Glossary of Geologic Terms

For further details, and for
introductory texts in volcanology,
see the additional sources following
the endnotes,

airfall: refers lo the origin of a volcanic
rock or deposit by being erupted into
the air, drifting downwind (such as an
ash cloud), and eventually ialling to
earth. Pyroclastic deposits are formed
mainly of either airfall ash ar scoria,
or by deposition from pryroclastic
flows.

andesite: magma having between about
55 percent and 63 percent by weight
SiOz (silicon dioxide); also refers to
lava flows, scoria, ash, etc., which
are the rocks and deposits formed of
andesitic magma. Andesitic lava
flows are typically medium to dark
grey or brown, reflecting intermediate
contents of iron and magnesium, and
have a viscosity intermediate
between that of basalt and rhyolite.
See Fig. BA.

ash: a volcanic particle the size of a sand
grain, that is, about 0.0025 to 0.16
inches (0.06 to 4 mm) diameter,
which consists of solidified magma.
Ash is commonly produced during
‘explosive eruptions, hence the term
“ash cloud.” Its name derives from
mistaken reference to the ash that
accompanies wood fires.

basalt; magma having between about 47
percent and 51 percent by weight
SiO» (silicon dioxide); also refers to.
lava flows, scoria, etc., which are the
rocks and deposits formed of basaltic
magma. Basaltic lava flows are
typically dark gray due to their high
contents of iron and magnesium. The-
viscosity of basaltic lavas is low due
to the low silica content, thus,
basaltic lavas flow readily and spread
laterally, See Fig, 8B.

basaltic andesite: magma having
between about 51 percent and 35
percent by weight SiO) (silicon
dioxide), that is, inte
between basalt and

breccia: a rock formed of a deposit of
angular fragments of older rocks and
mineral grains. Volcanic breccias
may be formed of airiall deposits of
pumice and ash or af mudflow
deposits (sec Fig. 12), or may form
bencath and adjacent to lava flows
that have shed broken fragments of
lava (see Fig, 5A).

cone: a volcano having the shape of a
cone; composite cones are
constructed of mixed lava flows and
ash, whereas scoria cones are:
constructed mainly of scoria and ash.

crust: with reference to the Earth, the

outermost 5 to 50 miles (8 to 80 km)
of rock, which differs in composition
and physical properties from the
underlying mantle. Crust beneath the
continents differs from that beneath
the oceans, but the larger difference
is between the crust and mantle.

dacite: magma having between about 63
percent and 70 percent by weight
SiOz (silicon dioxide); also refers to
lava flows, pumice, ctc., which are
the deposits formed of dacitic
magma. Dacitic pumice or lava is
typically medium to light gray or
brown due to its low contents of iron
and magni where the iron has
oxidized during or shortly after
eruption, the color can be pale
orange-yellow to dark red (see Fig,
138).

dike: a mass of lava having the shape of
a slab, formed when magma fills a
vertical crack in older rock.

dome: a mass of lava of which the
thickness (height) is about the same
as the horizontal dimensions. A dome

ion marks the site of a
volcanic vent, because domes form
where the lava is too viscous to flow
or is prevented by the surrounding
rocks from flowing horizontally away
from the vent. See Figs. 8A and 9.

fault: a break in rocks along which there
has been relative movement of the
rocks. The mavement can be purely
lateral (horizontal), in which case the
fault is called a transform; other types
of faults involve some vertical
movement

Kruzof Island pluton: granitic rocks on
Kruzof Island and adjacent islands,
which intruded the Sitka Graywacke
between about 60 and 30 million
years ago; the rocks are white to pale
grey with specks of dark minerals (sec
Fig. 14) and are easily distinguished
from the drab grey Sitka Graywacke
or from lavas of the Mount
Fdgecumbe volcanic fiold; lavas and
ash of the valcanic field are much
younger than the pluton and the
greywacke and bury these rocks over
most of southern Kruzof Island;
houlders of pluton and of graywacke
along the southern and eastern shores
of Kruzot Island were deposited an
the lava flows by southflowing
glaciers between about 14,000 and
25,000 years ago.

lava: magma that has erupted at or upon
the ground surface; also, a rock
formed af cooled, solidified magma:

magma: melted rock before it has
erupted; magma typically contains
dissolved gases (waler, sulfur
compounds, carbon dioxide, etc.) but
they are not formally considered
magma.

‘mantle: that part of the tarth between
the outer crust and the core. The top
of the mantle is typically buried by 5
to 10 miles (8 to 16 km) of oceanic
crust and by as much as 30 to 40
miles (48 to 64 km) of crust in the
continents. The crust, mantle, and
core differ significantly in chemical

ion; the mantle, for

low in silica and high in

iron and magnesium relative to the

‘crust. As a consequence of such

differences, the mantle typically
produces basalt magma if partly
melted whereas the crust produces
andesite, dacite, or rhyolite.

plate: in a geologic context, a piece of
the Earth’s crust and upper mantle
that moves as a single fragment
relative ta adjacent pieces. The outer
shell of the Earth is now believed to
consist of seven large plates and
another half-dozen smaller plates.
Plates form at mid-ocean ridges by
eruption of basalt, which adds new
material to the plates on either side of
the ridge. Plates accomodate to the
addition of material by moving
slowly away from the ridge. Besides
oceanic ridges, plates can be
bounded by transform faults, where
two adjacent plates slide laterally
past one another, or subduction
zones, where one plate slides
beneath another to be eventually
assimilated into the mantle deep
beneath the overlying plate.

plug: a mass of magma, typically
Cylindrical in shape; the central
conduits of composite volcanoes are
commonly filled by plugs (see Fig.
1)

pumice: a highly vesicular volcanic rock,
essentially a rapidly chilled magmatic
froth. The chilled magma is now
glass, so pumice breaks casily and is,
highly abrasive. The vesicles that
make the froth are the castings of gas
bubbles, which were dissolved in the
magma prior to its explosive
eruption. Generally only high-silica,
viscous magmas—dacite and
rhyolite form pumice, in wl
vesicles are typically small and
isolated by the intervening glass walls
(which is why pumice floats!). Low-
silica magmas, in contrast, allow for
easy merging of gas bubbles and tend
to make scoria.

h the

pyroclast: literally, “hot” (pyro) and
“fragment” (clast). Refers to volcanic
deposits formed of hot. broken
fragments; implies an explosive
eruption.

39


to melt crustal rocks is provided by basalt, which rises from the mantle at
temperatures in excess of 1150° C. Crustal rocks at depths of 5-10 miles
begin to melt at about 700° C, so basalt can melt a lot of crustal rocks. Small
pieces of partly melted Silka Graywacke and Kruzof Island pluton have been
found in basaltic andesite and andesitic lavas which supports this.
hypothesized process of mixing and melting. Other, unknown rock types
deeper in the crust may also be involved in melting and mixing with the
basalt.

