Obsidian
From Wikipedia, the free encyclopedia
Obsidian is a naturally occurring
volcanic glass formed as an
extrusive igneous rock.
It is produced when
felsic lava extruded from a
volcano cools rapidly with minimum crystal growth. Obsidian is commonly found within the margins of
rhyolitic lava flows known as
obsidian flows, where the chemical composition (high
silica
content) induces a high viscosity and polymerization degree of the
lava. The inhibition of atomic diffusion through this highly viscous and
polymerized lava explains the lack of crystal growth. Obsidian is hard
and brittle; it therefore fractures with very sharp edges, which had
been used in the past in cutting and piercing tools, and has been used
experimentally as surgical
scalpel blades.
[4]
Origin and properties
Obsidian talus at Obsidian Dome, California
... among the various forms of glass we may reckon Obsian glass, a
substance very similar to the stone found by Obsius in Ethiopia.[5]
The translation into English of
Natural History written by the elder
Pliny
of Rome shows a few sentences on the subject of a volcanic glass called
Obsian, so named from its resemblance to a stone found in Ethiopia by
Obsius (
obsiānus lapis).
[6][7][8]
Obsidian is the rock formed as a result of cooled
lava, which is the parent material.
[9][10][11] Having a low water content when newly formed typically less than 1% water by weight,
[12] becomes progressively hydrated when exposed to groundwater, forming
perlite.
Tektites were once thought by many to be obsidian produced by
lunar volcanic eruptions, though few scientists now adhere to this hypothesis.
Obsidian is
mineral-like, but not a true mineral because as a glass it is not
crystalline; in addition, its composition is too complex to comprise a single mineral. It is sometimes classified as a
mineraloid. Though obsidian is usually dark in color similar to
mafic rocks such as
basalt, obsidian's composition is extremely
felsic. Obsidian consists mainly of SiO
2 (
silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include
granite and
rhyolite. Because obsidian is
metastable
at the Earth's surface (over time the glass becomes fine-grained
mineral crystals), no obsidian has been found that is older than
Cretaceous age. This breakdown of obsidian is accelerated by the presence of water.
Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities.
Iron and
magnesium typically give the obsidian a dark brown to black color. Very few samples are nearly colorless. In some stones, the
inclusion of small, white, radially clustered crystals of
cristobalite in the black glass produce a blotchy or snowflake pattern (
snowflake obsidian).
It may contain patterns of gas bubbles remaining from the lava flow,
aligned along layers created as the molten rock was flowing before being
cooled. These bubbles can produce interesting effects such as a golden
sheen (
sheen obsidian). An
iridescent, rainbow-like sheen (
rainbow obsidian) is caused by inclusions of
magnetite nanoparticles (Nadin, 2007).
[13]
Occurrence
Obsidian can be found in locations which have experienced rhyolitic eruptions. It can be found in
Argentina,
Armenia,
Azerbaijan,
Canada,
Chile,
Georgia,
Greece,
El Salvador,
Guatemala,
Iceland,
Italy,
Japan,
Kenya,
Mexico,
New Zealand,
Peru,
Scotland,
Turkey and the
United States. Obsidian flows which may be
hiked on are found within the calderas of
Newberry Volcano and
Medicine Lake Volcano in the
Cascade Range of western North America, and at
Inyo Craters east of the Sierra Nevada in
California.
Yellowstone National Park has a mountainside containing obsidian located between
Mammoth Hot Springs and the
Norris Geyser Basin, and deposits can be found in many other western U.S. states including
Arizona,
Colorado,
New Mexico,
Texas,
Utah,
Washington,
[14] Oregon[15] and
Idaho. Obsidian can also be found in the eastern U.S. states of
Virginia, as well as
Pennsylvania and North Carolina.
There are only four major deposit areas in the central Mediterranean:
Lipari,
Pantelleria,
Palmarola and
Monte Arci.
[16]
Ancient sources in the Aegean were
Melos and
Giali.
[17]
Acigöl town and the
Göllü Dağ volcano were the most important sources in central Anatolia, one of the more important source areas in prehistoric Near East.
[18][19][20]
Historical use
The first archaeological evidence known of usage were made from within
Kariandusi and other sites of the
Acheulian
age (beginning 1.5 million years previously) dated 700,000 BC, although
the number of objects found at these sites were very low relative to
the Neolithic.
