As the continents have moved through time, they have repeatedly collided to form “supercontinents.”   Most of the rocks that make up continents are insulators -- they are reluctant to transfer thermal energy.   Eventually, heat builds up beneath the continent.  The continental crust swells, stretches, and finally ruptures.  New ocean floor begins to build within the rupture zones.  Fragments of the supercontinent spread as the ocean plate grows along a new seafloor spreading center.

 Because the Earth is a sphere, the moving continental fragments inevitably reassemble about every 500 million years.  As we will see, the creation and destruction of giant continents has played a major role in the geologic history of Washington and the Pacific Northwest.

The modern Atlantic Ocean is spreading at the expense of the Pacific.  As North America moves westward, the Pacific Ocean basin is getting smaller along subduction zones (convergent plate boundaries) under North and South America, and Japan, as western North America and Asia get closer together. Sometime in the future the Pacific ocean will close completely and Asia and North America will collide to form yet another supercontinent.

Before Pangaea, yet another supercontinent called Rodinia existed between 1.2 billion and 750 million years ago. It also formed from the accumulation of isolated continents, only to fragment 350 million years after its formation. There is growing evidence that even older supercontinents predated Rodinia early in Earth’s history.

 This supercontinent cycle – the assembly, rupture, breakup, spreading and reassembly  -- has left an indelible record on the geology of the Pacific Northwest.  We will consider the impact of the supercontinent cycle as we explore Washington’s geologic history.

Geologists think of time very differently from most people.  The Earth is nearly 4.7 billion years old.  Washington’s geologic history goes well beyond a billion years.   The most recent plate tectonic cycle that built the Pacific Northwest began 200 million years ago.  Clearly, we need a different type of calendar to order geologic events that occurred so long ago.

The geologic time scale has evolved over the last 200 years as geologists began to order events in Earth history.  The geologic time scale developed in the 1800’s based on the history of life preserved in fossils.  Each division of the time scale marked significant changes in the fossil record, such as the extinction of certain life forms and the appearance of new ones. 

Since the 1950’s, the modern science of geochronology has used the decay rates of radioactive isotopes to put absolute ages on the geologic time scale.  The time scale is a working document, often amended in detail as our ability to date rocks improves.

As we discuss Washington’s geologic history, we will make frequent reference to the periods of the geologic time scale.  This scale will help you keep the chronology of our ancient history in order.

The modern geologic time scale widely used in North America.  (Image: Geological Society of America)