The alignment of the ash vents in the center of southern Kruzof Island is
thought to mark a major fracture in the Earth’s crust. A fracture would
provide a path for magma to preferentially follow to the surface. The 50-
degree angle between the inferred fracture and the adjacent Fairweather
Queen Charlotte transform fault (Fig. 2) has been duplicated in laboratory
experiments to model transform faulting. The fracture is the result of pressure
in the edge of the continental plate, caused by friction on the nearby
transform fault. What is not understood is, why has magma not erupted from
fractures elsewhere along the Queen Charlotte-Fairweather fault?

oat

14(A&B). Basalt lava flowed over boulders of a light-colored granitic rock ithe Kruzof
Island pluton; see glossary) on the beach near the mouth of Fred's Creek. The boulders
had been transported to the site by glaciers; if, as is likely, the boulders were
deposited during the last ice advance and not during an earlier ice advance, then the
basalt is no more than about 14,000 years old. Young basalt like this could have
nain explosive eruptions by adding heat to the base of the silicic ma

ma

What triggered the explosive eruptions? It is not known for certain. One
possibility is unweighting of the crust beneath Kruzoi Island by deglaciation.
Such a trigger has been proposed to account for explosive eruptions at an
Italian volcano, where crustal unloading was caused by a fall of sea level.14
Another possibility is that newly risen basalt suddenly heated the base of
magma chamber, driving the more silicic magmas upwards to eruption. This
possibility is suggested by an occurrence of basalt lava that flowed over
boulders on the beach near the mouth of Fred’s Creek (Fig. 14). Attempts to
date this basalt by the potassium-argon methad failed due to the low argon
content of the basalt. Low argon could result from several causes, including,
that the lava is geologically very young. The boulders were deposited by
glaciers, most likely during the last major glaciation. Because the last
glaciation ended just before the onset of the explosive eruptions, this means
that the basalt flow is probably about the same age as the explosive
eruptions.



west. Movement of the Pacific plate northward causes earthquakes on this
boundary. One of the larger earthquakes to have been felt in Sitka was a
magnitude 7.2 in 1972. This earthquake was located along the Queen
Charlotte-Fairweather transform fault about 30 miles (48 km) south of Sitka.+

Volcanoes are rare along transform faults worldwide, and geologically
young volcanoes are uncommon in southeastern Alaska (Fig. 2). To
volcanologists, then, the Mount Edgecumbe volcanic field (MEVF) is unusual
for its geologically recent eruptions so near ta the Queen Charlotte-
Fairweather transform fault. Geophysical surveys of the seafloor near the
Queen Charlotte Islands, 200 miles (320 km) south of Kruzof Island,3 show
that the Queen Charlotte-Fairweather fault dips eastward beneath the earth’s

4

surface. Although similar surveys have not been done near Mount
Edgecumbe, it is likely that the fault continues to dip eastward beneath
Kruzof Island. Basalt on southwestern Kruzof Island has chemical
characteristics of oceanic basalt, while basalt from the northeastern part of
the MEVF has characteristics of continental basalt. This means that MEVF
basalt comes from a zone in the earth's mantle that may include both Pacific
oceanic plate and North American continental plate. Such a zone may
straddle the transform fault at depth where it dips beneath Kruzof Island (Fig.
3A). So while it is not known why the volcanic field formed at this particular
place along the transform fault, it is likely that MEVF basalt formed because
movement on the transform fault caused melting of some of the mantle.

Hypothetical views of the subsurface beneath southern Kruzof Island, along an east-west section.

West Oceanic.) Continental

East
plate MeVE plate
—Sea level
7 —10 miles
Oceanic crust Continental
‘Oceanic mantle Crust —20 miles
Transform Continental —30 miles
fault mantle

—40 miles

Zone of mantle melting
which produces basalt,
sain of which fas |
oceanic mantle

characteristics
Oceanic Continental
plate plate

HA). The Queen Charlotte-Fairweather transform fault probably dips east beneath
Kruzof Island; the exact locations of the fault and the base of continental crust
beneath Kruzof Island are uncertain, Basalt magma forms by melting of the mantle
beneath Kruzof Island and rises (large vertical arrows) to erupt in the Mount
Edgecumbe volcanic field (MEVF). Basalt that erupts in the southwestern part of the
MEVF has chemical characteristics of oceanic mantle, so this basalt (solid arrow) may
have originated in the oceanic plate on the west side of the transform fault. Basalt that
erupts in the northeastern part of the MEVF has characteristics of continental mantle
(open arrows). The red area below MEVF in the middle of the continental crust is a
crustal magma chamber where crustal rocks partly melt to form silicic magmas. The
transform fault splits into multiple strands see (fig. 2), not a single fault, as indicated by
the fork in the diagram above. The splitting signifies a poorly understood complexity
in the contact between the plates near Kruzof Island, which may some how be
respansible for the volcanism on the island.

8

MEVF

—Dip —Sealevel

—5 miles

1
1
1
1
1
1
1

Magma
chamber

Basalt magma

3(8), A clase view of the MEVF magma chamber, which is probably 5 to 10 miles (8 to
16 km) below the surface of the earth in the continental crust. In the chamber, silicic
magma (dats) overlies basalt (open area). The magma chamber is thickest on a major
crustal fracture (solid vertical line) because heating by basalt has been concentrated
on this fracture. On and near the fracture, silicic magma has risen (wavy arrows) to
erupt at the MEVF but basalt is blocked from rising further by the thick magma
chamber. At the margins of the MEVF where the magma chamber is thin, some basalt
has beer able to rise (open arrows} along small fractures (dashed lines; see, for
example, the fissure vent in Fig. 6} to erupt even after the magma chamber grew to its
present size.


pyroclastic flow (ash flow): a moving
mass of incandescent magmatic
fragments (ash and pumice) and hot,
expanding gases. Pyroclastic flows
range from small features formed by
nonexplosive collapse of slowly
growing domes, to large, fast-moving,
hurricanes of explosively erupted
magma and gas. In both cases, the
solid fragments concentrate at the
bottom of the flow where they are
channelled by irregularities in the
ground surface. The hat gases form
an overriding cloud, or surge, that
can become detached from the basal
fragments and even surmount hills
and ridges. A pyroclastic surge from
Mont Pelee killed 30,000 residents of
St. Pierre, on the Caribbean Island of
Martinique, in 1902.

rhyolite: magma having the highest
content of SiOz (silicon dioxide; from
about 70 percent to more than 77
percent by weight) and thus, the
highest viscosity. As a consequence
‘of high viscosity, low-gas rhyolitic
magma makes short, steep-siced
flows or domes and gas-rich thyolitic
magma can cause highly explosive
eruptions because it is so difficult for
the gas bubbles to escape the
magma. Because of low contents of
iron and magnesium and because the
iron is commonly oxidized (“rust”),
frothy rhyolitic pumice tends to be
light yellow or orange. However,
dense rhyolitic glass—called
“obsidian”—can be nearly black due
to the solid nature of the glass in
combination with the nonoxidized
state of the iron.

40

scoria: a fragment of vesicular lava in
which the vesicles tend to be coarse
and interconnected, thus, scoria does
not float as does pumice. Scoria is
more typical of low-silica, low-
viscosity magmas (andesite or basalt)
than of dacite or rhyolite.

2 one of the most abundant
elements in the Earth’s crust.
Combined with oxygen (SiOz), occurs
in noncrystalline form as “chert” or in
crystalline form as the mineral quartz.
The amount of silica dissolved in
magmas (silicate melts) strongly
influences melt behavior because
melt viscosity directly correlates with
silica content.

Sitka Graywacke: a rock unit consisting
of layers of dark gray sandstone and
mudstone and minor amounts of
conglomerate (hardened pebbles and
cobbles}, which underlies much of
Kruzof Island and nearby islands in
the region; the rock formed between
about 60 and 90 million years ago by
deposition of sediments in a deep
marine trench at the edge of the
continental slope.

‘transform fault: see “fault.”

vesicle: a hole in a volcanic rock that is
the casting of a gas bubble, formed
when the surrounding rock was
molten lava.

viscosity: a material property of fluids;
the preater the viscosity, the “stickier”
or “stiffer” the fluid.

Endnotes

Literature sources of information provided in the brachure

1. As handed down by Herman Kitka:
Sealaska Heritage Foundation, Sitka.

nN

- Notes on Russian America, Part I:
Novo-Arkhangelsk, by K.T.
Khlebnikov. Alaska History No. 43,
308 p. The Limestone Press,
Kingston, Ontario and Fairbanks,
Alaska, 1994. (S.G. Fedorova,
compiler; Richard Pierce, editor.)

3. Voyage round the world in the years
1803, 1804, 1805, and 1806, by

Australiana #42, Da Capo Press, New
York, 1968.

4. The Sitka, Alaska, earthquake of
1972—an expected visitor, by R.A.
Page. U.S. Geological Survey,
Earthquake Information Bulletin v. 5,
pp. 4-9, 1973,

5. Queen Charlotte Islands margin, by
R. Riddihough and R.D. Hyndman. In
The eastern Pacific Ocean and
Hawaii, E.L. Winterer, D.M, Hussong,
and R.W. Decker, editors, Geological
Society of America, The Geology of
North America, v.N, 1989, pp. 403-
4u.

