GLY3163: Olympic (Answered) 100%
Correct, 2024/2025.
The Crescent Formation rocks are largely basalts lava flows that were erupted on the seafloor before
being accreted to North America.
True
All of the sediment transported by rivers and deposited on the Juan de Fuca plate is subducted deep
into the mantle.
False
The Cascade Range and the Coast Range were formed by subduction of the Juan de Fuca plate under
North America.
True
Why are the rocks of the Core Area younger than the rocks of the Crescent Formation even though
they are beneath the Crescent Terrane?
The Core Area rocks were thrust under Crescent Formation by faulting
The Siletzia Terrane is oceanic lithosphere that was attached to North America
True
Where is the Olympic National Park located?
Northwest Washington
Quick facts about Olympic National Park
- Highest amount of annual precipitation in the U.S.
- Roughly 60 active glaciers on high slopes
- Temperate rain forest on lower part of lower western slopes
- Rain shadow keeps northeastern slopes dry
- Young sedimentary rocks with submarine lavas
- These rocks were scraped off the ocean floor by plate convergence and pushed into a heap on the
Olympic Peninsula
- Western shores exposed to violent and stormy surf
Glaciers actively erode resistant ______________________________________ rocks of the Olympic
massif.
metamorphosed deep marine rocks
Geographic Setting of Olympic
- Preservation for the habitat of the elk meant some of the Olympic rainforest was also saved
- Trees in the rain forest includes : Douglas fir, Sitka spruce, giant western hemlock, and red cedar
- Ferns, mosses, and smaller plants cover the forest floor
- Higher up are forests made up of subalpine firs and alpine meadows
,- Above the tree-line are bare ledges, glaciers, snowfields, and peaks of the massif
- Olympic Peninsula has snow-capped mountains
- Once inhabited by Makah Native Americans who whaled
- About 150 inches of precipitation per year
- Radial drainage from the high area near Mount Olympus (highest peak 7965 feet)
- Streams rush down narrow valleys between steep interfluves
- Glaciers can sometimes block drainage
The trees in Olympic usually range in height from __________ to __________ feet, and some even
reach __________ feet.
100 to 200, some as tall as 300 feet
Geologic Features of Olympic
- Miocene rock only about 15 million years old (western side of shore) turned vertical and upside
down
- Most sedimentary rocks were deposited in the deep ocean on the basaltic oceanic crust (some laid
down on partly submerged continental rock which then rode on oceanic crust as a microcontinent or
terrane)
- When Juan de Fuca (eastward moving segment of oceanic crust) meets the North American Plate
(westward moving), the heavier basaltic oceanic crust is subducted/overridden by the lighter one
- From the same process as stated above, after millions of years, marine sedimentary rock is
accumulated by being scraped off the subducting plate, folded, compressed, and thrust downward
or squeezed upward, fractured, then partially metamorphosed, then sliced and uplifted (this kind of
tectonic activity created the Olympic Mountains)
- These effects are seen in the more eastern part of the park due to the up-warping of the North
Cascade Range and creation of the northern Cascade volcanoes
- Piles of submarine basaltic lava are exposed on the Olympic Peninsula and erupted in Eocene time
at mid-oceanic spreading center
- Lava cooled underwater as either pillow lava or brecciated rocks, though some solidified slowly in
dikes and sills between sedimentary beds or within previously erupted lavas
- Basalt was altered by hot saline brines and by forces of the overriding North American plate
- Between pillow lava eruptions were few beds of fossiliferous red limestone deposited on the ocean
floor (fossils were Foraminifera)
What is Foraminifera?
Single celled protozoa that secrete calcite and whose tiny shells can barely be seen without the help
of a magnifying glass
Red color of rocks in the eruptions (pillow eruptions) are caused by what?
Iron leached out from the cooling lava by the hot sea water
What is associated with the iron in the rocks that are red?
Manganese and copper
Upper North Fork of the Skokomish River are had what kind of rocks?
