Geology Reference
In-Depth Information
Box 7.4
Basalt vesicles and paleoaltitude.
The search is ongoing for reliable
methodologies that are capable of converting
measurable attributes of a pre-Holocene rock
sample to the paleoaltitude at which it was
formed. One promising strategy utilizes
vesicles that are frozen into a lava flow at
the time of its eruption (Sahagian and
Proussevitch, 2007). Basaltic lava, for exam-
ple, emerges from a volcano as a bubbly liq-
uid in which bubbles nucleate and grow in
the magma due to decompression during rise
through the volcanic conduit. These bubbles
are well mixed when the magma emerges
from the vent and becomes a lava flow.
Hence, the bubble population contains equal
amounts of gas throughout the flow. As it is
emplaced, the flow cools from top and
bottom, thereby freezing in vesicles that
surround gassy bubbles, while bubbles rise
buoyantly through the core of still-molten
lava at a speed that depends on their
individual sizes. In the end (see figure A),
this process results in a lower vesicular zone,
a central massive zone (from which all bub-
bles were able to escape a very slowly rising
solidification front), and a highly vesicular
upper vesicular zone, the lower part of which
includes very large bubbles that coalesced
while rising up through the central zone.
Although the bubbles would thus have
identical mass distributions at top and base,
they are subject to different total pressures
due to differences in overburden. At the top
of the flow, only the overlying atmosphere
exerts pressure,
P
atm
, whereas at the base, the
lava itself exerts pressure as a function of
its thickness,
H
. Consequently, two factors
control the size of bubbles at the base of the
flow: atmospheric pressure and lava weight.
Thus, the atmospheric pressure dependence
of vesicle size can be expressed by the ratio
of vesicle size modes at the top and bottom
of a flow:
VP
P
+
r
H
t
=
atm
V
b
atm
where
V
t
and
V
b
are the volumes of the
modal bubble sizes at the quenched top and
bottom of the flow, respectively, and
r
is lava
density (2650 kg/m
3
for basalts). The atmos-
phere's paleopressure can thus be determined,
because all other variables can be measured,
and, based on the known relationship
of pressure as a function of elevation, a
paleoelevation can then be calculated.
In order to make reliable calculations of
atmospheric pressure with this approach,
however, the thickness of the lava flow
measured in the field today must be the
same as its thickness at the time of solidifi-
cation of the upper and lower parts of the
flow, as bubbles were “frozen in.” Even when
the flow top and bottom have been cooled,
the thickness of the still-molten flow interior
can change due to inflation or deflation.
Because evidence for either process can
usually be observed by an astute field geolo-
gist, much effort is commonly expended on
scouting for flows unaffected by inflation or
deflation.
After samples have been collected from
the top and base of flows with reliable
thicknesses, the size distribution of their
Vesicle
Distribution
final vesicularity:
after flow solidifies
initial vesicularity:
gradient due to mass
of overlying flow
Bottom
To p
A
Position in Flow
A. Predicted initial and final vesicle distribution in a
3 m thick flow. Modified after Sahagian
et al.
(2002a).