6. Geologic map of the Mount
Edgecumbe volcanic field, Kruzof
Island, southeastern Alaska, by
Riehle, |.R., Brew, D.A., and
Lanphere, M.A. U.S. Geological
Survey Miscellaneous Investigations
Map I-1983, 1989, scale 1:63,360, 1
plate.

7, The Mount Edgecumbe tephra
deposits, a marker horizon in
southeastern Alaska near the
Pleistocene-Holocene boundary, by
J.R. Richie, D.H. Mann, D.M. Peteet,
D.R. Engstrom, D.A. Brew, and C.E.
Meyer. Quaternary Research, v, 37,
1992, pp. 183-202, Univ. of
Washington.

8. Wisconsin and Holocene glaciation
of southeast Alaska, by D.H. Mann
In Glaciation in Alaska, the geologic
record, T.D. Hamilton, K.M. Reed,
and R.M. Thorson, editors. Alaska
Geological Society, Anchorage, 1986,
pp. 237-265.

9. Pyroclastic deposits of the Mount
Edgecumbe volcanic field, southeast
Alaska: eruptions of a stratified
magma chamber, by |.R. Richle, D.E.
Champion, D.A. Brew, and M.A.
Lanphere. Journal of Volcanology and
Geothermal Research, v. 53, 1992,
pp. 117-143

10. Rare earth element contents and
multiple mantle sources of the
transform-related Mount Edgecumbe
basalts, southeastern Alaska, by J.R.
Riehle, J.R. Budahn, M.A. Lanphere,
and D.A. Brew. Canadian Journal of
Farth Sciences, v. 31, 1994, pp. 852-
864.

11. The Mount Edgecumbe volcanic
field, Alaska: an example of tholeiitic
and cale-alkaline volcanism, by D.G.
Kasco. Journal of Geology, v. 89,
1981, pp. 459-477.

12. Geology and petrogenesis of the
Edgecumbe volcanic field, SE Alaska:
the interaction of basalt and sialic
crust, by J.D. Myers and B.D. Marsh.
Contributions to Mineralogy and
Petrology, v. 77, 1981, pp. 272-287.

13. Assimilation of crustal material by
basaltic magma: strontium isotopic
and trace-element data from the
Edgecumbe volcanic field, SE Alaska,
by J.D. Myers, A.K. Sinha, and B.D.
Marsh. Journal of Petrology, v. 25,
1984, pp. 1-26.

41


19(A). View from below of the lower 19(B). Above timberline, vegetation is

flanks of Mount Edgecumbe. The outer low and scattered, occurring in leeward
part of the cone consists of thick deposits areas protected from wind and
of airfall pumice and ash, which are desiccation.

visible as reddish grey layers in the leit
bank of the gully. The layers dip
downslope because airtall deposits
follow the underlying slopes, like a
blanket, rather than filling them with a
horizontal layer as pyroclastic-flow
deposits would do.

possible from along the central part of the trail (Figs. 17A and 178).
Timberline, or the upper limit of tree growth, on Mount Edgecumbe occurs
at about 1,800 feet (549 m). In southeast Alaska, timberline generally occurs
at elevations from 2,500 to 3,000 feet due mainly to cold and wind that
restrict tree survival.18 Timberline is lower on Mount Edgecumbe due to
several factors. First, the cone is isolated on an outer island and exposed to
winds, which restrict accumulation of organic material for soil development
and plant growth sites. The effects of wind can be seen near the summit,
where plants occur primarily in protected gullies or in the shelter of
boulders, and where scoured areas can be seen on the windward side of
boulders. Second, the cone is very steep, which limits accumulation of
weathered minerals and organic material for plants to grow in. Steep slopes
are often unstable, especially in ash soils, resulting in slumps, landslides,
and avalanching. Third, the materials making up the cone (pumice and ash)
drain rapidly, leaving little moisture for plants. These materials are not well
weathered and do not readily release phosphate, an essential nutrient, into
soils for plants to use. These factors combine to limit tree growth on the
upper cone.

20KA). The hiking trail ends at the summit on the northeastern side of the crater. The
summit of Mount Fdgecumbe consists of windswept deposits of ash and pumice and
blocks of dense, fused volcanic glas:

20(B). View down the flanks of Mount Edgecumbe. The ash and pumice deposits,
visible in the left bank of each gully, are susceptible to erosion unless stabilized by
vegetation,



9. A view to the northeast from the summit of Mount Fdgecumbe, showing the
thyolitic domes of Crater Ridge. The rhyolitic magma was so viscous as it extruded
fram the ground that it formed steep-sided domes rather than flaws. Also, the gas
content of the magma was low, or else the magma would have erupted explosively to
make ash and pumice. Three complete domes —the three conical peaks —can be
seen; pact of a fourth is out of view to the lefl. The tree-covered cidge that extends
toward Mount Edgecumbe from the conical peaks is the remains of the fifth dome,
which was largely destroyed in the explosive eruptions that made. Crater Ridge crater.
The dark mound next to the person in the foreground is an andesitic plug af Mount
Edgecumbe.

Stage Three: More Recent Explosive
Eruptions of Ash and Pumice

Volcanic ash beds are easily seen in roadcuts and streambanks in Sitka and
elsewhere in the region. These deposits formed during a series of explosive
eruptions at the MEVF between about 12,000 and 14,000 years ago? The
deposits are about 36 inches (1 m) thick in Sitka, but are more than 100 ft
(30 m) thick near their source vents on southern Kruzof Island (Fig. 10 A&B)
The beds are dark grey or red at the bottom, brownish yellow in the middle,
and light yellow or light grey at the top. The colors reflect the different
compositions of the magmas that were erupted by different vents. Generally,
the darker colors indicate a lower silica content.

10(A), Ash and pumice deposits produced by a series of explosive eruptions at the
Mount Edgecumbe volcanic field between about 12,000 and 14,000 years ago.

In Sitka, the deposits are a total of about 36 inches (Im) thick. The bottom layers are
dark gray and red, basaltic andesite scoria, those in the middle are gray-brown
andesitic scoria, and those at the top are pale gray or yellow, rhyolitic ash and
pumice. The deposits rest directly on glacial till (boulders), with no intervening soil.

19


can be seen in sea cliffs between Kamenoi Point and Mountain Point. A
partly eroded cone that consists of basalt flows and breccia marks a former
vent just west of the head of Mud Bay. Most early basalt flows, however,
erupted from many fissures across southern Kruzof Island. The only
evidence that remains of such fissure vents are dikes like ane visible in the
sea cliff north of Crab Bay (Fig. 6A)

6(A). Fissure vent on the eastern shore of Kruzot Island is marked by a vertical slab of
chilled basaltic lava called a dike. The dike formed when lava was squeezed up to fill
a facture. Here, the fracture cuts older lava flows and breccia, which are now eroded
below the top of the dike. Elsewhere on Kruzot Island, dikes also fill fractures in Sitka
Graywacke.

Stage Two: Different Lavas Erupted
Near the Center of the Volcanic Field

After basalt had erupted for a time, other kinds of lava that have a higher
silica content than basalt began to erupt along with the basalt (Fig. 6B).
Eruptions began to concentrate in a central zone that extends northeast from
Cape Edgecumbe to Mud Bay. For example, the low, forested maund 1 mile
(1.6 km) southeast of Shell Mountain (Fig. 7A) is the eroded remains of a
cone-shaped vent built of andesitic lava flows. Basaltic andesite lava
erupted from near the tree-covered hills southwest of Mount Edgecumbe,
and andesitic domes formed west of Shaals Point (Fig. 8A) and at St. Lazaria
Island

<oseain

Basalt Basaltic Andesite Dacite Rhyolite

Andesite
—__ Viscosity
Low Silica content High

6(B). Vacious types of lavas that have crupted from the MEVF showing the least
viscous fo the mast viscous and haw viscosity, effects topagraphy. Colors fou
bars in this chart represent the same lavas indicated in Figure 7A, 7B, and 7C

d in the

The higher the silica content of lava, the higher its. viscosity (stiffness). The
effect of viscosity on lava behavior can be seen by comparing a steep-sided,
andesitic lava flow (Fig. 8A) with a thin basaltic lava flow of lower relief

(Fig. 8B). IL is lava viscosity, then, that helps to explain major landforms of
southern Kruzof Island. The low ground of limited relief along the coast from
Mud Bay south to Shoals Point and west nearly to Sitka Point (see Fig. 7B.) is
underlain by thin, flat-lying lava flows of low-viscosity basalt or basaltic
andesite. Higher ground inland from this coastal strip, having relief to 150 ft
(50 m), is underlain by more silicic, viscous lava flows of andesite.