Red limestones associated with pillow lavas
, In figure 32.6 (page 544), there are large rocks that are very bumpy looking and a grayish-brown
color. Explain what is seen -
Eocene pillow lavas from the basalt horseshoe or Crescent Terrane along the Hurricane Ridge Road.
They appear as bulbous pillow forms because the lava is erupted underwater and quenches at the
lava-water interface
Most basalts cropped out in what kind of pattern?
Horseshoe
Explain the horseshoe pattern of the basalts
The horseshoe partly rings the peninsula and the arc portion of it is on the east side and the arms
extend westward
- This horseshoe shaped rock unit is named the Crescent Formation and is also referred to as the "
basaltic horseshoe "
The oldest rocks are at the _________________ of Hurricane ridge and the youngest are at the
____________.
oldest = summit
youngest = base
Fossils of Foraminifera in the red interbedded limestone were dated as _____________________
age.
Eocene
What is indicated by the red limestones being dated in the Eocene age?
The basalt lavas are Eocene, too
The Eocene basalts are from a stranded block of ____________________________________ that
became stuck in and older subduction zone further east.
oceanic Farallon plate
What is the name of the stranded Farallon plate?
Siletzia terrane and makes up much of the basement rocks of western Washington, Oregon, and
Vancouver Island
The sedimentary layers between the Crescent Formation and the western coast become
progressively _____________ from east to west due to being scrapped off the subducting plate and
accreted from below and at the front of an accretionary prism above the _____________________.
younger, Juan de Fuca plate
Series of plate tectonic events that lead to the jumble of rocks on the Olympic Peninsula
1. 50 MYA - Shift in tectonic forces in Eocene time leaves part of the Farallon plate stranded and
attached to North America
2. 35 MYA - Subduction of Farallon plate under Selitzia Terrane, growth of new accretionary prism
3. 10 MYA - Crescent Formation basalts at edge of Selitzia Terrane up folded by underthrust accreted
sediments
Correct, 2024/2025.
The Crescent Formation rocks are largely basalts lava flows that were erupted on the seafloor before
being accreted to North America.
True
All of the sediment transported by rivers and deposited on the Juan de Fuca plate is subducted deep
into the mantle.
False
The Cascade Range and the Coast Range were formed by subduction of the Juan de Fuca plate under
North America.
True
Why are the rocks of the Core Area younger than the rocks of the Crescent Formation even though
they are beneath the Crescent Terrane?
The Core Area rocks were thrust under Crescent Formation by faulting
The Siletzia Terrane is oceanic lithosphere that was attached to North America
True
Where is the Olympic National Park located?
Northwest Washington
Quick facts about Olympic National Park
- Highest amount of annual precipitation in the U.S.
- Roughly 60 active glaciers on high slopes
- Temperate rain forest on lower part of lower western slopes
- Rain shadow keeps northeastern slopes dry
- Young sedimentary rocks with submarine lavas
- These rocks were scraped off the ocean floor by plate convergence and pushed into a heap on the
Olympic Peninsula
- Western shores exposed to violent and stormy surf
Glaciers actively erode resistant ______________________________________ rocks of the Olympic
massif.
metamorphosed deep marine rocks
Geographic Setting of Olympic
- Preservation for the habitat of the elk meant some of the Olympic rainforest was also saved
- Trees in the rain forest includes : Douglas fir, Sitka spruce, giant western hemlock, and red cedar
- Ferns, mosses, and smaller plants cover the forest floor
- Higher up are forests made up of subalpine firs and alpine meadows
,- Above the tree-line are bare ledges, glaciers, snowfields, and peaks of the massif
- Olympic Peninsula has snow-capped mountains
- Once inhabited by Makah Native Americans who whaled
- About 150 inches of precipitation per year
- Radial drainage from the high area near Mount Olympus (highest peak 7965 feet)
- Streams rush down narrow valleys between steep interfluves
- Glaciers can sometimes block drainage
The trees in Olympic usually range in height from __________ to __________ feet, and some even
reach __________ feet.
