Multimedia Mania/abacus-multimedia-mania.iso/dp/0099/00999.txt

Text File | 1993-07-27 | 21KB | 351 lines

$Unique_ID{bob00999}
$Pretitle{}
$Title{Glacier Bay
Galloping, Calving, Advancing, Retreating}
$Subtitle{}
$Author{Kirk, Ruth}
$Affiliation{National Park Service;U.S. Department Of The Interior}
$Subject{ice
glacier
glaciers
water
hopkins
johns
inlet
bergs
advancing
feet
see
pictures
see
figures
}
$Date{1983}
$Log{See Fairweather Range*0099901.scf
}
Title:       Glacier Bay
Book:        Part II: Of Time And Ice
Author:      Kirk, Ruth
Affiliation: National Park Service;U.S. Department Of The Interior
Date:        1983

Galloping, Calving, Advancing, Retreating

Johns Hopkins

     All morning we have been charting in upper Johns Hopkins Inlet.  The high
peaks of the Fairweather Range thrust like white fangs above us.  Beside us
rise gray, bare, abrupt rock walls.  We arrived here aboard Growler about 2100
last evening.  Sunlight still flooded the upper walls but the water already
stood in twilight, lending an eerie quality to this cathedral-like fjord.
Eager to see whether the Tyeen Glacier had surged forward since last summer,
we barely noticed, however.  Austin, Dave, Emily, Charles, and I all crowded
into the wheelhouse, with last year's aerial photograph on the chart table for
comparison with what we hoped to see ahead, a glacier that galloped.  Alas, no
drama greeted us.  The ice still hung near the top of the cliff, poised to
surge, perhaps, but far from having done so.

[See Fairweather Range: Climbers find the Fairweather Range, with its quixotic
and severe weather, misnamed. This immense land seems to triple in size
immediately when you get in a tight spot.]

     Two hundred galloping glaciers are known in Alaska and northwestern
Canada, some occasionally surging several kilometers in a single year.  These
extraordinary advances occur only on certain glaciers.  No glaciers overlying
granitic bedrock are given to surging.  Many that do surge are associated with
geologic faults, but not all.  Water beneath the ice has been advanced as an
explanation for galloping glaciers, but this can't be the whole answer.

     A mountain glacier is usually rushing if it moves a meter or two (4 to 7
feet) a day.  Deformation permits the ice to bend and slide around obstacles,
and the enormous pressure against any such protrusion produces enough heat to
melt a fraction of the glacier's undersurface.  Lubricated by this minute film
of meltwater, the ice jerks forward.  That relieves the pressure and the
melt-film refreezes.  The process starts anew.

     I once watched this happen where University of Washington researchers had
dug a 25-meter (85-foot) tunnel to bedrock beneath the Blue Glacier in
Washington's Olympic Mountains.  Gauges imbedded in the tunnel walls measured
the pressure the ice exerted against irregularities in its bed and the rate of
its jerky flow over and around them.  Dials dispassionately registered what
was happening, but you could see it without them.  A knob of bedrock might
have ice pressed against it.  Then a momentary wetness would darken the rock
and an additional fraction of the knob would be engulfed.  The process was
silent and, but for the glaciologists' lights, would haven taken place in
utter blackness.

     Water beneath ice may not fully explain why some glaciers gallop, but
meltwater - with land runoff - surely affects the rate at which Glacier Bay's
glaciers perform their greatest scenic wonder: calving icebergs off their
tidewater snouts.

     The water works down through the ice and momentarily lifts the glacier
off bedrock during brief periods of exceptional hydrostatic pressure.  The
lifting weakens the ice and accelerates collapse.  Some feel that low tide may
also step up calving.  Ice in contact with saltwater melts more rapidly than
ice exposed only to air, producing undercutting - and reduced support - at the
high-tide line.  Others find this erosive undercutting inconsequential to
calving.  Geological Survey monitoring establishes no relation between tide
and calving rate.

     Icebergs themselves are far from uniform.  Those that look white hold
myriad trapped air bubbles.  Blue means denser ice.  Greenish-black ice is
from the bottom, or sole, of a glacier and such bergs may also be grooved
where bedrock knobs have gouged the glacier.  Morainal rubble stripes some
icebergs with brown, or totally darkens them.  Rocks ride atop bergs and plop
into the water from their sides.