Basaltic eruptions began about 600,000 years ago and continucd to at least
140,000 years ago, making basalt the dominant type of lava erupted during
the first 500,000 years of activity at the MEVF (Fig. 7C).



Stage One: Early Basalt Eruptions of
the Mount Edgecumbe Volcanic Field

The oldest volcanic rocks in the Mount Edgecumbe volcanic field (MEVF)
erupted from fissures (cracks) in sedimentary and igneous rocks that
comprise the older part of Kruzof Island.6 The oldest racks are basalt, which
is the same type of /ava that erupts in the Hawaiian Islands. MEVF basalt

e and rapid

flows have smooth, ropy tops that indicate the fluid ne
typical of this type of lava (Fi

Ww
4). Basall flows are easy to see along the

4(A), The surface of a basalt flow that can be se the be.
coast of Kruzat Island. The top of the flow was mounded wh
within the chilled crust. The pe

e crust, called “columnar joit

eastern

the lava was still

ix-sided fractures

caused by

ing, “is ch

acteristic of s

2 outer surface inw

4(B). A lava tunnel forms a cave

seacliff | mile (1.6 km) east of Cape
Edgecu sults whe

fluic or of a lava flow drai

leaving the chilled roof and sides.

10

beach from Mud Point south to the Army camp. The first eruptions occurred
in part on the seafloor, as shown by breccia—deposits of broken pieces of
ms (Fi

lava—that contain shells of small ocean organi: 5). Marine breccia

fs near Port Krestof. The dark block

is basalt, similar to nearby lava flows that erupted during the earliest valeanic activity
at the Mount Edgecumbe volcanic field. The shells of small marine organisms can be
found in the mud and sand between the lava blocks. The breccia is formed mainly of

pieces of lava broken from flows that erupted on the seafloor.

5(A). Volcanic breccia can be seen in the seac

5(B). The voleanic breccia was deposited on a surface erode
Graywacke (note the person for scale). The an
waves and weathe centrated at the

graywacke and the less resistant brece

is the

ing, which is ce hard



Above timberline, typical subalpine vegetation includes deer cabbage,
luetkea, moss heather, dwarf blueberry, alpine blueberry, hawkweed, and
buttercup. These plants are growing under marginal conditions (wind, cold,
short growing season) and are, therefore, slow-growing. Repeated trampling
by hikers will destroy foliage and even roots. Loss of vegetative cover opens
the substrate to erosion. Shrubs and sedges tend to be hardier than herbs,
but once damaged, may be less able to recover the following growing
season. Twenty-five one-way passes through a subalpine herb meadow were
sufficient to decrease relative vegetation cover by 50 percent in a study in
the Washington Cascades.19 Therefore, hikers are asked to spread out and
walk in multiple paths above timberline. While there is a trail marked with
poles and cairns above treeline, this is for use only when visibility is
restricted (fog, storm, whiteout). Avoid areas where vegetation appears
damaged or soil rutting has occurred. Do not pull up plants, and avoid
dislodging rocks to further protect this habitat.

Although the top of the mountain appears to be devoid of vegetation, it is
nat. Much of the area high on the crater is blanketed by a brown crust,
which is composed of tiny, intertwined liverwort plants. The plant
(GYMNOMITRION APICLATUM) is of particular interest since it is only
known from two other places in North America; one is in Mt. McKinley
National Park, and the other is on Yakobi Island. Organic crusts such as this
are an important component to Mt. Edgecumbe’s ecosystem; they stabilize
soil and help prevent erosion. This interesting crust is very fragile, so please
make every effort not to disturb the crust’s integrity.

The trail ends on the northeastern rim of the summit crater
(Figs. 20A and 208).

In the northwestern wall of the summit crater, a dark grey plug of andesite
marks the vent from which andesitic airfall and pyroclastic flows were
erupted (Fig. 11). The summit crater was formed after emplacement of the
andesitic plug, during the dacitic eruption that was the culminating
explosive eruption on Mount Edgecumbe. Dacitic ash and pumice is more
than 150 ft (45 m) thick in the northern, eastern, and southern walls of the
crater; in the northern wall of the vent, dacitic airfall deposits can be seen
overlying andesitic airfall deposits. Ash and pumice that fell so close to the
vent atop Mount Fdgecumbe retained enough heat to compact under their
weight and fuse to nearly solid, red-brown to dark grey glass. Fused glass
can be seen, both in the airfall layers that have not yet eroded and as loose
blocks around the rim of Mount Edgecumbe. On clear days, excellent views.
of Sitka Sound and Baranof Island are afforded from the flanks of Mount
Edgecumbe (Figs. 18A and 18B).

As on all trails - Pack out what you pack in.

36

A Brief History of the Hiking Trail

The Mount Edgecumbe trail was initially constructed in the 1930s by the
Civilian Conservation Corps (CCC), to provide recreational access to the top
of the volcano. Unlike most CCC activities throughout the contiguous
United States, projects in Alaska were completed by local citizens who had
joined the Corps. With the onset of World War II, CCC activities stopped
and the program ended in 1942

Several examples of CCC work can be seen along the trail. The woodshed at
the start of the trail near the Fred’s Creek cabin is a CCC structure. The shed
was built in the Adirondack style: the roof has a double pitch and the
overlap of these slopes creates a vent which extends the length of the
ridgepole. This design allows for warming fires to be built under the dripline
of the shelter, yet smoke can escape from the vent. Several primitive sign
posts marking the route of the trail remain standing in the muskeg, some
partially covered by moss. Near the new midway shelter are the remains of
the original, Adirondack-style CCC shelter.

37


==

What is Underneath
Explanation of Physiographic Units

1. Summit crater of Mount Edgecumbe, formed about 12,500 years ago
during major explosive eruption of dacitic pumice

2. Volcanic cone of Mount Edgecumbe; constructed of andestic lava flaws,
now heavily covered by andesitic and dacitic pumice and ash erupted by
Mount Edgecumbe.

3. Andesitic lava flows erupted by Mount Edgecumbe, thinly covered by
airfall pumice and ash.

4. Andesitic lava flaws erupted by Mount Edgecumbe, heavily covered by
air fall ash and pumice and by pyroclastic-flow deposits erupted by
Crater Ridge crater. Arrows indicate paths followed by pyroclastic flows
after their explosive eruptions from Crater Ridge crater.

5. Rhyolitic domes of Crater Ridge; 3 of 4 remaining domes are visable as
rounded peaks.

6. Andesitic lava flows erupted by Crater Ridge before the rhyolitic domes
formed; these lava flows were shielded by the rhyolitic domes from the
pyroclastic flows erupted at Crater Ridge crater

7. Low-lying, basalt lava flows erupted from local fissure vents scattered
across southern Kruzof Island early in the history of the volcanic field;
now, are buried by pyroclastic-flow deposits and by airfall ash and
pumice deposits.

8. Basalt lava flows like those in unit 7, but less deeply buried and only by
airfall ash and pumice deposits;

16

7(B). A digital elevation model (DEM)
emphasizing the geomorphic features
along the hiking trail to the summit of
Mount Edgecumbe. The relations
between terrain (physiography) and
underlying geology (color shading) are
well illustrated by this type of image.

