Page 26 - (DK Eyewitness) Travel Guide - Pacific Northwest

24  INTRODUCING THE P A CIFIC NOR THWEST

Geology of the Pacific Northwest

No small amount of geological activity has shaped the
present-day Pacific Northwest. One hundred and fifty
million years ago, much of the western part of the
region was at the bottom of the sea. Over the eons, the
North American continental landmass crept westward
and collided with the landmass moving eastward across
the Pacific Ocean, forcing the Earth’s crust upward and
creating the coastline of the Pacific Northwest as we
know it today. Meanwhile, the eruption of volcanoes
thrusted up mountain peaks, and glaciers and ice
sheets advanced and retreated, carving out deep
gorges and canyons. As recent volcanic eruptions and
earthquakes in the area attest, the Pacific Northwest is
still a geologically active region, and its topography will Washington’s Mount Rainier – the most
continue to change as a result.
active volcano of the Cascades

Fossil records are found throughout the
Pacific Northwest, with its sedimentary rock
bearing traces of plant, marine, and animal
life from as long ago as 136 million years.
The world-renowned John Day Fossil Beds
National Monument in Kimberly, Oregon,
and the fossil beds at Burgess Shale near
Field, British Columbia, are both extensive
repositories of this ancient past.

Sedimentary Rock
As the Pacific plate periodically lurched eastward,
sedimentary rock from older coastal mountains was
uplifted to form the peaks of the Rocky and Cascade
mountain ranges. Layers of the sedimentary rock,
such as sandstone and shale, that were formed
about 15 to 20 million years ago can be seen when
visiting the ranges.
Volcanoes such as Mount St. Helens are formed when a plate
descends (subducts) beneath another plate and it begins to melt.
The molten rock rises to the surface to form a volcano. In the Pacific
Northwest, volcanoes began erupting about 55 million years ago.
The Cascade Mountains in Oregon and Washington, the Blue
Mountains in Oregon, and the Olympic Mountains in Washington
are in the Ring of Fire, a zone of volcanic activity that partially
encircles the Pacific Ocean.

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