought to be a iron meteorite, but no preserved fragments have ever been identified. However, some samples of Monturaqui Impactite are slightly magnetic, apparently due to incorporated (but now altered) meteoritic material.
Lonar Impactite (1.7 cm across) from Maharashtra State, India. This scoria-looking rock is the result of a Late Pleistocene impact in Maharashtra State, India. The impact occurred at about 52 ky. The Lonar Crater is ~1.1 miles in diameter and is filled with water (Lonar Lake). The target bedrock consists of Cretaceous-Tertiary aged flood basalts - the famous Deccan Traps. The texture and chemistry of Lonar Impactite samples show that the original solid basalt has been converted to a frothy, glassy-like material. If I didn't know where this came from, I'd swear it was scoria from a cinder cone.
IMPACTITES IN ARGENTINA
The late Cenozoic-aged Pampeano Formation of Argentina has been found to host several (up to 8) impactite horizons. Specimens from two of these horizons are shown below. The impactites are glassy, weathered, and highly vesiculated.
Chapadmalal Impactite (6.8 cm across), derived from coastal-eastern Buenos Aires Province in eastern Argentina. It's from the lower Upper Pliocene and dates to 3.3 million years. This correlates with an extinction event in South America (Schultz et al., 1998, 2002).
Chasic— Impactite (4.8 cm across), derived from southwestern Buenos Aires Province, eastern Argentina. Based on local drainage patterns, the impact site appears to be a buried crater 15 kilometers in diameter (Schultz et al., 2002).
Info. synthesized from:
Schultz et al. (1998) - A 3.3-Ma impact in Argentina and possible consequences. Science 282(5396): 2061-2063.
Schultz et al. (2002) - Argentine impact record. Geological Society of America Abstracts with Programs 34(6): 401.
Harris & Schultz (2007) - Preservation of floral and faunal remains in impact melts. Geological Society of America Abstracts with Programs 39(6): 126.