Basalt

Basaltic
Andesite

WBee18

Rhyolite [EJ J

Thousands of Years Ago

0 100 «200 300 400 500 600 700
PJEruptions begin at MEVE J Construction of Mt. Edgecumbe
BJvomes of Crater Ridge

7(C). A summary of the ages of the different lava types of the MEVF; blocks are
potassium-argon ages of specific samples. The “4.” attached to each potassium-argon
age is a measure of the uncertainty duc to imprecision in analyses of potassiurtr and
argon; for example, “189451” means that there is a. 67% probability that the actual
age of the sample is within 51,000 years of 189,000 years old.



aad oeesweentv ~The Mount Edgecumbe

rorestseviee ~~ - WOllcanic Field

Alaska Region

) RI0-RG-114 K Geologic History



>osits of airtall pumice an
Ridge. Ihe pu ule
The dark blocks are solid rhyolitic
site of the crater

and wh
caught up int
explosive eru
of the pumice.
Note how the

lice is vesi ama th.

lass (obsidian), farmen

‘h was

layers of ash and
pumice were
deforme

by the

force of the faili
block. The de

were
posited cold
the pu

blocks—the
magma that
ally caused the
eruption—were

ted he

depos

Clearly this was a major eruption. What is remarkable, is that no dacitic
pumice or ash was deposited only 10 miles (16 km) north of the Shelikof
the region to the east of this site including on the

Bay site, nor anywhere i
ge domes! Instead, only rhyolitic ash deposits a

found east of a

at extends north from Mount Edgecumbe. This phenomenon can be
ned if a rhyolitic eruption a
same time as the dacitic eruption at Mount Fdgecumbe. Little mixir

t Crater Ridge occurred at precisely the

of the

ash clouds would have occurred near the vent, where the clouds were

dense aiding. As the clouds moved northward, dacitic

id vigorously expe

ash was confined to the west of the rhyolitic ash cloud. The dacitic ash

cloud would eventually have mixed with the rhyolitic ash cloud, and

indeed, a single deposit of mixed, dacitic and rhyolitic ash occurs at sites

200 miles (320 km) to the north, between Lituya

Bay and Yakutat

Deposits of rhyolitic or dacitic ash occur in peat deposits away from Kruzoi
Island. Radiocarbon dates of the peat indicate that the ash deposits are
between about 12,500 and 13,300 years old.* The interpretation of this

3A Shelikot Bay, 4 miles (6.4
northwest of Dacitic ice extends fre
base of the cli prominent yellow band near the top of the bank
pumice is overlain by a dark grey, rhyc aclastic-flow deposit. Even
the vent, the sit is still more than k

is that the many rhyolitic eruptions occurred over an interval

of about 800 years, The simultaneous dacitic and rhyolitic eruptions

occurred near the end of this period

Al

least 11,900 years ol

ash deposits found more than 30 miles (48 km) from Kruzof Island are at

d, which means that the major eruptions ended by this

time. Evidenc 's found on Kruzof Island

for eruptive activity after this time

at Shelikof Bay where a tree 10,200 years old was rooted in pyroclastic
tlor

streamchar

and buried by a volcanic mudflow. The t

2e was not in

1, so it is likely that the mudilow that buried

tree was

'y eruptive activity and was not simply a flood. Such
sation of activity after the

triggered £

was probably a minor ¢

eruption

ent during the gradual

large, dacitic-rhyolitic eruptions.



10(8). The ash and pumice
deposits near their source
vents on Kruzof Island are
locally up to 100 ft (30 m)
thick! Here, most of the
quarry wall is dark grey-
brown, basaltic andesite ash
erupted from a nearby scoria
cone. Yellow-orange layers at
the tap of the bank arc
rhyolitic pumice, the same as
the pale yellow deposits in
Fig. 100A)

The explosive eruptions began soon aiter retreat of glaciers from the region
about 14,000 years ago.8 The oldest layers of volcanic ash in a pond on
northern Baranof Island are just above the base of pond sediments that rest
on glacial till. By tracing ash deposits in the direction that they become
thicker and coarser, it has been found that this basaltic andesite ash came
fram small scoria cones at the northeastern and southwestern ends of the
MEVE. The mixture of dark grey and red fragments in these basaltic andesite
depasits resulted from alteration of some of the magma shortly before
eruption, perhaps by groundwater that entered the magma beneath the
volcano. The grey fragments are fresh lava, whereas the red fragments—
which are otherwise identical to the grey fragments—contain more water.
The red color is due to oxidized iron, similar to rust.

As cruptions of basaltic andesite ash ended, eruptions of brown andesitic
ash began at Shell Mountain, at a scoria cone 5 miles (8 km) northeast of
Crater Ridge, and at Mount Edgecumbe itself. The andesitic vent on Mount
Edgecumbe is marked today by a small, dark gray plug visible in the
northwestern wall of the summit crater (Fig. 11). In addition to ash clouds,
Mount Edgecumbe produced small avalanches of hot andesitic blocks that
tumbled down the northern side of the volcano. While andesitic eruptions
were occurring at Mount Fdgecumbe, rhyolitic ash and pumice began to
erupt at Crater Ridge. Depasits of some of the andesitic avalanches cover
rhyolitic pumice layers between the two volcanoes, indicating (1) that both
andesitic and rhyolitic eruptions occurred multiple times, and (2) that some
andesitic eruptions occurred at about the same time as some rhyolitic
eruptions.

Rhyolitic eruptions from Crater Ridge became increasingly violent, breaking
off pieces of old rhyalitic dome and blowing them skyward with ash and
pumice (Fig. 12). In addition to airborne ash, rhyolitic pyroclastic flaws were
produced. These fast-moving masses of expanding magmatic gas, hot ash,
pumice, and pieces of cold rhyolitic dome swept to the sea both to the east
and west of the low saddle between Crater Ridge and Mount Edgecumbe
(Fig. 7A). The flows were, however, blocked from passing to the north by
Crater Ridge and ta the southwest by Mount Edgecumbe. This means that

20

11. The summit of Mount Edgecumbe, viewed! from the south; the crater and domes of
Crater Ridge are in the background. A dark plug in the northwestern wall of the
summit crater marks the source vent of andesitic eruptions by Mount Fdgecumbe
between about 13,000 and 14,000 years ago. The crater itself was the source of a

major explosive ccuption af dacitic ash and pumice that occurred after the end of the
andesitic eruptions, between about 12,600 and 13,000 years ago. Halfway up the
northern wall of the crater is the contact between dark gray, andesitic airtall pumice
and overlying, red-gray dacitic pumice. The pumice compressed and fused to a solid
mass of glass under its awn weight, due to its high temperature and low viscosity
where it fell clase to the vent (compare with uncompressed pumice in Fig. 13)

the flows formed during low-energy eruptions and/or as horizontal wind
blasts from the base of rising eruption columns and were unable to surmount
these topographic barriers

The amount of rhyolitic dome material in deposits of rhyolitic airfall and
pyroclastic flows is roughly equal to the volume of the Crater Ridge crater,
indicating that the crater formed as the rhyolitic eruptions blew apart a
dome.? Not all craters form by such “excavation.” Crater Lake—a famous
volcanic crater in Oregon—formed when the former summit of the volcano
collapsed downward as a result of the sudden eruption of a large volume of
magma from beneath the volcano. One reason that there was no such
collapse at the MEVF may be that the subsurface magma chamber is deeper
than that beneath Crater Lake volcano.

One last, remarkable eruption occurred at Mount Edgecumbe, late in the
sequence of rhyolitic eruptions at Crater Ridge. The andesitic eruptions at
Mount Edgecumbe had ended with emplacement of a plug. A sudden, violent
eruption of dacitic pumice occurred alongside the plug, producing the
present summit crater and making an airfall deposit of pumice that is 30 feet
(9 m) thick 4 miles (6.4 km) to the northwest at Shelikof Bay (Fig. 13 A&B).


