Welcome to Pacific Northwest Geology. The topics of this week's lecture are:

1. Geologic activity the Pacific Northwest today
2. Landscape regions of the Pacific Northwest
3. How the landforms and landscape regions of today originate from the geologic past
4. Geologic time -Relative and Absolute

Related Basics Pages: Absolute Ages of Geologic Materials; Stratigraphy & Relative Ages
Related Focus Pages: #1--Landscape Regions and Landforms of the Pacific Northwest

1. Geologic activity of the Pacific Northwest today

If you compare the landscape of the Pacific Northwest to Florida or North Dakota, you immediately see that the Pacific Northwest is more geologically active. The Pacific Northwest exhibits the following geological processes, which range in pace from gradual and everyday to abrupt and, for humans, potentially catastrophic:

• Sideways movements of the crust.
• The moving North American tectonic plate stresses and strains as it builds towards earthquakes.
• Up and down movements of the crust.
  • Partly from tectonic plate motions but also caused by other processes inside the earth.
• Active volcanoes.
  • We have over a dozen active volcanoes, Each and every one of them will erupt again in the future.
• Earthquakes.
  • Small earthquakes happen every day in the Pacific Northwest. Every so often, totally catastrophic earthquakes occur here.
• Mountain ranges that are still rising, uplifting.
  • At the same time, erosion is eating away at them.
• Weathering.
  • Rocks and minerals at the Earth's surface are being chemically altered, discolored, and physically broken apart.
• Streams (creeks, rivers).
  • Streams erode the land, occasionally flood, transport sediments, and create new sediment layers.
• Glaciers.
  • Like streams except in a different way, glaciers erode the land, transport sediments, and leave distinctive sediment deposits on Earth's surface.
• Shorelines, lakes, seas.
  • Waves, tides, currents, and underwater landslides erode, transport, and deposit sediment.
• Landslides.
  • From soil creep to sudden, giant avalanches and slides, any hill or slope is subject to gravity trying to pull it down.

The geological activities listed above can be thought of as the result of several forces that in some ways oppose each other and in some ways work together:

• the force of the internal heat of the earth
  • which causes volcanoes to erupt, mountain ranges to lift up, and earthquakes to shake and deform the earth's crust
• the force of heat from sunlight and the force of earth's gravity
  • which work to erode down and flatten the mountains, volcanoes, hills and ridges. Heat from the sun drives the weather systems of the earth, creating the wind, rain and snow that form rivers, glaciers and waves.

The land around you is buzzing with the geological activity that results from these forces acting on the earth. The hillsides, rivers, and glaciers of the Pacific Northwest are constantly demonstrating that the earth is anything but stable.

The goals for this class are to develop your understanding of the scientific explanation of geologic activity in the Pacific Northwest, to enable you to gain insight into how the geology of the Pacific Northwest today derives from the geologic history of the region, and to learn how that geologic history has unfolded chapter by chapter.

2. Landscape Regions of the Pacific Northwest and overview of the geology of the Pacific Northwest today, as seen in its landscape regions

There are various landscapes around the Pacific Northwest, which we shall refer to as landscape regions. Go to Focus Page #1--Landscape Regions to read a classification of the landscape regions of the Pacific Northwest and an inventory of the geology of the Pacific Northwest today in terms of each of its landscape regions.

3. How the geology and landscape regions of today originate from the geologic past

The geology you see in the Pacific Northwest today is the result of its geologic history. Working from today back into the past, we can identify 10 key stages in the geologic history of the Pacific Northwest:

1. Recent activity, after the Pleistocene Ice Ages (0 to 11,700 years before present, or BP)
   (Note that the absolute time span of the Holocene epoch is often listed as 0 to 10,000 years ago,
   based on less well-calibrated radiocarbon ages from older research along with rounding off.)
2. Pleistocene Ice Ages (11,700 to 2.6 million years BP)
   (Note that you may see this listed in some places as 10,000 to 1.8, 2.0, or 2.4 million years ago,
   based on older research - a good example of more refined measurement and scientific advance.)
3. Neogene plate convergence, tectonics and volcanism, including Cascades arc continuing, Columbia River Basalt erupting, and Yellowstone hotspot track forming (2 - 23 million years ago, or Ma)
4. Paleogene most recent large terrane accretion, strike-slip tectonics, rapid sediment accumulation in basins, igneous intrusions and volcanoes all the way inland to Montana, Wyoming, and South Dakota (23 - 66 Ma), and subsequent initiation of Cascades volcanic arc
5. Cretaceous pate convergence, terrane accretion and orogeny (66 - 145 Ma)
6. Jurassic initiation of subduction and terrane accretion (145 - 201 Ma)
7. Late Proterozoic to Triassic passive plate margin (201 - 600 Ma)
8. Late Proterozoic rifting of the continental margin (600 - 700 Ma)
9. Middle to Late Proterozoic Belt Supergroup (700 Ma -1.5 billion years ago, or Ga)
10. Archean to Early Proterozoic Continental Basement (1.5 - 3.5 Ga)

(Reminder: BP means years before present; Ma means millions of years ago; and Ga means billions of years ago.)