100 to 200, some as tall as 300 feet
Geologic Features of Olympic
- Miocene rock only about 15 million years old (western side of shore) turned vertical and upside
down
- Most sedimentary rocks were deposited in the deep ocean on the basaltic oceanic crust (some laid
down on partly submerged continental rock which then rode on oceanic crust as a microcontinent or
terrane)
- When Juan de Fuca (eastward moving segment of oceanic crust) meets the North American Plate
(westward moving), the heavier basaltic oceanic crust is subducted/overridden by the lighter one
- From the same process as stated above, after millions of years, marine sedimentary rock is
accumulated by being scraped off the subducting plate, folded, compressed, and thrust downward
or squeezed upward, fractured, then partially metamorphosed, then sliced and uplifted (this kind of
tectonic activity created the Olympic Mountains)
- These effects are seen in the more eastern part of the park due to the up-warping of the North
Cascade Range and creation of the northern Cascade volcanoes
- Piles of submarine basaltic lava are exposed on the Olympic Peninsula and erupted in Eocene time
at mid-oceanic spreading center
- Lava cooled underwater as either pillow lava or brecciated rocks, though some solidified slowly in
dikes and sills between sedimentary beds or within previously erupted lavas
- Basalt was altered by hot saline brines and by forces of the overriding North American plate
- Between pillow lava eruptions were few beds of fossiliferous red limestone deposited on the ocean
floor (fossils were Foraminifera)
What is Foraminifera?
Single celled protozoa that secrete calcite and whose tiny shells can barely be seen without the help
of a magnifying glass
Red color of rocks in the eruptions (pillow eruptions) are caused by what?
Iron leached out from the cooling lava by the hot sea water
What is associated with the iron in the rocks that are red?
Manganese and copper
Upper North Fork of the Skokomish River are had what kind of rocks?
Red limestones associated with pillow lavas
, In figure 32.6 (page 544), there are large rocks that are very bumpy looking and a grayish-brown
color. Explain what is seen -
Eocene pillow lavas from the basalt horseshoe or Crescent Terrane along the Hurricane Ridge Road.
They appear as bulbous pillow forms because the lava is erupted underwater and quenches at the
lava-water interface
Most basalts cropped out in what kind of pattern?
Horseshoe
Explain the horseshoe pattern of the basalts
The horseshoe partly rings the peninsula and the arc portion of it is on the east side and the arms
extend westward
- This horseshoe shaped rock unit is named the Crescent Formation and is also referred to as the "
basaltic horseshoe "
The oldest rocks are at the _________________ of Hurricane ridge and the youngest are at the
____________.
oldest = summit
youngest = base
Fossils of Foraminifera in the red interbedded limestone were dated as _____________________
age.
Eocene
What is indicated by the red limestones being dated in the Eocene age?
The basalt lavas are Eocene, too
The Eocene basalts are from a stranded block of ____________________________________ that
became stuck in and older subduction zone further east.
oceanic Farallon plate
What is the name of the stranded Farallon plate?
Siletzia terrane and makes up much of the basement rocks of western Washington, Oregon, and
Vancouver Island
The sedimentary layers between the Crescent Formation and the western coast become
progressively _____________ from east to west due to being scrapped off the subducting plate and
accreted from below and at the front of an accretionary prism above the _____________________.
younger, Juan de Fuca plate
Series of plate tectonic events that lead to the jumble of rocks on the Olympic Peninsula
1. 50 MYA - Shift in tectonic forces in Eocene time leaves part of the Farallon plate stranded and
attached to North America
2. 35 MYA - Subduction of Farallon plate under Selitzia Terrane, growth of new accretionary prism
3. 10 MYA - Crescent Formation basalts at edge of Selitzia Terrane up folded by underthrust accreted
sediments