(A), An andesitic dome
on the coast 1.5 miles
(2.4 km) west of Shoals
Point. Note the massive
interior and a chilled
outer “rind” that
consists of columnar
jointed lava (compare
with Fig. 5A). The dome
formed by upwelling of
lava immediately above
a vent; the lava was so
viscous that it could not
flow away from the
vent

8(B). The surface of a
chilled basalt flow
about 6 feet (2M) wide
an the beach south of
Fred's Creek, on the east
side of Kruzof Island. In
contrast to the viscous
andesitic lava in Fig.
BA, the basaltic lava
was highly mobile and
made jong, thin flows.

Silicic lava flows that are less than 100,000 years old are found at Sitka
Point and Cape Edgecumbe, around the base of Mount Edgecumbe, and
near Crater Ridge. Some older silicic lavas may be buried beneath these
young lavas, but the important fact in the history of the MEVF is that s
lavas had begun to construct the higher ground at the center of southern
Kruzof Island by about 100,000 years ago. Andesitic lava flows on the flanks
of Mount Edgecumbe are between 20,000 and 40,000 years old; the interior
of the Mount Edgecumbe cone undoubtedly contains older lava flows.

ic

Among the most silicic lavas at the MEVF are the domes of Crater Ridge.
These rhyolitic lavas were so viscous due to their high silica content that
they could not flow at all and instead mounded up over their vents (Fig. 9),
The domes are each about 20,000 years old, indicating that the most silica
rich lava at the MEVF appeared late in the geologic lifespan of the field.
Based on observations of actively growing domes elsewhere, each of the
Crater Ridge domes probably required years to decades to grow to its
present size.

What is on Top

Surface Rocks and Deposits.

(A), Geologic map of the Mount Edgecumbe volcanic field, southern Kruzof island.
Alaska. Unit symbols are:

pre-volcanic bedrock (Sitka Graywacke and Kruzot Island pluton)
IB fava flows of basaltic or basaltic andesite composition; typically low and flat-lying
due ta low viscosity

andesitic lava flows, typically steep-sided and having hummocky surfaces due to
high viscosity; erupted from vents other than Mount Edgecumbe
B andesitic lava flows erupted from Mount Edgecumbe

rhyolitic domes of Crater Ridge; there are jour nearly complete domes and the
remains of a fifth, which was largely destroyed during formation of Crater Ridge crater
Bash, pumice, and scoria erupted from vents identified by * symbol during major
series of explosive eruptions 12,000 to 14,000 years ayo; airiall deposits are thickest
on Mount Edgecumbe and on cones to the northeast and southwest; also includes an
andesitic ave flow on the northeast side of Shell Mountain cone and a dacitic lava
flow on the floor of Crater Ridge crater

thyolitic pyroctastic- flow deposits erupted trom Crater Ridge crater 12,000 to 13,000
years ago; up to 300 ft (90 m) thick in the southwest wall of Crater Ridge crater, more
typically 30 to 50 [1 (10 to 15 m) thick where they cover andesitic or basaltic lava flows
(bright yellow) or low-lying, basaltic lava flows near the coast (dull yellow)


14, Mechanical models for correlation of
ring-fracture eruptions at Pantelleria,
Strait of Sicily, with glacial sea level
drawdown, by P.C. Wallmann, G. A.
Mahood, and D.D. Pollard. Bulletin
of Volcanology, v. 50, p, 327-339,
1988.

«LR. Martin, S.J. Trull, WW. Brady,
R.A. West, and |.M. Downs, 1995,
Forest Plant Association Management
Guide, Chatham area, Tongass
National Forest, U.$.Dept, of
Agriculture, Forest =e, Alaska
Region, report R10-TP-57.

L.A. Viereck, C.T, Dyrness, A.R.
Batten, and K.J. Wenzlick, 1992. The
Alaska Vegetation Classification, U.S.
Dept. of Agriculture, Forest Service,
Pacific NW Research Station report
PNW- GTR-286.

Additional Sources

Glossary of Geology, by R.L, Bates and
1.A. Jackson. American Geological
Institute, Falls Church VA, 751 p., 1980,

Voleanoes, by G.A, Macdonald.
Prentice-Hall, Inc., Englewood Cliffs, NJ,
510 p., 1972

Volcanology, by Howel Williams and
Alexander R. McBirney. Freeman,
Cooper & Co, San Francisca, CA, 397
p., 1979

42

17. Harris, A.S. and WA. Farr. 1974, The
forest ecosystem of southeast Alaska.
7. Forest ecology and timber
management. USDA Forest Service
GTR-PNW-25, Pacific Northwest
Forest and Range Experiment Station,
Portland, OR

18. Harris, A.S. and 6 others. 1974. The
forest ecosystem of southeast Alaska
1, The setting. USDA Forest Service
GTR-PNW:12, Pacific Northwest
Forest and Range Experiment Station,
Portland, OR.

19. Cale, D.N, and $,J. Trull, 1992,
Quantifying vegetation response to
recreational disturbance in the North
Cascades, Washington. Northwest
Sci, 66(4):229-236,


1(B), Southern Kruzof
Island viewed to the
northeast. Cone-shaped
Mount Edgecumbe, which
when snow-covered bears
a striking resemblance to
Japan’s Mount Fuji, is at
the lower leit. Aligned to
the northeast are, in
succession, the crater and
domes of Crater Ridge, a
small unnamed scoria
cone, the Shell Mountain
scoria cone, the partly
eroded remains of an old
cone to the right of Shell
Mountain, a young lava
flow that erupted from
Shell Mountain and flowed
northward, and another
unnamed scoria cone.

the site of Mount Edgecumbe. By 30,000 years ago, construction of Mount
Edgecumbe itself was well underway. Crater Ridge, the cluster of peaks
immediately northeast of Mount Edgecumbe, was formed about 20,000
years ago by five lava domes that grew over a period of years.

Southern Kruzof Island had a series of major explosive eruptions about
12,000 to 14,000 years ago, just after glaciers had retreated from the region.
Ash deposits resulting from these eruptions can be seen today in
streambanks and roadcuts in Sitka and as far as Cape Fairweather, 150 miles
(240 km) to the north. The last known eruptions on Kruzof Island were two
minor ash eruptions 5,000 to 6,000 years ago. Volcanoes are categorized by
the length of time since their last activity. “Active” volcanoes have erupted
during historic time or within the past 2,000 years, and “extinct” volcanoes
have been inactive for at least 10,000 years, so Mount Edgecumbe andl its
neighboring vents are considered “dormant.” There is presently no evidence
that another eruption on southern Kruzof Island is imminent, but other
dormant volcanoes have awakened after thousands of years of inactivity and
Mount Edgecumbe could do so again as well.

Mount Edgecumbe and the other vents in the Mount Edgecumbe volcanic
field are today part of the Tongass National Forest, which is administered by
the United States Department of Agriculture Forest Service. This booklet
summarizes the volcano’s geologic history and presents a guide to the hiking
trail leading to the summit of Mount Edgecumbe. The geologic terms used
are marked by italics the first time they occur, and definitions are given in a
glossary at the end of the booklet.

6

Regional Setting: What Has Caused the
Volcanic Activity?

Maunt Edgecumbe is on southern Kruzof Island, 15 miles (24 km) west of
Sitka (Fig. 2). The island is part of the North American plate. The boundary
between the North American and Pacific plates is the Queen Charlotte-
Fairweather transform fault, which is only 10 miles (16 km) offshore to the

NORTH
AMERICA,

Kruzof
Island

MOUNT
EDGECUMBE
VOLCANIC
FIELD

Chatham
steatfaul

GULF

ALASKA,

srk 0 0 Om

2. Map of southeastern Alaska showing the Mount
Edgecumbe volcanic field on southern Kruzof Island (black).
Other, smal! volcanic fields in the region that have been
active in the past 2 million years are indicated by dots (on
land) and by stars (submarine seamounts). The Queen
Charlotte-fairweather transtorm fault (heavy lines) is the
boundary between the North American and Pacific plates
The Pacific plate is moving northward past Kruzof Island
(solid arrows) in response to spreading away trom the Juan
de Fuca ridge (open arrows) where new ocean crust is
forming.