As an example of the present-day landscape originating from its geologic history, let's look at the greater Puget Sound area. If you live in the Puget Sound region, the land you see results only from stages 1 through 5 on the list above. That is because the Puget Sound area, including the Cascade Range and Olympic Mountains, was added to North America in the last 120 Ma. Before that the west coast of the Pacific Northwest was roughly east of where the Cascades Mountains are now.

The rocks of the Cascade Range and Puget Lowland began being added to North America during stage 5. These landscape regions rearranged and shuffled closer to where they are now as a result of stage 4.

The Cascade Range were built up into a volcanic mountain range more similar to how they are today, and the Olympic Mountains formed to the west of the Cascades, during stage 3.

All of this Puget Sound region landscape was extensively eroded, and a thin veneer of new sediment layers was added, during stage 2. Also during this interval the modern volcanoes of the Cascade Range, such as Mt. Rainier, started forming.

Finally, Puget Sound formed when the sea entered the troughs left by the last giant Ice Age glacier, and all of the surrounding landscape has undergone recent, local geologic modification, such as erosion by small alpine glaciers, volcanic eruptions, and so on, during stage 1.

If you live in Spokane, it is obvious that the landscape around you is quite different from the landscape around Puget Sound. The land you see around Spokane originates from a geologic history that goes all the way back to stages 9 or 10 on the list above (whether we include 10 is open to further geologic research in the area around Spokane). The land from Spokane and east of there across the Rocky Mountains has been part of North America since long ago in geologic time, over a billion years ago. This contrasts with the land around Puget Sound, which was added to North America only in the last 120 million years or less.

In sum, even though landscapes tend to show most prominently the effects of recent geological activity, they are also the result of longer geological histories, which contribute in various ways to the nature of the present-day landscape.

4. Geologic Time - Relative and Absolute

As you have seen in this lecture so far, references to geologic time must be made in any discussion of the geology of the Pacific Northwest. Therefore, we shall finish week 1 by studying the geologic timescale, and how geologists determine the geologic age of rocks, fossils and geologic events.

Geologists analyze geologic time in two different ways: relative and absolute.

Relative geologic age is how geologic rocks and events fit into an age sequence relative to other geologic rocks and events, such as this layer of rock came first, followed by this lava flow, and topped off by this layer of gravel.

Absolute geologic age is how old something is, in years, thousands of years (ka), millions of years (Ma), or billions of years (Ga). To say that a rock layer is 66 Ma is to give it an absolute age.

Relative Geologic Ages

To determine the relative geologic age of a layer of rock or a geologic event, the principles of relative geologic age determination are used. Go to Stratigraphy & Relative Ages to see the principles of relative geologic age determination.

Here is an example of how to determine relative geologic age. The following sequence of sediments is found throughout much of the central Puget Sound area:

The geologic principle of superposition allows you to deduce that the layer on the bottom is the oldest layer of the sequence, the clay layer is younger, the cross-bedded sand and gravel younger yet and the unsorted, unbedded sediment layer on top formed last.

Further analysis of the actual sediment layers represented above would reveal that the Puget Sound area underwent a change from

1. streams and rivers meandering through flood plains toward the sea, which deposited the thin beds of silt, sand and clay, to
2. extensive lakes that resulted from the streams being dammed by an approaching continental glacier; clay was deposited in the stagnant water of the lake; to
3. a zone of sand and gravel being deposited by glacial meltwater in front of the approaching glacier, to
4. the presence of the glacial ice itself, which deposited glacial till consisting of unsorted, unbedded sediments.

By putting the sequence of sediment layers in the right order, the chapters of geologic history that they represent are also placed in the order they occurred, and the story told by the sediments about how environments changed over time can be reconstructed in correct sequence.

Absolute Geologic Ages

Absolute age is how old something is in terms of numbers of years ago. For example, to say that a lava flow is 17.5 Ma is to specify its absolute geologic age.

The most commonly used method of determining an absolute geologic age is to analyze the radioactive parent isotopes in the minerals of a rock, and the stable daughter product isotopes that are produced by the decay of the radioactive parent isotopes. To see what these terms mean and to learn more about the methods of determining absolute geologic ages, see Absolute Ages in Geologic Materials.

As an example of absolute age determination in geology, fossil plant stems and leaves from the bottom layer in the sequence shown above (the sediments deposited by rivers) have yielded carbon-14 ages of between 20,000 and 30,000 years BP. Fossil stumps in the top layer (the till deposited directly from glacial ice) have yielded carbon-14 ages of between 14,000 and 16,000 years BP. This indicates that the Puget Sound area was unglaciated between 20,000 and 30,000 ago, and then the last large glacier came across the area and retreated from the Puget Sound area between approximately 14,000 and 16,000 years ago.

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