1.2 Billion Years Ago to 750 Million Years Ago
Around 1.2 billion years ago, plate tectonic motion pushed together fragments of
continental crust which created the supercontinent Rodinia ("homeland" in Russian).
Rodinia was the dominant landmass on Earth for around 350 million years. The exact
size and configuration of Rodinia is not known and is debated by scientists. It is believed that
what is now North America's east coast was adjacent to western South America. The west coast
of North America was next to what is now Australia and Antarctica.
During this time the area that would become Washington was a vast floodplain that
accumulated thick sequences of sand and silt. The rust red colored landscape would have been
devoid of plants as life on Earth had not progressed beyond single-celled algae. The atmosphere
would have only had 5% of the oxygen it now holds and lacked an ozone layer. The only single
ocean of earth would have been spawning violent storms over the water. There would have been
no protection for the landscape from the forces of water. Great floods would have been common
on Rodina.
Under these desolate and violent conditions, the first rocks of Washington formed.
Between 700 million and 1.5 billion years ago a distinct group of sedimentary rocks accumulated
in a giant basin over part of what are now Washington, as well as Alberta, British Columbia,
Montana, and Idaho. Scientists named these rocks the "Belt Supergroup" after a nearby town.
The Belt Supergroup rocks are multicolored sandstone, siltstone and limestone. They are noted
for their beautiful preservation of sedimentary features such as mud cracks, ripple marks and
abundant "stromatolites." Stromatolites are cabbage shaped fossils left by the action of bluegreen algae know as cyanobacteria. In Washington, the Belt rocks are exposed extensively in the
eastern Okanogan Highlands. The Belt Group also makes up most of the spectacular mountains
in Glacier National Park in Montana. The Belt rocks are remarkably thick. They accumulated to
perhaps 50,000 feet near the Washington-Idaho line, and thin eastward toward central Montana.
The rupture that split Rodinia abruptly truncated the basin in which the Belt sediments
accumulated. The missing western margin of the Belt basin is now thought to be part of either
Australia or Asia.
750-200 million years ago
After dominating Earth for 350 million years, Rodinia began to break apart. The Earth's
internal heat was slowly building up beneath the supercontinent. This caused the crust to dome,
stretch and weaken. Eventually the entire continent would rupture and tear itself apart.
The breakup of Rodinia began about 750 million years ago in now what is east-central
Washington. The continent began to rupture a couple inches a year. The line ran from what is
now north to south. After ten millions years, the waters of the ocean flowed into the new rift
valley. As the rift continued to grow, it would eventually form a vast ocean basin called the
"Panthalassic Ocean," which is Greek for "all the seas." The Panthalassic Ocean separated the
Americas, Siberia and Scandinavia from Antarctica, Australia, and the rest of the eastern
hemisphere. The new coastline of ancestral North America now ran through what is today
eastern Washington, not too far east of modern-day Pullman. As the great rift separated what is
now Eastern Washington from Antarctica, Australia and northern China, it did so by pushing
both halves of the supercontinent away from each other. As Rodinia split, the Panthalassic Ocean
became ever wider.
The broken edge of Rodinia through what is now eastern Washington, was a quiet,
passive continental margin and would remain that way for around 400 million years. Everything
west of present day Pullman and Spokane was ocean. Between 670 and 600 million years ago,
the region hosted a major episode of glaciations, the earliest known ice age. Fossil record from
the Cambrian Period (540 million years ago), show that most of the major groups of animals
were around what is now the Pacific Northwest. During the Ordovician Period (500 million years
ago) the first fish appeared. Plants first entered the land in the Silurian Period (240 million years
ago). While Amphibians arrived during the Devonian Period (370 million years ago), it wasn't
until the Carboniferous Period (320 million years ago) that reptiles were around. There are
preserved rocks in Eastern Washington representing each of these times periods.
It was during the Devonian period that the oceanic plate previously fixed to the
continental margin, plunged underneath the continent. This caused there to be a volcanic arc
along the coast of Eastern Washington and British Columbia. This arc developed inland and
appears to have lasted from about 350 to 280 million years ago.
During the Paleozoic Era, the continental fragments dispersed during the break up of
Rodinia slowly began to come back together. About 300 million years ago, a new supercontinent
formed called "Pangaea" which is Greek for "all lands."
During Pangaea's 100 million year span, the western shoreline curved through western
Idaho. Sediment made of mud, silt, sand and calcium carbonate would accumulate in thick layers
beneath shallow seas along the continental shelf of Eastern Washington.
