The Chugach Mountains
Prince William Sound
Prince William Sound with its
3,000 miles of shoreline is surrounded by the
Chugach Mountains to the east, west and north. Fifty-mile long Montague Island
and several smaller islands form natural breakwaters between the Sound and the
Gulf of Alaska. Between the barrier islands stretch underwater sills separating
the Sound's deep waters from the much shallower waters of the Gulf. Deep water
renewal occurs during the winter when cold winds from interior Alaska cool the
surface waters causing them to sink, while the warmer bottom water rises to the
surface bringing rich nutrients which support huge plankton blooms in the
Millions of years of glaciation gradually carved away a coastal plateau
creating the sound with its many tributary fiords and passageways, islands and
rocky shores. Fewer than 10,000 people live in the three towns-Whittier, Valdez,
and Cordova- and two native villages-Chenega and Tatitlek situated on the shores
of the Sound. Because the Sound was formed by millions of years of glaciation,
its shorelines are heavily indented by deep fiords and many smaller bays. No
roads connect these communities.
Geology of the Prince William Sound and the
Columbia Bay Area
The geological story of Prince William Sound and Columbia Bay begins off the
coast of northern California some sixty million years ago when rain and floods,
freezing and thawing eroded the continental landscape. Rivers carried loads of
sediments seaward depositing them on the edges of a deep, submarine canyon.
Periodic earthquakes dislodged the mud and boulders, sending massive turbidity
currents-a slurryof mixed water and sediments-cascading down the canyon walls to
spread out in a fan below. Partial subduction of these unconsolidated sediments
slowly transformed them into sedimentary rocks. Turbidites from this period
occur along the western shore of Columbia Bay.
About 50 million years ago, off the northwest coast of the American continent,
the Kula and Farallon ocean plates were spreading apart, magma rose and cooled
underwater forming pillow basalts. The pillow basalts and sheeted dikes
composing Glacier and Growler Islands were formed in this spreading center. It
is here that prospectors would seek copper, gold and silver.
Meanwhile, the Pacific plate pushed the Kula and Farallon plates northward
where they encountered the North American plate. The lighter continental plate
rode up over the heavier oceanic plates forcing them down into a deep subduction
trench. Movement of plates along this subduction trench creates many tiny and a
rare few, catastrophic earthquakes. In 1964, the plates about 12 miles beneath
Miners Lake in Unakwik Inlet unlocked causing the largest Earthquake recorded in
North America. The quake was so strong that it was the first earthquake since
the invention of the Richter scale NOT to be recorded. It went off the scale
within the first few seconds. Geologists have since calculated that it would
have registered 9.2 on the Richter Scale. The earthquake was so powerful it
uplifted many areas of Prince William Sound from just a few to 36 feet, while
other places sank 8 feet. Growler Island and the Columbia Bay area uplifted
about 4 feet. Evidence of this uplift can still be seen along the shorelines.
About 40 million years ago, the heat of the Earth's interior melted the
subducted sediments forming pools of magma which periodically rose towards the
Earth's surface. These plutons (named for Pluto, Greek Lord of the Underworld)
cooled slowly forming granites. Granites at Granite Point and Granite Bay on the
west side of Columbia Bay represent this era.
Thrust faulting from subduction in the Aleutian trench over the past 20
million years gradually uplifted the Chugach Range in the Prince William Sound
area. This range at first was most probably an uplifted plateau. But the
combined physical forces of uplift and glaciation created the highly sculpted
Chugach Mountains. The only higher range of coastal mountains is the Andes.
Links to other pages on geology: