Tephrochronology

 

     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

before settling.

     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

processes.

     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 Pompeii. Tephra ties the various deposit types

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

characteristics.