[21][22][23][24][25]
Use of obsidian in pottery of the Neolithic in the area around Lipari
was found to be significantly less at a distance representing two weeks
journeying.
[17]
Anatolian sources of obsidian are known to have been the material used
in the Levant and modern-day Iraqi Kurdistan from a time beginning
sometime about 12,500 BC.
[26] The first attested civilized use is from excavations at
Tell Brak dated the late fifth millennia.
[27] Obsidian was valued in
Stone Age cultures because, like
flint,
it could be fractured to produce sharp blades or arrowheads. Like all
glass and some other types of naturally occurring rocks, obsidian breaks
with a characteristic
conchoidal fracture. It was also polished to create early
mirrors. Modern
archaeologists have developed a
relative dating system,
obsidian hydration dating, to calculate the age of obsidian artifacts.
Middle East
In
Ubaid in the
5th millennium BC, blades were manufactured from obsidian mined in what is now
Turkey.
[28] Ancient Egyptians used obsidian imported from the eastern Mediterranean and southern
Red Sea regions. Obsidian was also used in ritual circumcisions because of its deftness and sharpness.
[29] In the east of the Mediterranean the material was used to make tools, mirrors and decorative objects.
[30]
Americas
Lithic analysis can be instrumental in understanding prehispanic
groups in Mesoamerica. A careful analysis of obsidian in a culture or
place can be of considerable use to reconstruct commerce, production,
distribution and thereby understand economic, social and political
aspects of a civilization. This is the case in
Yaxchilán, a Maya city where even warfare implications have been studied linked with obsidian use and its debris.
[31] Another example is the
archeological recovery at coastal
Chumash sites in California indicating considerable trade with the distant site of
Casa Diablo, California in the
Sierra Nevada Mountains.
[32]
Pre-Columbian Mesoamericans'
use of obsidian was extensive and sophisticated; including carved and worked obsidian for
tools and decorative objects. Mesoamericans also made a type of
sword with obsidian blades mounted in a wooden body. Called a
macuahuitl,
the weapon was capable of inflicting terrible injuries, combining the
sharp cutting edge of an obsidian blade with the ragged cut of a
serrated weapon.
Raw obsidian and obsidian blades from the Mayan site of Takalik Abaj
Native American people traded obsidian throughout the Americas. Each
volcano
and in some cases each volcanic eruption produces a distinguishable
type of obsidian, making it possible for archaeologists to trace the
origins of a particular artifact. Similar tracing techniques have
allowed obsidian to be identified in Greece also as coming from
Melos,
Nisyros or
Yiali, islands in the
Aegean Sea. Obsidian cores and blades were traded great distances inland from the coast.
[citation needed]
In Chile obsidian tools from
Chaitén Volcano have been found as far away as in
Chan-Chan 400 km north of the volcano and also in sites 400 km south of it.
[33][34]
Easter Island
Obsidian was also used on Rapa Nui (
Easter Island) for edged tools such as
Mataia and the pupils of the eyes of their
Moai (statues).
Current use
Though not approved by the
US Food and Drug Administration (FDA) for use on humans, obsidian is used by some surgeons for
scalpel
blades, as well-crafted obsidian blades have a cutting edge many times
sharper than high-quality steel surgical scalpels, the cutting edge of
the blade being only about 3
nanometers thick.
[35] Even the sharpest metal knife has a jagged, irregular blade when viewed under a strong enough
microscope; when examined even under an
electron microscope an obsidian blade is still smooth and even.
[36] One study found that obsidian incisions produced fewer inflammatory cells and less
granulation tissue at 7 days, in a group of rats.
[37] Don Crabtree produced obsidian blades for
surgery and other purposes,
[35] and has written articles on the subject. Obsidian scalpels may currently be purchased for surgical use on research animals.
[38]
Pig carved in snowflake obsidian, 10 centimeters (4 in) long. The markings are
spherulites.
Obsidian is also used for ornamental purposes and as a
gemstone.
It possesses the property of presenting a different appearance
according to the manner in which it is cut: when cut in one direction it
is jet black; in another it is glistening gray. "
Apache tears" are small rounded obsidian nuggets embedded within a grayish-white
perlite matrix.
Plinths for
audio turntables have been made of obsidian since the 1970s; e.g. the grayish-black SH-10B3 plinth by Technics.
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