A tephrochronology is a series of dated layers of ash, or tephra left in
sedimentary deposits and representing volcanic events. Tephrochronologists
determine the tephra layers' physical, chemical, spatial, and temporal
characteristics. With this information, we can reconstruct regional volcanic
activity and apply tephrostratigraphic markers to other temporal studies.
Explosive eruptions occur when the temperature of blocked lava rises to the
point at which the lava vaporization pressure is sufficient to remove the block.
Clouds of vaporized lava are then released which rise in the cooler air until
temperatures drop enough for coalescence into glass particles. These glass
shards are carried by the prevailing winds according to their mass/size and
aerodynamic shape. Volcanic ash often covers hundreds of square miles,
sometimes thousands, and if the cloud rises high enough, it can circle the globe
When the ash settles, it is subject to local environmental perturbation.
Ground winds and ocean currents direct some into various catchments.
Rains wash it off hillsides and floods clear out valleys. Landslides, glacial
motion, and tectonics can place it back into erosive conditions long after
settling and burial. Ice breakup and floating on rivers and oceans can carry it
great distances. Plant and animal burrowing stirs it up some.
With perturbation taken into consideration, certain types of sedimentary
deposits provide the best, though by no means perfect, records of volcanic
activity. These include peat deposits, ice buildups, lake sediments (particularly
kettle lakes), sea floors, bog sediments, and windborn loess deposits.
Cores through these deposits show the dominant types of sedimentary
activity occuring in a timely sequence and provide clues to many natural
Tephra layers range in thickness from meters to micrometers. They are
found in ice, snow, peat and other organic matrices, and many types of
mineralogical settings. Though some are easily seen, most are not,
particularly the extremely small and unconcentrated cryptotephra. Techniques
used to locate them rely on their insolubility in acid, inflammability, opacity
to x-rays, and susceptibility to magnetism. Density and magnetism is used
to separate the glass shards from other rock and minerals.
Microscopy is used to determine shard size, color, shape, and texture.
Polished thin sections provide refractive index, mineralogy, and hydration
since formation. Mass spectrometry by electron and ion microprobe of
individual shards can show major oxide and trace element concentrations.
Standards, such as Lipari obsidian, and interlab comparisons are used to
control variation in lab technique and instrumentation.
The data is sent to a database, such as Tephrabase, where algorithms
provide possible matches. When matches are verified, another spatial
dimension allows terrain mapping across land, sea, lake, and ice deposits.
Fission track or 40Ar/39Ar analysis of the tephra, or radiocarbon analysis of
carbon held in the layers directly above or below the tephra, adds a temporal
dimension to this map.
Durability, preservative properties, wide areal distribution, and datability
ensure tephra studies an eminent role in reconstructing the past. Barring
long-lasting abrasive conditions, tephra shards survive millions of years
with little alteration. By preventing oxidation, thick layers of tephra assist in
the preservation of fossils. Blankets of tephra also preserve historical
disaster scenes, as at
together, and helps calibrate other dating mechanisms.
Tephrochronology is strongly involved in paleoclimate reconstruction,
particularly in the global climate change occuring at the end of the last
ice age. Climate affects erosion, which affects sedimentation rate, both of
which are strongly affected by the addition of millions of tons of volcanic
glass onto the landscape. Wind direction and velocity are indicated by
the tephra spread. Paleotemperatures at the time of ash deposition are
assumed from the life forms above and below the ash. Massive injections
of tephra into the atmosphere have a definite impact on global climate,
and transitional ice age surface shifting of water weight may affect
volcanic frequency and magnitude.
Paleoenvironmental reconstruction is assisted by tephrochronological
markers delineating ecosystem change and preserving the life forms for
study. Local volcanic impacts on the environment include the release of
toxic gases, heat and tephra blanket thickness, possible associated
tsunamis and earthquakes, and enhanced erosion with removal of plant
cover. Impacts on past human activity include forced migrations and
land use changes, while current impacts require civil defense planning
and aircraft rerouting.
Geomorphological rate change is measured with tephrochronological
input along with landscape reconstruction. Erosion and accumulation,
river valley changes, and glacial scouring are monitored, as are block
and plate motion.
Vulcanology is strongly tied to tephrochronology. Volcanic zones,
arcs, traps, systems, and fields display growth, endurance, subsidence,
activity, density, and age. Individual volcanoes exhibit eruption types,
frequency, magnitude, age, thermal energy, explosivity, and magma