"Grand Teton, Moose Entrance" by U.S. National Park Service , public domain
NatureGeologic Tour |
Journey Through the Past: A Geologic Tour in Grand Teton National Park (NP) in Wyoming. Published by the National Park Service (NPS).
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Grand Teton
National Park Service
U.S. Department of the Interior
Grand Teton National Park
John D. Rockefeller, Jr.
Memorial Parkway
Journey Through the Past:
A Geologic Tour
The Big Picture
When visitors catch their first glimpse of the Teton Range, the jagged skyline sparks
wonder. What natural forces shaped this magnificent landscape? Some of these
forces began and ended long ago, but some of these forces are still changing the
landscape today.
The rocks found in the core of the mountains are some of the oldest in North
America; whereas, the forces that lifted the Teton Range and formed the Jackson
Hole valley began very recently in geologic time. Our journey through the past
explores these stories of the Teton Range.
The Rocks
Figure 1. Mount Moran is composed of
metamorphic gneiss, igneous granite and
diabase, and sedimentary sandstone. The
summit is flanked by five glaciers.
The geologic story of the Teton Range began
more than 2.7 billion years ago. Sand, mud
and volcanic sediment sank into an ancient
sea. The collision of tectonic plates, moving
sections of the Earth's crust, buried these
sediments up to 20 miles deep. Heat and
pressure changed these sediments into a
metamorphic rock called gneiss. In this rock,
light and dark minerals separated into layers
as seen along the trail to Inspiration Point,
or sometimes into “eyes” as seen in Death
Canyon.
Inland seas flooded the region about 510
million years ago, depositing sand, mud,
and forming coral reefs during the next 400
million years. With burial, these sediments
compressed into layered sedimentary rocks
such as sandstone, shale, limestone and
dolomite. These rocks flank the Teton Range
to the south, west, and north and outcrop on
Blacktail Butte. (Figure 2)
Around 2.5 billion years ago, molten rock
called magma squeezed into weak zones
or cracks in the gneiss. Crystals grew as the
magma slowly cooled to form an igneous
rock called granite. These bodies of granite
are inches to hundreds of feet thick slicing
through the gneiss. Granite appears speckled
in contrast to the layers seen in gneiss.
Granite is harder than gneiss and forms the
jagged summits of the Cathedral Group such
as the Grand Teton.
Roughly 775 million years ago, iron-rich
magma similar to basalt squeezed into
vertical cracks in the granite and gneiss
and cooled to form dikes. These igneous
dikes are made of a rock called diabase. The
“Black Dike” on Mount Moran is roughly
150 feet wide, sticks out from the face of the
mountain 200 feet and continues west for
six or seven miles before being buried under
younger sedimentary rocks. This dike sticks
out from the face of Mount Moran because
diabase is harder than gneiss. (Figure 1)
Mountain Building
Figure 3. Regional map tracing the path of the
magma hotspot that lies under Yellowstone
National Park today.
The dike on the face of the Middle Teton,
however, forms a slot because granite is
harder than diabase.
Starting 120 million years ago, a tectonic plate
under the Pacific Ocean collided into the
west coast of North America. This collision
built mountains by crumpling the Earth’s
surface from the west coast progressing
eastward. Mountain building reached the
Rocky Mountains and Gros Ventre Range
around 70 million years ago by thrusting large
blocks of bedrock skyward. (Figure 3)
Formation
2.7
Figure 2. Stratigraphic column shows the age,
relative thickness and hardness of rocks found
in the core of the Teton Range.
As the Rocky Mountain uplift ended, lava
erupted from volcanoes across the region.
Layers of lava and volcanic debris deposited
to form the Absaroka Range. Lingering heat
from this molten rock left the Earth’s crust
hot and bulged up like a hot-air balloon. In
places, the crust stretched past the breaking
point. Huge blocks of the Earth's crust broke
and slipped past each other along faults such
as the Teton fault.
Teton Fault
Movement on the Teton fault accounts for the
dramatic uplift of the Teton Range. Starting
10 million years ago, a series of massive
earthquakes triggered by movement on the
Teton fault tilted the mountain block skyward
and dropped the valley block. Each of these
earthquakes, up to magnitude 7.5, broke or
offset the Earth’s surface by up to ten feet.
Figure 4. Each major earthquake breaks the
Earth's crust forming a vertical face of raw dirt
and rock called a scarp.
Today, the total offset on the Teton fault
approaches 30,000 feet. The Flathead
Sandstone caps Mount Moran 6,000 feet
above the valley floor. This same sandstone
layer lies buried more than 20,000 feet
beneath the valley floor.
