in glaciated regions. Glaciers
are large, flowing bodies of ice that form where snow accumulates
faster than it can melt away. The ice gets thicker and thicker,
until gravity's steady pull on the massive weight of ice causes the ice
to flow like a viscous glob of icing oozing off a cake. The ice
abrades the ground over which it flows, carving deep valleys.
Fjords form when glaciers carve those deep, u-shaped valleys into the
rocky earth near coastlines. After the glaciers melt back a bit,
seawater flows into the valley, submerging it and forming a
steep-walled cove. Kenai Fjords National Park has many fjords
from glaciers that sprout off the Harding Icefield and flow down into
the Gulf of Alaska. The park is a great place to see modern
glacial geology in action! Just look at all of those glaciers
fingering out in all directions from the central ice sheet!
Exit Glacier and
There are two hiking trails in Kenai Fjords National
Park that are readily accessible from the park entrance near
Seward: the Exit Glacier trail and the Harding Icefield
The Exit Glacier loop is a very easy hike along the glacial outwash to
the toe of Exit Glacier. The trail works its way up a short
distance alongside the glacier, as well, where this photo was taken.
(Melania is not actually wearing a
wilderness tent. Dan loaned her his jacket because her luggage
was delayed by the airline.)
Exit Glacier terminates at the edge of the valley.
Melting snow feeds a creek that carries sediment eroded by the glacier
as the ice scrapes along the rock. When the creek opens up into
the the valley, the water slows and deposits the silt, sand, and
cobbles in a broad plain called the outwash plain.
One can approach the toe of the glacier by hopping over the little
rivulets running between the muddy cobbles. We visited very early
in the season, before the winter's thin mantle of snow covering the ice
could melt away, so the blue glacial ice and potentially hazardous
overhangs were not exposed.
The glacier cuts through poorly-sorted marine wackes and
conglomerates. The white and brown spots in the rock pictured
here are pebbles that were embedded in the gray seafloor mud before the
stuff hardened ("lithified") into solid rock.
The white lines cutting across the rock are quartz veins that formed
when the hard rock fractured, then groundwater flowed through the
cracks and deposited the mineral quartz.
The dark streaks on this rock are glacial striations - scratch marks
left by the glacier as the ice flowed over the surface.
There are two types of lines in this rock:
sedimentary bedding and glacial striations. Sedimentary bedding
reflects compositional variations in the layers of sediment as it
deposited on the sea floor. The bands of rock have slightly
different colors because some layers contain a little more quartz silt
Glacial striations, on the other hand, are scratches that cut straight
across the surface
of the rock.
One of these features crosses the photo side-to-side and the other
crossed diagonally. Can you recognize which is which?
This is a close-up view of the quartz veins that cut the
marine sedimentary rocks along the Harding Icefield trail. These
veins have sharply-defined walls, which means they formed when the
siltstone was a hard rock - long after the silt deposited on the ocean
The white quartz that fills the veins is a mineral made of silicon and
). The sand grains in the marine
sedimentary rocks contain a lot of quartz, too. It is likely that
the quartz in the veins formed when deep groundwaters flowed through
cracks, dissolving silica from silt grains in the sedimentary rocks,
then re-depositing the silica when the water cooled in the veins.
If you get a chance to go on this hike, pay attention to the
orientation of the veins in outcrop
(i.e., not in this photo). There is a very systematic pattern
with two dominant orientations. The pairing of those two
particular orientations suggest the fractures/veins formed as conjugate
shears in the same stress field.
go more than a kilometer along the Harding Icefield
trail because it was still snowed in. The trail should be
passable by June, but I wouldn't recommend it in early May.
in our group to discover devil's club. You can see
in this photo that the clubs are just starting to bud. The spines
break off once their in ones skin and are quite fine, making them
tricky to extract. Devil's club grows in large patches and
usually has big leaves, but not this early in the season.
the valley reveals a nice overlook of the toe of
Exit Glacier peeking out of its valley (bottom right), as well as the
lateral moraine sediments that deposited alongside the glacier when it
used to stretch out beyond its current confines. It's sort of
like a bathtub ring visible here about halfway up the picture.
The subhorizontal shelf in this photo is the lateral
moraine. The mountainside below the line is covered with glacial
sediment (the lateral moraine), which is a much better place for plants
to grow than the rocky ledges above that lack soil.