     Stranding icebergs leave tracks as they half float, half drag along the
beach.  And they grind, squash, and rip seaweeds and mussels pioneering rocky
shores.  Floating bergs offer perches favored by bald eagles, cormorants, and
gulls.  For eagles the bergs seem to serve as movable vantage points for
spotting opportunities to prey or scavenge.  Cormorants often hold out their
wings to dry while they ride.  Most gulls just rest.  Kittiwakes - gulls that
come ashore only to nest - briefly ride Glacier Bay icebergs during their
August transition from nesting colonies to life at sea.  Guillemots and
puffins never ride the bergs, perhaps because of difficulties landing on ice.
Their legs, set far back and fine for swimming, are awkward out of water. Land
birds, except for eagles, generally ignore icebergs.

     As you kayak among bergs, paddling silently, you hear melt take its toll.
Water drops and cascades.  Air bubbles pop and ice cracks constantly as it
adjusts to changing pressures and temperatures.  Even with your eyes closed,
you can tell icebergs are close.  How high bergs float depends on their size
and ice density and on the density of the water.  Where runoff or rainwater
floats atop saltwater, bergs sink lower than if freshwater is absent.  The
burden of rock and sediment in the ice sometimes weighs a small berg below the
surface.  A faint shadowy presence is all that gives it away.

     Huge bergs, recognizable by distinctive shape or patterning, may last a
week or more, though they split or turn over as reshaping melt affects
balance.  What had seemed a modest floating crag may, when rolling over,
suddenly loom as an enormous hazard if you've paddled too near.

     Studying a beached iceberg reveals its fabric and susceptibility to melt.
Ice crystals that measure a centimeter (0.4 inches) or more across interlock
as in a three-dimensional puzzle.  Along such interfaces sun warmth and
saltwater attack.  Grasp a projection and wiggle it.  You will hear a
squeaking as the crystals rub one another along these junctions.

     Last evening Dave stood near Growler's bow as we approached the upper end
of Johns Hopkins Inlet.  Net in hand, he scooped up icebergs for the
refrigerator.  We had run close to the Johns Hopkins and Gilman glacier faces
to take bottom readings.  For these, Austin used Bergy-bit, the little
radio-controlled boat which amounts to a sleek hull fitted with a tight lid.
Only its three-horsepower electric motor projects vulnerably.  We placed one
of Growler's depth sounders inside Bergy-bit.

     Mid channel approaching the Johns Hopkins snout, Growler consistently
recorded a water depth of 400 meters (1,300 feet) and a flat bottom, the sort
of uniform contour expected of fine-grained sediments deposited in deep water.
The water is so deep that there is no anchorage in this inlet.  The bottom
lies far beyond an anchor's reach even along the sidewalls.  To our surprise,
however, about one kilometer (1.5 miles) from the glacier face we measured
water "only" 150 meters (500 feet) deep.  The glacier is pushing a steep-sided
submarine plug far out ahead of its front.  Austin has found nothing like this
elsewhere.

     We sent Bergy-bit along the east side of the Johns Hopkins ice front, and
the entire front of the Gilman Glacier, and then, barely before midnight, quit
for dinner.  For the past two hours I had supposed we would stop, so I kept
spinach noodles hot on the stove, and they turned into a startling green goo.
Rather than admit culinary defeat I topped the mass with Parmesan cheese and
croutons and baked it.  Camaraderie and hunger sufficed to prompt praise for
my baked goo.  By the time we finished dinner it was technically already
morning.

     We drifted all night.  With the water too deep for anchorage, we had to
depend on pack ice to hold us safely away from the fjord wall.  We took turns
standing watch, a long boat hook in hand for pushing off bergs that might
cause trouble.  At one point Emily roused Austin to start the engine and work
free of encircling ice that brought with it an iceberg towering higher than
Growlers rail.

     Mostly it was a night of ethereal peace.  There was no moon but the
floating ice reflected enough light so that on watch you could make out
closeby bergs and the seals circling us like dark phantoms.  Occasionally a
seal would signal the sudden end of its curiosity and slap the water with its
hind flippers, then dive.  Otherwise, the only sounds were a faint roar from
distant waterfalls, the sporadic grinding of ice against Growler's hull, and
once the splash of an iceberg rolling over.

     This morning we resumed readings with Bergy-bit.  I sit out of the way
atop the wheelhouse while Dave controls the skiff with the radio transmitter
and Emily watches with binoculars, telling him which way to turn so as to
steer Bergy through leads in the ice pack.  Falling ice strikes the little
boat with a loud clonk and for a while Berg vanishes from sight amid a welter
of falling and surging bergs.  Then we see the dot of its brilliant red hull
and know it has survived.  Bottom readings are clear.  They show a depth of
350 meters (1,100 feet) close to the west side of the Johns Hopkins ice front.
We have just charted an underwater canyon.

