Transcript pacific northwest major landforms

PACIFIC NORTHWEST
MAJOR LANDFORMS
A Brief Geological History
700 to 800 million years ago
• Western border of Idaho was coastline,
all areas to the west were open ocean.
Inland areas were deserts
• This arrangement survived until ~200
million years ago
During Last 200 million years
• Northern Rocky Mountains rose along the western
margin of the continent, areas E. of Rockies were inland
shallow sea. (handout 1)
• Large and small scraps of coastal rocks attached
themselves to the W. Coastline, assembling the
geologic mosaic of the Pacific Northwest. Key features:
• Klamath, Blue and Wallowa Mountains were groups of
islands (terranes) far out in the Pacific that lodged
against the western edge of North America (~100-90
mya)
• At ~ same time, other terranes jammed against the
western edge of the continent, forming the North
Cascades & one large mass that became Vancouver
Island and W. Coast of B.C.
More recently…
• N. America collides with Pacific seamounts
(undersea volcanoes) to form core of Coast
Range (OR) ~40 mya
• Eocene (35-55 mya) tropical period leaves
world class fossils (John Day formations)
• Western Cascades (volcanic) form ~42-17
mya, as the oceanic plate sank through the
trench off the west coast.
Major Events 17 Million Yrs. Ago
• Western Cascades volcanoes go extinct
• Faults began to break former plains of
Nevada, Utah into broad valleys & jagged
mountain ranges (Basin & Range Province)
see animation
• Immense flows of basalt spread across E.
WA and OR and into Willamette Valley &
Coast (Cape Lookout is example)
• 15 million years ago, Olympic Mountains
rise, mostly of oceanic crust
<17 million years ago (cont)
• Climate transitions from dry to wet
12 million years ago:
High Cascades rise in line east of
Western Cascades (Oregon) and on
nearly same axis (Washington)
• 15,000 years ago: The Great Lake
Bonneville Flood
Glacial Lake Missoula Floods
• http://www.opb.org/television/programs/ofg/s
egment/missoula-floods/
• Purcell lobe of regional ice blocks Clark Fork
River, then breaks, at least 41 separate
events
• Lake Missoula’s volume exceeded that of
Erie or Ontario
• Wall of water >2000’ high in places, moving
perhaps 45+ mph…evidence?
Erratics in high and far away places,
shorelines of Lake Missoula, Channeled
Scablands…
This is site of the Purcell Lobe,
now Lake Pend Orielle, Idaho
(Sandpoint in the distance). In
some places the Lake is more
than 1000’ deep. At the
southernmost extent of the lobe,
a large dam known as a moraine
was left behind, blocking the St.
Joe River and thus forming Lake
Coeur d’ Alene.