IMPACTITES
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
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
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.