     The Johns Hopkins Glacier started advancing more than 50 years ago. South
of here the Brady Glacier extends a full 70 kilometers (43 miles) through the
Fairweather Range to Taylor Bay.  Indeed, the Reid Glacier and the Lamplugh,
near the mouth of this inlet, are lobes of the Brady.  It is an ice mass today
choking a fjord, much as ice a few centuries ago sealed the Glacier Bay fjord,
forcing out the Tchukanedi Tlingits and denying entrance to Captain Vancouver.
Why the asynchrony?  Why, of the national park's 17 current tidewater
glaciers, are six advancing, three retreating, and eight holding their own?

Photo Station 3

     We have rowed ashore on the west side of Johns Hopkins Inlet to
photograph the glaciers from a position first used decades ago by Dr. William
O. Field, of the American Geographical Society.  This station is simply a
rounded, glacier-polished outcrop of white rock partly veneered by a mat of
dryas runners rooted nearby.  A low stone cairn holds a jar with a registry of
those who have made official photographs here.  It requests anyone who takes
unofficial pictures to send copies to the Society to enhance the record.
There are only four entries, beginning with 1958.  The position is stunning.
We see the Johns Hopkins and Gilman Glaciers clearly and half a dozen high
peaks, including Mount Crillon, almost 4,000 meters (13,000 feet) high.

     I talked with Dr. Field a few years ago in New York City.  White haired,
the epitome of a gentleman-scholar, he is dean of those who have studied
Alaskan glaciers.  From memory he recited which glaciers were advancing, which
retreating, and in what years.  As a young geographer he had pondered the
small amount of ice left in the United States compared to its dominant role in
shaping the land.  That's when I got hooked," he told me.

     In 1926 on his first trip to Glacier Bay he noticed immense changes in
the ice positions documented by pioneering glaciologists beginning in the late
1800's.  Harry Reid, for example, had written about "changes expected in the
next 50 years."  Where Reid's map showed solid ice, Field watched whales and
seals.  The ice was gone.

     "You need continuity in a record," he told me.  "Otherwise there's no way
to see what's happening.  The Johns Hopkins Glacier, for example, has advanced
a mile since I first saw it in 1926 and it's still coming.  Small glaciers
show change more quickly than vast icefields can.  Greater accumulation than
normal, or more melting, and they respond almost right away.  Yet glaciers
aren't simply barometers of climate.  There's more to it, especially with
tidewater glaciers."

     The lack of glacier documentation had launched Field's career.  Getting
data takes remarkable persistence, partly because of the mammoth compilation
needed and partly because of isolated and difficult working conditions.  "You
need triangulation to keep track of what an ice front is doing, but
maintaining usable triangulation points gets tough at times," Dr. Field
reminisced.  "You may go back and find a station worthless because alder has
grown so much you can't see out, let alone do any surveying or even take a
picture.

     "Or if the ice is advancing, you have to move the station out of its way.
If it's receding, you still have to move so as to stay close enough to do any
good.  In the 1940's we watched the Grand Pacific Glacier advance from Canada
back into the U.S.  We'd set up a station and it'd be obliterated before we
could get back on another trip.  Access was a problem, too, even if the
station was still there.  We had a real battle getting to the photo point
between the Margerie and the Grand Pacific.  The beach we needed to land on
often was completely blocked by floating icebergs.  And the calving of new
ones set up shock waves that kept us alert the times we did go ashore."

     Field said that tidewater glaciers "confuse the whole picture" in
measuring past climates.  As an oversimplification, assume the steady
nourishing of a glacier by yearly snowfall.  Once equilibrium is reached, this
ice should neither thicken nor thin, advance nor retreat.  Given present
climate, this fairly well describes most ice tongues in Glacier Bay National
Park and Preserve except for those that reach saltwater.  These cause the
confusion, but research aboard Growler has contributed to understanding them.
Receding tidewater glaciers reach into deep water.  Advancing or stable
tongues end either on marine shoals or where the heads of inlets rise above
sea level.

     If deep water spells retreat, what's the depth where tidal glaciers are
advancing?  Shallow.  Usually less than 80 meters (260 feet).

     Why?  The glaciers themselves make it so.  They advance only if they've
built a protective shoal at the snout, by dumping rock debris.  This forms an
underwater terminal moraine and provides a partial barrier between the ice and
the erosive action of sea water.  By plucking material from the up-slope of
this ridge and redepositing it on the down-slope, a glacier can keep advancing
along even a very deep waterway.