How the Geology of Mount
Edgecumbe Influences Its Vegetation

Following is a brief summary of the main types of vegetation to be seen along
the Mount Edgecumbe trail and elsewhere on southern Kruzof Island.15.16

‘Compared to development of the volcanic field on a geologic time scale,
plant community development is very rapid. Plant colonization and
community development on a new surface, such as an ash deposit or lava
flow, is called primary succession. This process has been much studied on
bare surfaces left by glaciers in southeastern Alaska. Typically early colonizers
include plants such as. willows, fireweed, horsetail, alder, lichens, and
mosses, These plants help build up the soil by trapping wind and water-borne
sediments, adding organic material to the substrate, and, in some cases, fixing
atmospheric nitrogen and thereby enriching the soil. These plants may
become established very quickly after a surface is exposed, depending on
factors such as seed availability, type of surface, moisture, temperature,
etc.17,18

Typically, these pioneer plants are replaced by willow and alder shrubland
with occasional cottonwood or spruce trees. Spruce and hemlock forest with
a diverse understory af shrubs and herbs begin to develop. Eventually,
conifers dominate on most sites, unless the site is frequently disturbed (for
example by floads), poorly drained, or at high elevation. The conifer forest
may persist indefinitely on many sites, while, on others, forest may gradually
be replaced by wet soils and muskeg vegetation. Spruce forest may develop
on bare ground in as little as 100 years.

Succession on lava flows and ash deposits can be expected to follow a similar
pattern, with mosses, lichens, and herbs arriving first, followed by shrubs and
trees. Where the lava has flowed over relatively level ground, drainage is
typically impaired, and muskeg vegetation may develop. On steeper, more
well-drained sites, conifers are likely.

Through their effect on soil drainage, the volcanic racks and ash deposits of
southern Kruzof Island influence vegetation patterns. This influence can be
seen from Sitka asa radial pattern of vegetation on the flanks of Mount
Edgecumbe: dark green forest stringers on low ridges and along gullies, and
tawny muskeg in between. Soils on steeper topography—the low ridges and
along the edges of gullies are better drained, which favars forest vegetation.
The lava flows between the gullies and ridges underlie poorly drained soils,
which support nonforested peatland (see Fig. 1B). Such a pattern of mixed
forest and peatland is characteristic of the volcanic portion of Kruzof Island.
In contrast, the northern part of the island—and better drained, steeper slopes
in the southern island such as Crater Ridge—support more productive,
western hemlock forest types.

28

The Mount Edgecumbe Trail

The trail from Fred’s Creek to the top of Mount Edgecumbe is 7 miles (11.2
km) in length, and involves an elevation gain of 3,189 ft (966 m). Much of
the lower trail is over wet muskeg, whereas the trail above treeline to the
summit of the volcano is steep and rocky. Thus, footwear should provide
both ankle support and moisture protection.

At the Fred’s Creek trailhead, the influence of volcanic deposits on
vegetation is not dominant, and the vegetation is similar to coastal forest
elsewhere in the Sitka area. Sandy soils that occur in a narrow shoreline
fringe formed from marine deposition followed by centuries of weathering
This type of well drained, mineral soil (without much accumulated organic
material) favors Sitka spruce growth. Thus, a tall spruce forest occurs on the
approximately 100-yard-wide (91 m) fringe. Trees in this forest are often
widely spaced, and the vegetation under the trees is not well developed.

As the trail moves inland, the terrain slopes gently because it is underlain by
flat-lying, basaltic lava flows that are covered by ash and pumice (Fig. 15).
The sandy deposits are left behind, and the volcanic deposits dominate the
substrate. Drainage is impeded and tree heights decrease. Organic material
builds up on the soil surface, favoring growth of western hemlock over
spruce. The coastal spruce forest gives way to a western hemlock and mixed
conifer forest. The tree canopy typically covers 60 percent of the sky, so this
is referred to as open forest. Trees in this forest grow relatively slowly, and
are referred to as scrub or low productivity forests. The overstory is a mix of
western and mountain hemlock, yellow cedar, Sitka spruce, and occasional
shorepine. Rusty menziesia and tall blueberry are the mast common shrubs.
Understory herbs typically include fern-leaf goldthread, skunk cabbage, deer
cabbage, bunchberry, false hellebore, sedges, and ferns. The ground cover is
often moss-dominated. As drainage decreases (farther from the beach and
small streams), the tree cover thins even more, and nonforested peatland
becomes dominant.

Nonforested peatland, or “muskeg,” is dominated by grass-like plants such
as tufted clubrush, many-flowered sedge, and cottongrass, and by dwarf
shrubs such as Labrador tea, crowberry, bog laurel, bog rosemary, juniper,
cranberry, and low blueberry. Scattered stunted shorepine, yellow cedar,
and mountain hemlock trees also occur. Herbs on the peatland include
shooting star, sundew, trifoliate goldthread, deer cabbage, skunk cabbage,
bog orchid, fleabane, and swamp gentian. Peat moss (Sphagnum) is
Common; and is one of the main ingredients in the accumulated organic
ayer.

29



slopes of Mount Edgecumbe proper. The steeper, well drained slopes allow
better tree growth. Forests along the lower cone are nearly as productive as
those along the beach, Western hemlock dominates the forest canopy and the
shrubs are dominated by blueberry and rusty menziesia.

Lava flows from Mount Edgecumbe near the foot of the cane overlie the lava
flows that extend further down the trail and so are younger than the lower
flows. The potassium-argon age of one flow on the flanks of Mount
Edgecumbe is 36,000 years.

Mount Edgecumbe consists of andesitic lava flows that are interlayered with
blocks and ash, so the volcano is called a “composite cone.” The deposits on
the flanks of Mount Edgecumbe are mainly red-orange, dacitic airfall pumice
and ash that lie on dark grey or brown andesitic airfall pumice and ash (Figs.
194 and 19B). Both the dacitic and the andesitic deposits were erupted by
Mount Edgecumbe during its most recent activity 12,000 to 14,000 years ago.
Close up views of both Mount Edgecumbe and the Crater Ridge domes are

17(A). The domes of Crater Ridge as seen from the middle part of the hiking trail.

f % me ee ee bs kg
17(B). Mount Edgecumbe trail climbs the left edge of the broad ridge in the center of
the photo.

32

18(A). During clear weather views on the trail from the flanks of Mt. Fdgecumbe are
breath taking. View to the west of Sitka Sound and Baranof Island; the City of Sitka is
near the shore of Baranof Island to the right of center. Ihe small islands and a low
coastal strip on Baranof Island are underlain by the Sitka Graywacke. The dark, craggy
ridges to the [eft (north) of Sitka are underlain by the Kelp Bay Group of volcanic and
sedimentary rocks, whereas the lighter ridges and peaks to the right (south) of Sitka are
underlain by granitic rocks that intruded the Sitka Greywacke and Kelp Bay Group
about 50 million years ago. The valleys were carved by glaciers during several glacial
stages, the most recent between aboul 14,000 and 25,000 years ago.

16(B). St. Lazaria Island off the
south shore of Kruzof Island. The
island consists of massive
andesitic lava, some of which has
domical steuctures (like that
shown in Fig. 8A) suggesting that
the island is the site of a volcanic
vent.

33


Tongass
National

“Sy Forest

Mount Edgecumbe

Introduction

Mount Edgecumbe, a beautifully symmetric cone on southern Kruzof Island
(Fig. 1A), is one of southeastern Alaska’s striking landmarks. The volcanic
origin of the mountain has been long recognized: it is referred to even in
Tlingit legends as a smoking volcana.1 Russian explorers and settlers referred
to the volcano by several names, including St. Lazarus? (a name now given
to a small island off the southern coast of Kruzof Island). The mountain was
given its present name in 1778 by Captain James Cook. In 1803, Urey
Lisianski climbed the volcano and deduced that, because ash and pumice
on the mountain appeared so weathered, the volcano must have been
inactive for some time.

