Transcript Document 7329950

HAWAIIAN LAVA FLOWS
Scott K. Rowland, University of Hawai‘i at Manoa
Lava flows: ‘a‘a and pahoehoe
From Hawaiian Dictionary by M.K. Puku‘i & S.H. Elbert
- 1. To burn, blaze, glow; fire; staring, as eyes. Fig., angry,
‘a‘a:
fury. Ua ‘a‘a- ‘ia au i ke aloha (I burn with love). 2. stony,
abounding with ‘a‘a- lava.
pahoehoe:
1. Smooth, unbroken type of lava. 2. Satin.
3. to drive fish into a net by beating the paddles
rhythmically against the canoe.
Lava flows: ‘a‘a- and pahoehoe
These two types of flow are identical chemically, a single
eruption will often produce both and commonly even a
single lava flow has parts that are ‘a‘a- and parts that are
pahoehoe.
Their differences are due to different emplacement
processes (flow rates, cooling rates, etc.).
- - - to gush, ooze forth
Pahuhu:
Ho‘opohaku: to harden, as lava
(Courtesy of Ululani Makue)
‘a‘a-high volumetric flow rate
(10-103 m3 s-1)
-high flow-front velocity
(102-104 m hr-1)
-develops large channels
-few, large flow units
-thick (1-10 m) flow units
-higher viscosity
-higher yield strength
-slightly cooler
-“apaluhraun” (rough lava)
in Icelandic
pahoehoe
-low volumetric flow rate
(<10 m3 s-1)
-low flow-front velocity
(1-10 m hr-1)
-develops lava tubes
-innumerable flow units
-thin (10-100 cm) flow units
-lower viscosity
-lower yield strength
-slightly hotter
-“helluhraun” (pavement lava)
in Icelandic
‘a‘a- and pahoehoe
flows on the north flank of Mauna Loa
~3 m
Wave-cut cliff section, Makapu‘u, O‘ahu
For every lava rheology, there is a critical disruption rate
beyond which it cannot flow as a fluid
slow-moving
lava in a
tube
slow-moving
lava that
encounters
a steep slope
Fast-moving
lava in an open
channel
Qualitative graph after Peterson & Tilling (1980) and Kilburn (1981)
- flows
‘a‘a
- flows
Proximal-type ‘a‘a
-relatively thin
-vesicular interior
-thin top and bottom
clinker layers
When moving, the incandescent interior is deforms fluidly
~1 m
The top surface consists of relatively small, spiny clinkers
Distal-type ‘a‘a- flows
standard geologists
for scale
Distal-type ‘a‘a- flows:
-dense, angular, large surface
blocks
-incandescent lava has a high
yield strength
-advance is by avalanching of
loose blocks and fine material
George Walker, walking around on the surface of the (active)
1984 Mauna Loa distal-type ‘a‘a- lava flow
- A small, partially-crusted channel on the flank of Pu‘u ‘O‘o,
feeding an ‘a‘a- flow front
~5 m
A big, fast-moving channel (1984 Mauna Loa eruption)
~10 m
Channels develop downflow, from distinct near the vent to
indistinct (or non-existent) at the flow front.
flow
front
Lipman & Banks (1987)
A velocity gradient sets up relative rotation and shear,
tearing viscous lumps of surface and near-surface
lava into ragged clinkers.
Air photo of the distal end of the 1942 Mauna Loa flow
Transition of distinct channel to zone of dispersed flow
Three relatively thin ‘a‘a- flows in cross section
clinker
flow cores
Accretionary lava balls
~1.5 m diameter
pahoehoe
flows
Flow field of pahoehoe
“toes”
(individual flow units)
~0.5 m
Old pahoehoe
flows in cliff section, Kaua‘i
~2 m
pahoehoe
issuing from a crack in a previous toe
~40 cm
active pahoehoe
toe
~20 cm
Inflation of a pahoehoe
flows
Inflated flows exposed at the coastline of Pinta, Galápagos
pelican for scale
Shelly pahoehoe:
formed by coalescence and expansion of
gas bubbles
Shelly pahoehoe
is a treacherous near-vent facies
Folding of the plastic surface crust: ropy pahoehoe
Multiple generations of ropes
1
2
3
4
Side view into an active lava tube
a ~1 m x ~2 m skylight in the roof of an active tube
Master lava tubes can be
many meters high and wide.
Note that during a tube-fed
eruption, the main tube is
essentially as long as the flow,
(the distance from vent to flowfront), but afterwards it
almost never drains completely.
Like a large channel, a large tube develops downflow
(same time, different
locations along the flow)
One mechanism of tube formation is coalescence of many
small, single flow-unit tubes (i.e., toes) into a master tube.
(same location, different times)
Another tube-formation mechanism is roofing of a lava channel.
Here, the surface skin is growing in a down-flow direction.
~1 m
Here, a surface skin growing inward from the channel margins
~1 m
A partially drained lava tube
-showing evidence of
thermal erosion of the
substrate
Sources of blue fume mark
the line of an active tube
Tumulus on 1859 Mauna Loa flow, on the coastal plain
George Walker for scale
Tumulus on an old Mauna Loa SW rift zone flow,
Pu‘u Hou (1868 littoral cone) in background
A leaky tumulus
~2 m
Tumuli are everywhere on tube-fed pahoehoe
flows
Comparison of s-type and p-type pahoehoe
Spongy pahoehoe
Pipe-vesicle-bearing
pahoehoe
Stretched vesicle walls on the skin of s-type pahoehoe
~10 cm
The interior of an s-type (“spongy”) pahoehoe
flow
fresh p-type (p) and s-type (s) pahoehoe
s
p
p
p
s
s
An ochre-colored secondary mineral (some sort of clay?)
commonly forms on non-vesicular glassy basalt surfaces
(“blue-glassy” pahoehoe
doesn’t stay blue for ever)
Water inhibits formation of
this secondary coating:
-it does not form near waterretaining fractures
-it does not form where rain
drips from plants
-shading of sunlight is also
involved
Pipe vesicles near the base of a p-type pahoehoe
flow unit
Lava trees (~2 m high)
A forest of lava trees, partially buried by scoria
Toothpaste lava: the rheology of ‘a‘a- at the volumetric flowrate of pahoehoe
‘Olelo no‘eau involving lava (from M. K. Pukui; compiled by Ululani Makue)
O ka la- ko luna, o ka pahoehoe
ko lalo. The sun above, the
smooth lava below. Said of a journey in which the traveler
suffers the heat of the sun above and the reflected heat
from the lava below (a difficult trip).
- uwe- ka mamane.
- - ke ‘ala,
Kike
When the boulders clash, the
mamane
tree weeps. Meaning that when two people fight,
those that are dear to them often weep.
PAU