     How fast?  Perhaps one to three kilometers (0.5 to 2 miles) per century.
Eventually the ice may become so extended that the amount lost from the
surface melt and calving matches the snowfall feeding the upper glacier.  At
this stage, balance is so precarious that even a slight retreat causes the
snout to back off its shoal and re-enter deep water.  Irreversible retreat
then continues until the glacier reaches shallow water, usually at the head of
tidewater.  There it stabilizes, at least until it builds enough shoal to
begin anew, slow advance.

     Sometimes I resent the name Johns Hopkins for this inlet.  It comes from
an early-day university expedition here.  It struck me as audacious to make an
institutional trophy of such scenic magnificence.  Bob Howe, park
superintendent when I first visited here, clamped a moratorium on further
naming of peaks, valleys, waterfalls - or anything.  He felt there should be
places where humans experience the pristine without presuming to label.  The
gift shop manager of a cruise ship told me she put up a closed sign during her
first trip into Johns Hopkins Inlet.  "Come to the upper deck if you need
film," her note read.  "The shop will reopen after we leave Johns Hopkins."
It's that beautiful.

Reid Inlet

     We anchored Growler about 0200 this morning.  We'd eaten another midnight
dinner after finishing the Johns Hopkins depth readings and hiking across the
Topeka Glacier outwash, looking for fossil wood.  We debated whether to stay
in Johns Hopkins or run to Reid Inlet.  Austin decided to run because we might
be too tired to stand effective watch through the night. There was too little
pack ice in Johns Hopkins Inlet to hold Growler safely free of the sidewalls
as we drifted.  Two other vessels also were running, their distant lights
ghostly companions for the late, weary hour.  One must have been Explorer, the
park concession boat that drifts in the pack ice off the Margerie Glacier
through half the night, giving passengers a unique experience of the upper
bay.  The other probably was a commercial fishing boat.

     Harry Reid's 1890 map of this inlet now bearing his name shows nothing
but ice here.  No land at all.  Even in the 1940's, when Joe and Muz Ibach
built a distinctive little cabin and began mining pockets of gold ore high on
the cliffs, the Reid Glacier had drawn back no farther than the toe of their
beach.  Now you can boat 6 kilometers (4 miles) into the inlet.

     After breakfast this morning we motored Growler's dory across from our
anchorage, following as close as is prudent to the bulging ice face.  "The
glacier must be advancing," Austin said.  "Look at the push moraines."  He
pointed out low ridges of rock and gravel slightly ahead of where ice is
pressing against the inlet's sidewall.  Circular mats of dryas are half
swallowed by the advance.  Sheer crevasses split the ice where its leading
edge has thrust across the land.  They form 50-meter (164-foot) slits clearly
visible against the sky.

     Aboard Growler I have been seeing advancing or stable glaciers, yet other
glaciers in the park are rapidly withdrawing.  Muir Glacier has gone back 40
kilometers (25 miles) since 1890 when Reid mapped its terminus barely above
the inlet's junction with Glacier Bay.  In the years my husband, Louis, and I
have been coming to the park we have seen the Muir front separate from the
Riggs Glacier and retreat far up the inlet.  Austin says it has only a few
kilometers to go to reach the head of tidewater.

     Elevation explains why some glaciers advance here while others withdraw.
Tarr, Johns Hopkins, and Reid Inlets all finger from exceedingly high peaks.
Plateaus feeding their ice typically stand 2,000 meters (6,500 feet) high and
are subject to prodigious snowfall.  The park's retreating glaciers, on the
other hand, derive from elevations averaging about half that high.  The
uplands near Glacier Bay's mouth, where ice is gone, rise little more than 350
meters (1,100 feet) overall.  This difference in park elevations separates
northwestern advancing ice from eastern receding ice.  And the Brady
Icefield's immensity seems to influence its own weather.  The icefield chills
moisture-laden clouds from the Pacific and triggers their glacier-nourishing
release.

     Surprisingly small temperature differences account for radically varying
glacial effects.  The Wisconsinan Ice Age was only 5 to 6 degrees Celsius
cooler than today.  The following warm period averaged perhaps one degree
warmer than today.  During the Little Ice Age here, the elevation above which
more snow fell in winter than melted in summer was about 830 meters (2,700
feet).  Dr. Field places this point today at 1,600 meters (5,200 feet) -
except for the Brady Icefield where it's half that.

     No wonder the only glaciers here likely to advance now are those with
their heads high in the mountains.  The dice are hopelessly loaded against the
others, aside from the peculiarities of tidewater ice.  Viewed on a time scale
of millennia, all glaciers are responding to climate.  They are asynchronous
only in terms of centuries and decades, time scales more comprehensible
because they better match our lifespan.  What we view as significant events
may be minute fluctuations on the millennial scale, which is, for glaciers,
the more true scale.