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Lecture 16

Anthropology 2229F/G Lecture Notes - Lecture 16: Seafloor Spreading, Volcanic Glass, Magnetite


Department
Anthropology
Course Code
ANTH 2229F/G
Professor
Christopher Ellis
Lecture
16

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Lecture 16 Archaeomagnetism, Obsidian Hydration, Amino Acid
Racemization
Archaeomagnetism
o Done through paleoorientation
o Magnetic field of the Earth has varied over time
Location of magnetic north is not always the same
E.g. magnetite in Atlantic seafloor spreading
o Strength/intensity of magnetism has also varied over time
o Clay or clay soils are heated to certain temperatures
Naturally contain iron oxides
Acquire a weak permanent magnetism when heated
Iron aligns to where magnetic north is at the time, reflects the
direction/location of magnetic north
Also reflects the intensity
Point downwards, determined by strength
o At any one point of magnetic north and intensity of the magnetic field
reflects a specific/unique point in time
o Best in locations like the southwest where hearths are often lined
with clay, clay pottery, ovens, kilns, etc.
Will preserve magnetism
o Do have to know over time where magnetic north was; need a
master chronology to compare it to
Similar to dendrochronology
o Can get a very accurate date, within 10-15 years
Obsidian Hydration
o Obsidian volcanic glass
Formed when lava cools very quickly, minerals are quenched
and crystals don’t have time to form/grow
Creates very sharp edges; good for cutting/scraping tools, etc.
o Hydration rind/layers form on the surface from the reaction with the
obsidian with the air
Rind measured in microns
Long it is exposed to moisture in the air, the thicker the rind
will be
Called the hydration layer; water penetrates/diffuses into the
surface
Can provide a date when it was flaked
Assumes that rind develops at a consistent rate over time
Example
o Hydration rate is 4 microns per 1000 years
o Layer is 2 microns thick, therefore is 500 years
old
Thickness of layer is directly linked to time
Now know it is more complex than that
Thickness also depends on other variables:
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