Jökull - 01.01.2012, Page 45

The use of tephrochronology in geomorphology
Figure 3. The surface covered by the pre-historic tephra
layer Bj defines an isochron of the same age as the overly-
ing tephra (location shown in Figure 1). The silicic tephra
Ey H is also an isochronous primary tephra deposit, but in
this case the surface in contact with the base of the layer is
not all of the same age. The dotted lines highlight places
where the surface is most probably the same as the tephra.
In the centre of the profile, however, the layer has been dis-
torted by post-depositional frost hummock formation and
to the left of centre the layer has been over folded. Thus,
the hummock form post-dates the deposition of the tephra
and the surface defined by the tephra as a whole is not
isochronous. The early 5th century SILK-YN tephra has
also been distorted after deposition to form a vertical finger
of sediment in line with the hummock peak, and so in this
case too the surface defined by the tephra is not the same
age as the tephra. – Yfirborð jarðvegsins sem forsögulega
gjóskulagið Bj féll á er jafntímaflötur af sama aldri og
gjóskulagið. Yfirborðið sem súra, ljósleita gjóskulagið Ey
H féll á er hins vegar ekki allt af sama aldri þar sem gjósku-
lagið hefur aflagast vegna frostverkunar eftir að það féll.
Þúfan sem frostlyftingin myndaði er yngri en gjóskulagið
og flöturinn undir gjóskunni er ekki alls staðar af sama
aldri. Punktalínurnar sýna staði þar sem yfirborð jarðveg-
sins er af sama aldri og gjóskan. Gjóskulagið SILK-YN frá
5. öld aflagaðist einnig eftir að það féll en bútur úr því er
lóðréttur innan í þúfunni.
Other examples of post depositional morpholog-
ical modification of tephra layers occur in glaciers
where ice flow distorts and relocates tephra layers,
changing their geometry as well as changing their lo-
cation, but leaving their stratigraphical setting unal-
tered. Tephra layers currently outcropping around the
margins of Vatnajökull were originally deposited in
different locations above the snowline (Larsen et al.,
1998). They have been moved many kilometres by ice
flow and have been subject to complex sequences of
morphological modifications. The stratigraphical re-
lationships of the tephra and the frozen water exposed
at the time of tephra deposition does, however, remain
the same, even though the tephra fell on snow and that
snow has been transformed to glacier ice, which has
flowed from the accumulation areas of the glacier to
its ablation zone.
Tephra melting from a glacier may create a sec-
ondary isochron, one unrelated to the age of the tephra
itself. For example, in the North Atlantic during
the last glacial-interglacial transition the distribution
of Icelandic tephra along the southern margin of the
melting sea ice created an extensive ocean-floor strati-
graphic marker horizon, but one with a tenuous re-
lationship to the age of the eruption that created the
tephra (e.g. Ruddiman and McIntyre, 1973; Brend-
ryen et al., 2011).
A second and even more extensive secondary
dispersal of tephra has been achieved by ocean-
transported pumice of Icelandic origin during the
Holocene (e.g. Binns, 1972; Newton, 1999). These
pumice forming eruptions created comparatively lit-
tle atmospheric fallout (Larsen et al., 2001), but large
volumes of cobble-grade pumice. This was deliv-
JÖKULL No. 62, 2012 43