The formation of Pangaea caused large-scale climatic disruption and the destruction of
terrestrial and oceanic ecosystems. Much of North America would become a windswept desert.
During this time, known as the Permian Period (245 million years ago), there would be a mass
extinction and 90% of all species on earth would go extinct. It wouldn't be until the Triassic
Period that life returned in a significant degree. The first dinosaurs appeared as well as the
earliest mammals.
Similar to the fate Rodinia, Pangaea would break apart due to the Earth's internal heat.
Around 205 million years ago, Pangaea would rupture and form the Atlantic Ocean. The breakup
began rift between what are now the western and eastern hemispheres. The rift would evolve into
a spreading center that pushed the two hemispheres apart. During this period much of the Pacific
Northwest is under shallow water. As North America was forced westward by the spreading of
the Atlantic Ocean, Washington and the Pacific Northwest became a tectonically active
continental margin. This was an enormous change for the Pacific Northwest. Previous the area
had been a quiet and passive margin. Now the new tectonic activity began to affect the edge of
the continent as the floor of the Pacific Ocean basin floundered against the western continental
margin.
200 Million Years Ago to Present Day
Around 200 million years ago, the breakup of Pangaea had an enormous effect on
Washington and the Pacific Northwest. The North American Plate being forced westward
created massive friction between the westward-moving continent and the eastward-moving floor
of the Pacific Ocean. The floor of the eastern Pacific Ocean was composed of an oceanic plate
known as the Farallon Plate. As North America moved westward, the eastward-moving Farallon
Plate was forced beneath the edge of the continent to form an active subduction zone. This
situation was entirely unstable. The subduction of the eastward-moving Farallon Plate was
unable to keep pace with the westward motion of North America. The Farallon Plate was forced
into fragmenting into several "microplates" which are called Intermontane Plate and the Insular
Plate.
The subduction in the Pacific Northwest caused major geological phenomena. The first
was the collision of exotic lands onto the ancient continental margin. In the Pacific Northwest,
many terranes (for large blocks of crust that share a common geologic history) are exotic. In
geological terms they have moved great distances from where they originated) and were added to
our land by plate collision and subduction. At least four major terranes of oceanic rock were
added to the Pacific Northwest over the last 200 million years. These were The Intermontane
Belt, The Insular Belt, The Melange Belt, and the The Crescent Belt. As these terranes collided,
the edge of the continent expanded some 400 miles westward.
The other major phenomenon that shaped Washington's history was the formation of a
series of continental volcanic arcs. These continental arcs are regions where molten rock
(magma) intrudes and moves toward the surface, often to form volcanoes. In the Pacific
Northwest, magma intruded through the added terranes formed giant plutons of granite at depth
and volcanic rocks at the surface.
Scientists have organized the last 200 million years of Washington's geological history
into four episodes. Each is named after the continental arc that dominated that time period.
The Omineca Episode (195-115 million years)
The Omineca Episode started with a chain of volcanic islands colliding with the western edge of
North America. The islands were "welded" to the edge of the continent by molten rocks that
solidified as granite. The remnants of these ancient islands, along with the granite, form much of
the Okanogan Highlands of north-central Washington and southern British Columbia.
The Coast Range Episode (115-57 million years)
The Coast Range Episode began when a second chain of volcanic islands collided along the
expanding western shoreline. These islands welded to the edge of the continent by molten rocks
that formed the largest body of granitic rocks in North America.
The Challis Episode (57-37 million years)
During the Challis Episode large regions of the Pacific Northwest were crushed, all the while
hosting a chain of volcanoes running diagonally across Washington and Idaho. At the end of this
period, a large piece of ocean floor (now the Olympic Peninsula) was uplifted and forced beneath
the edge of the continent, extending the continental margin to its modern western extent.
The Cascade Episode (37 million years to the present)
The Cascade Episode began as the Juan de Fuca Plate advanced into the area and was forced
underneath the western edge of the continent. This gave rise to a chain of volcanoes that has been
erupting here for the past 36 million years. Between 17 and 12 million years ago, great floods of
molten rock erupted from cracks in the crust of Washington and Oregon to form the basalts of
the Columbia Plateau. The modern Cascade Range has risen over just the last 5 to 7 million
years. For the last two million years, vast continental glaciers have repeatedly scoured the Pacific
Northwest, creating some of the most spectacular landscapes on the continent.