There were some interesting conglomerates along the
The conglomerate on the left consists of large cobbles set in a matrix
of fine mud. What sedimentary environment would drop such large
rocks into such fine mud?
The conglomerate on the right also contains cobbles with mud. The
cobbles in this rock, though, are very long because the source rock
broke in a platy manner - like slate.
conglomerate are big at the bottom, medium-sized in
middle, and fine-grained at the top. Geologists call this "graded
bedding." Graded bedding tells geologists that the sediment
deposited quickly by a big rush of turbid, sediment-filled water in a
submarine landslide. The biggest pebbles drop out of the water
column first, then finer ones later. Graded bedding also tells
geologists the original orientation of rocks when they've been uplifted
and tilted (big pebbles were originally on the bottom.)
far on the Harding Icefield trail, we retreated and
hiked a nice little state park
trail to Tonsina Creek. It was a beautiful day. The beach
was fascinating because the sand was all derived from weathering of
phyllite (a weakly-metamorphosed siltstone that breaks into tiny
chips). The hike was easy and the scenery rewarding.
hiking in Chugach
and knowledgeable. The ranger suggested we hike the Ptarmigan
. She had hiked it the previous day and so knew
that it was dry (i.e., not snow-bound). It was great advice!
Ken and Dan looking over the waterfall
Ptarmigan Lake was still partially frozen over.
What a beautiful day!
Orcas, whales, and pillow
We took one of the boat
one day. The price seemed a little steep when we
signed up, but most things are a little more expensive in Alaska.
We were very
the tour, though, and had no buyers remorse at
all. The people running the boat were very friendly,
professional, and knowledgeable.
We even bought the lunch, which turned out to be a really good deal -
excellent food (prime rib and salmon) and all you can eat.
Seward Harbor - about a block from our hotel.
Considering the size of the town of Seward, everywhere is within a
block or two of the harbor!
sea in Aialik Bay. As the ice pours out of the
mountains into the ocean, big chunks of ice break off and splash into
the water - a process called "calving." The ice is constantly
popping as it bends and cracks.
icy Aialik Bay water. They're much
tougher than I am!
That's salt water, so the temperature is actually below 0ºC (below
deep fractures in the ice that form as the ice
flows unevenly - are clearly visible along the glacier's skyline.
The glacier's face is a vertical wall because the fracture ice calves
easily when the ice meets the water, losing support from the underlying
rock while simultaneously experiencing an upward buoyancy as the ice
floats on the seawater.
(killer whales) swam around the boat for a while.
They are extraordinary creatures!
but the geologic sights were more my style. In
addition to seeing the calving glacier, we also saw two outstanding
outcrops of the Resurrection
Ophiolites are big sections of oceanic crust (lithosphere) that get
pushed up onto the land. They are pretty rare geologic features
and not usually so beautifully exposed. This cliff consists of
millions of lobes of pillow basalt.
The USGS published a geologic
National Forest here
There is a draft
Kenai Fjords National Park here
bulbous basalt pillows. Pillow basalts form
when mafic lava erupts on the sea floor. The water is cold, which
causes the blob of magma to crystallize quickly into a "pillow."
As more lava escapes to the earth's surface, it must flow around the
old solidified pillows, quickly cooling to form new pillows.
shape of the basalt pillows while the birds stand
for scale so you can see how big these pillows are.
tops and bottoms that taper more or less to a
point. This is the normal shape of fresh basalt pillows, so
geologists know that these pillows are in their original orientation
(not tilted or overturned).
ophiolite sequences is made of pillow basalts and
deep water oceanic sediments. The lava needs some way to get from
deep in the earth up to the surface. Underground lava (we call it
"magma" until it reaches the surface) flows up through fractures in the
oceanic crust. A lot of the magma freezes solid in the fractures,
forming tabular bodies of basalt called "dikes." The dikes can be
very closely spaced because, once an old fracture is "healed" with
solidified basalt, the oceanic crust continues to break as it is pulled
apart, forming new fractures that fill with magma, ... that cools to
form new dikes, and the process continues. Imagine breaking a
dish, gooping some superglue into the fracture to mend it, then
breaking the dish again, over and over again many times (as has
happened with my favorite coffee mug). The closely-spaced
swarms of dikes in ophiolites are called "sheeted dikes."
as a scale for these sheeted dikes that were exposed
along the shoreline on our tour. There is a little cross-cutting
evident if you follow individual dikes up from the bottom of the photo
to the top.
A new take on the origins of sheeted dikes is posted in this