Natural Science in Archaeology

3.3 Igneous Rocks 49
Paleoindian, Archaic, and Woodland sites in Maine, New Hampshire, Massachusetts,
Vermont, and easternmost Quebec (Pollock et al. 2008).
Trachytes are fine-grained igneous rocks composed largely of alkali feldspar. In
Table 3.1 trachytes would lie between rhyolite and dacite. They are the extrusive
equivalent of syenite. In central Italy trachyte was used for flagstone pavements and
reused in Roman city walls (Renzulli et al. 1999). In Protohistoric times trachyte
from near Padua, Italy, was exploited for the manufacture of grinding tools that
were used in the karst plateau area that extends from Italy to Slovenia and Croatia
(Antonelli et al. 2004).
The name “basalt” is one of the few rock names widely familiar to non-earth scientists.
It is also one of the oldest names. Typical basalt is composed of plagioclase
(Ca-rich), pyroxene, magnetite, and, frequently, olivine [(Mg,Fe) 2 SiO 4 ]. Chemically,
basalt is the equivalent of gabbro. Basalts are generally fine-grained, but some
are medium-grained and some are glassy.
Basalt was the major rock type utilized by the prehistoric peoples of northwestern
Chihuahua, Mexico, for implements including grindstones, mauls, mortars, and
pestles (Gerald 1983). The second most common rock utilized was dacite, especially
for stone axes. Small, smooth dacite pebbles were used to rub the surface
of ceramic vessels to a glossy finish before decoration and firing. The third most
common rock used was felsite.
As in Egypt, the Olmec culture of Mesoamerica carved colossal basalt heads
more than 3 m tall. For a description of some of these monuments, see Stuart (1993).
Basalt was also used widely for stone tools in the Lower Rhine and Meuse valleys,
in the Netherlands and Germany. In Hawaii basalt was the main raw material, and
basalt axe quarries were abundant (McCoy and Gould 1977).
In eastern Sicily, the production of basalt axes was long-lived, beginning in
the Neolithic and reaching a climax in the Copper and Early Bronze ages. The
axes from Serra Orlando, dating from the third millennium BCE to the Hellenistic
Period, were made of basalt, serpentinite, tremolite-bearing rocks, and pyroxenite.
The lithology and sources of these materials are given by Leighton et al. (1989).
Basaltic Roman rotary millstones are commonly found in ancient mines and ironmaking
centers. The millstones from the iron-making site at Les Martys, France,
have been found in slag piles dated to the first century BCE. They are of two types.
One type was made of basalt imported from Agde in southern France, and the other
type was made of a phonolite imported from Orvieto in central Italy (Oliva et al.
1999).
Rapa Nui (Easter Island) is a string of volcanic sea mounts composed of basalt.
The majority of artifacts such as adzes and hand axes found on Rapa Nui are basalt.
The Moai statues were carved from a basaltic tuff. Basalt adzes have been traced to
a Neolithic production site on the Penghu Islands, lying between mainland China
and Taiwan (Rolett et al. 2007).
Andesites consist essentially of plagioclase and pyroxene, but many also contain
biotite and hornblende. Andesites are the most abundant volcanic rocks of island
arcs and active continental margins. For example, around the Pacific Ocean, andesites
predominate in the Andes, Central America, northwestern United States, Japan,