The best view of the Teton fault is from the
Cathedral Group Turnout along the Jenny
Lake Scenic Loop. From this vantage point,
the fault “scarp” or break in the Earth’s crust
represents up to a dozen earthquakes since
the end of the Pleistocene Ice Age. (Figure 4 ;
Figure 5)
Every day seismic instruments record
earthquakes up to magnitude 5 in the
Teton – Yellowstone region. Few if any of
these earthquakes occur on the Teton fault.
Geoscientists discovered that the last two
major earthquakes were around 4,800 and
8,000 years ago. Each of these earthquakes
offset the Earth’s surface by 4 – 10 feet.
Someday another major earthquake will
shake the ground, break the Earth’s surface,
and lift the mountains skyward once more.
Figure 5. Regional stretching has generated thousands of
earthquakes over the past ten million years; tilting the mountains
skyward and hinging the valley down.
The Teton/Yellowstone
Connection
Today a plume of magma or “hotspot” lies
beneath the Yellowstone Plateau (Figure
3). The magma heats the overlying rock and
water to generate spectacular hot springs and
geysers found in Yellowstone National Park.
Five million years ago, the hotspot erupted
west of the Teton Range sending clouds of
volcanic ash into Jackson Hole. This heat
caused the area to stretch more rapidly
triggering earthquakes on the Teton fault
and continuing to uplift the Teton Range.
Glaciation
Today’s landscape preserves evidence of the
last two glacial advances. The most recent
glacial advance, called the Pinedale, lasted
from about 50,000 to 14,000 years ago. This
ice sheet wrapped around Signal Mountain
and dug out Jackson Lake. The older glacial
advance, called the Bull Lake, buried the
town of Jackson under 1,500 feet of ice and
pushed south toward Hoback Junction.
peaks. Glaciers slid on a film of meltwater
picking up rocky debris in their bases.
Debris acted as a belt-sander to polish and
groove the bedrock. Glaciers also broadened
V-shaped stream drainages into U-shaped
valleys as seen in Cascade Canyon. When
the glaciers reached the valley floor, they
bulldozed out depressions and left behind
ridges of rocky debris called moraines.
Terminal moraines mark the furthest extent
of a glacier’s flow and form natural dams for
valley lakes such as Phelps, Taggart, Bradley,
Jenny, Leigh and Jackson. (Figure 6)
Ice, water and wind sculpted the stunning
Teton Range. The Pleistocene Ice Age began
2 million years ago as the Earth’s climate
cooled. Snow accumulated across the high
Yellowstone Plateau and compressed into
ice. Gravity caused the large ice sheet, up
to 3,500 feet thick, to flow down from the
high plateau. As the climate warmed, glaciers
melted and retreated and the cycle repeated.
While ice sheets flowed from the north,
alpine glaciers flowed eastward from the high
Figure 6. A glacier flowed out of Cascade Canyon gouging out a depression
and depositing a terminal moraine forming Jenny Lake today.
Today's Landscape
Ice age glaciers melted and flooded Jackson
Hole. The meltwater carved channels across
the valley floor, washed away soil, and
deposited glacial outwash plains of sand,
gravel, and cobbles. As time passed, the
Snake River cut through these plains leaving
behind benches or terraces that step down
to today’s channel. On the outwash plains,
sagebrush, arrowleaf balsamroot and scarlet
gilia have adapted to thrive in this sandy dry
soil. Silt in glacial moraines holds rainwater to
support lodgepole pine forests. Today, these
forests cover moraines such as Timbered
Island, Burned Ridge, and around Jenny
EXPERIENCE YOUR AMERICA
Between two million and 640 thousand years
ago, the Yellowstone hotspot exploded three
times. These eruptions destroyed mountain
ranges and sent fiery clouds of gaseous lava
south along both sides of the Teton Range.
Deposits from these eruptions cap Signal
Mountain and the north end of the Teton
Range.
Today the Teton Range hosts a number
of small glaciers. These glaciers are not
remnants of the Pleistocene Ice Age but
formed during a cool period called the Little
Ice Age, 1400 to 1850. Today, Skillet and
Falling Ice glaciers continue to carve Mount
Moran, and the Teton Glacier flows down
the north flank of the Grand Teton. Even
as these glaciers flow down due to gravity,
warming temperatures cause them to shrink
and retreat. During the past 40 years, these
glaciers have retreated 20 to 25 perscent.
Lake. Geology influences the vegetation and
in turn, the wildlife found here.
As you enjoy the scenic beauty of the Teton
Range and Jackson Hole, remember that
geologic forces are still at work. Mountains
continue to rise, while wind, water and ice
erode the mountains as part of a neverending story.
Park law prohibits collecting. Please leave
rocks where you find them so that others may
enjoy this geologic story.
02/2012