Mount Edgecumbe is actually one of several volcanoes on southern Kruzof
Island (1B). Volcanic activity started about 600,000 years ago when basalt
began to erupt quietly from ground cracks scattered across the island. By
about 400,000 years ago, other types of magma as well as basalt had begun
fo erupt. Fruptions eventually concentrated in a narrow zone that extends
from the southwestern tip of the island to the northeast and passes beneath

T(A), The symmetric cone of Mount Edgecumbe, viewed from the east. The high point
on the summit rim is 3,201 ft (970 m).

wu


The Author

Jim Riehle is a volcanologist with the Alaska Section of the U.S. Geological
Survey. He has studied Alaskan volcanoes for the past 20 years.

Purpose

This booklet is a compilation of technical information collected from many
hours of research done on the Mount Edgecumbe Volcanic Field. It is
presented here for the reader who has an interest-but not necessarily a
background-in geology. The booklet includes information that will help
hikers understand significant features of the Mount Edgecumbe Trail from
the east shore of Kruzof Island to the summit of Mount Edgecumbe.

The United States Department of Agriculture (USDA) prohibits discrimination in its
programs on the basis of race, color, national origin, sex, religion, age, disability,
political beliefs and marital or familial status. (Not all prohibited bases apply to all
programs.) Persons with disabilities who require alternative means for communication
of program information (Braille, large print, audiotape, etc.) should contact the
USDA's TARGET Center at (202) 720-2600 (voice and TDD)

To file a complaint, write the Secretary of Agriculture, U.S. Department of Agriculture,

Washington, D.C., 20250, or call (202) 1-800-245-6340 (voice) or
(202) 720-1127 (TDD). USDA is an equal employment opportunity employer.

November 1996.

Contents

Introduction

Regional Setting: What Has Caused the Volcanic Activity? ....

Stage One: Early Basalt Eruptions of the
Mount Edgecumbe Volcanic Field ....

Stage Two: Different Lavas Erupted Near the Center
of the Volcanic Field .

Stage Three: More Recent Explosive Eruptions of Ash and Pumice ....

Significance of the Ash Eruptions .....

How the Geology of Mount Edgecumbe Influences Its Vegetation. ....

The Mount Edgecumbe Trail

Maps.

A Brief History of the Hiking Trail....

Glossary of Geologic Terms ..

Endnotes ....



13(B). A close view of the dacitic
pumice, which is angular due to its
transport in an ash cloud rather than by
tumbling in an ash flow. Here, the
pumice blocks were cool enough
when they fell that they did not
compress and fuse, unlike those at the
summit of Mount Edgecumbe.

Two thin deposits of ash occur above, and separate from, the main sequence
of ash deposits on Kruzof Island and the nearby parts of Baranof and
Chichagof Islands. In many places only one deposit occurs, probably
because the two eruptions closely followed one another and the deposits
have become mixed. The deposits consist of rhyolitic pumice, like rhyolitic
pumice of the main sequence, and small pieces of Sitka Graywacke. A layer
of graywacke and pumice up to 3 inches (7.5 cm) thick is on the floor of
Crater Ridge crater, which is a possible source of the young ash deposits.
Radiocarbon dates of peat above and below these ash deposits indicate that
the two eruptions occurred closely together between 5,000 and 6,000 years
ago. Because there are 4,700 years between the last eruption of the main
sequence (10,200 years old) and these young, minor eruptions (about 5,500
years old), the young eruptions are probably separate eruptive activity
unrelated to the older sequence of eruptions.

Significance of the Ash Eruptions

The major ash eruptions of the Mount Edgecumbe volcanic field (MEVF) are
notable to volcanologists for several reasons: (1) many types of magma were
erupted in a geologically short span of time; (2) the eruptive activity moved
from vents at opposite ends of the MEVF, to Crater Ridge and Mount
Edgecumbe in the center of the field; and (3) the ash vents align
northeasterly, along the center of southern Kruzof Island (see Figs. 1A and 7).

Most of the ash vents had erupted lava flows of the same composition as the
ash a short time before the ash eruptions. For example, rhyolitic ash was
erupted from the Crater Ridge rhyolitic domes, and scoria cones that erupted
basaltic andesite ash had earlier extruded basaltic anclesite lava flows. All of
the magma types involved in the ash eruptions existed in the subsurface in a
pool or magma chamber, at the beginning of the explosive eruptions.

The ash eruptions show that these different magmas were not distributed
uniformly in the magma chamber beneath Kruzof Island. Basaltic andesite
magma is beneath the opposite ends of the MEVF, and rhyolite—the most
silicic magma—is beneath the center of the field. Some of the rhyolite in the
center of the magma chamber erupted quietly as domes shortly before the
main explosive eruptions. Chemical and isotopic studies show that MEVF
basalt formed in the mantle!9 and that the silicic magmas are a mixture of
basalt and partly melted crustal rocks.11,12 The basaltic andesite is mostly
basalt and a small amount of crustal melt, while the rhyolite is mostly crustal
melt and a small amount of basalt.

The progression of the ash eruptions, from low-silica magma at the
beginning to high-silica magma at the end, is due to the viscosity of the
different magmas: low-silica basaltic andesite magma could rise to the
surface much faster (230 times faster by some calculations) than viscous
rhyolitic magma.13

The subsurface beneath southern Kruzof Island may look something like
Figure 3. Some magma may be spread throughout a large volume of solid
rock, but most of the magma is concentrated in a magma chamber because,
in laboratory experiments, sil magma is too viscous to be easily removed
from rock. Beneath the northern and southern ends of the MEVF, basalt
overlain by basaltic andesite or andesite. Only minor crustal melting and
mixing with basalt have occurred there. The most silica-rich magma, and the
greatest amount of crustal melting, are beneath the center of the volcanic
field, where the rise of basalt and heat from the mantle is concentrated

along the major fracture. There is no direct evidence for the depth at which
this melting and mixing occur. Indirect evidence trom mineral compositions
suggests that it is between 5 and 10 miles (8 and 16 km). The heat required

25


At 2 miles (3.2 km), the trail encounters the first of several short, steep
climbs onto andesitic lava flows erupted by Mount Edgecumbe (Fig. 158).

Here mixed conifer, open forest is favored over muskeg by the improved

15(A). The hiking ly slopin
surface of low re The surtace is underlain by flat-lying basalt
flows that have been blanketed by pyroclastic deposits. The in the background is
the front of an andesitic lava flow

At about 2 miles (3.2

nts onto andesitic fav

drainage on these steeper slopes. The andesitic flows are deeply covered by
pyroclastic-flow deposits, which accounts for the flat, gentle slopes of
muskeg meadows on the andesitic flows (Fig. 16). The poorly sorted
pyroclastic-flow deposits can be seen in the creekbank just above the
footbridge near the shelter. The pyroclastic flows are known to have erupted
from Crater Ridge when the crater formed, because dense, dark fragments of
Crater Ridge dome rock are mixed with yellow rhyolitic pumice in the
pyroclastic-flow deposits. (The dark grey rhyolitic domes of Crater Ridge are
an exception to the rule that the darker the color of volcanic rocks, the
lower their silica content. For an explanation, see “rhyolite” in the glossary.)
The pyroclastic flows hugged the floors of gullies, so their deposits are
restricted to lower ground both northwest and southeast of the saddle
between Crater Ridge crater and the north flank of Mount Edgecumbe (see
Fig. 7A). Likewise, wet soil communities hug the gully floors, while forest
communities parallel the streams and steeper ridges.

At the shelter, midway from the coast to the summit of Mount Edgecumbe,
the trail encounters the steep front of another andesitic flow (Fig. 16). About
1.2 miles (1.9 km) above the shelter, the trail begins to

scend the lower

16. A muskeg meadow near the midway shelter, typical of much of the trail below the
base of Mount Edgecumbe. The muskeg is underlain by pyroclastic flows and airfall
deposits, which increase in thickness from about 15 ft (3 m) at the coast, to 66 tt |
m) at the base of Mount Edgecumbe. The deposits of ash and pumice bury and
subdue the hummocky surfaces of andesitic lava flows; the front of such a flow
underlies the hill in the background.