"Self-guided" Lecture
Additional information that will help you to complete your study guide is included in the descriptions of the sites we have visited on our trips (Highlights from Previous Trips). The number of questions you have about this information will depend on the depth of your background in natural history. Contact the instructor (meberle@fhsu.edu) with all of your questions as you read this material and complete your study guide.

Information for this on-line "lecture" and the summaries in the "Highlights from Previous Trips" webpages was derived from the references listed below (and listed on individual webpages), as well as from educational materials and observations at the parks and other public sites we have visited.

GEOLOGY

Geography of the Northwestern USA (see handouts)
    We will travel through several physiographic provinces on our trip, and they are labeled on the map handout. You should be familiar with the location of the physiographic features on the map. Our focus will be on the coastal ecosystems (rain forest, tidepools, sand dunes, and estuary), but we will also compare the communities on the various mountain ranges that we will cross. Information on these communities is available elsewhere within this website (Highlights from Previous Trips). What follows here is a general outline of selected general concepts and processes.
Plate Tectonics (see handout)
    The surface of the Earth is broken into several plates. Radioactive decay in the core of the Earth creates convection currents that circulate in the molten mantle below the plates. These currents pull the plates in various directions: moving toward each other (as along the Pacific Northwest coast), moving away from each other (as along the mid-oceanic ridge), and sliding past each other in opposite directions (e.g., as along the southern California coast). Mid-oceanic ridges form where convection currents diverge at the surface; basalt is deposited at the ridge, which pushes the adjoining plates apart (see handout).

    In areas where the plates are moving toward each other, trenches (subduction zones) form where the heavier basalt of the oceanic plate moves under the lighter materials of the continental plate. For example, the East Pacific Plate (which is comprised of the Juan de Fuca and Gorda plates shown on the map handout) is moving under the continental plate along the Pacific Northwest coast north of Cape Mendocino. As the subducting plate is deflected at an angle under the continental plate, it reaches a depth at which the high temperature and pressure cause it to melt. This molten material then rises to the surface, where it can flow over the surface (e.g., basalt lava flows) or solidify beneath the surface (e.g., granite batholiths) in volcanically active areas, such as the Cascade Mountains.

    The coast of a continent that is separated from a mid-oceanic ridge by oceanic crust, as occurs along the eastern coast of North America, is known as a passive margin where erosion dominates. The coast of a continent near the leading edge of the plate, as occurs along the coast of the Pacific Northwest, creates an active margin where volcanic activity and accretion dominate.

Geological Processes of Western North America (Cordillera)
    Volcanism

      Lava flows, cinders, and ash are examples of extrusive deposits, in which the molten material solidifies when it reaches the surface. Cascade Mountain volcanoes are composite volcanoes, which are built of alternating layers of flowing lava and deposits of cinders and other airborne debris.

      Batholiths (large volumes) and plutons (small volumes) are examples of intrusive deposits, in which the molten material solidifies before reaching the surface. For example, the granite peaks in Sierra Nevada were subsurface volcanic deposits that were later exposed by uplift and erosion of the overlying geological materials (see handouts).

    Extension and Compression

      In addition to volcanic activity, the movement of the plates and the underlying convection currents produce strains of extension and compression on the surface that can also create mountain ranges.

      Extension can cause elongated blocks of land to rise (mountain ranges) or fall (intervening basins). These mountain ranges tend to run parallel to each other, as seen in the north-south ranges of the Great Basin from the Sierra Nevada of eastern California to the Wasatch Mountains of eastern Utah (see handouts).

      Compression can fold and lift layers of rock into upward bulges. The Olympic Mountains of northwestern Washington and the Coast Ranges of western Oregon and northern California are comprised of uplifted material scraped from the surface of the oceanic crust as it slides under the continental plate (see handouts).

    General Sequence of Geological Events
    1. 200-80 mybp: Pangea begins to break into northern and southern continental masses.
      1. Sierra Nevada volcanic activity occurs (oceanic plate subduction) = Cordilleran Orogeny: Nevadan Phase.
      2. Volcanic activity also occurs in the region around present-day Idaho; western Oregon and Washington were an oceanic embayment.
    2. 80-40 mybp: North America separates from Eurasia and rotates clockwise, centered on the Colorado Plateau.
      1. Subduction rate increases.
        • Oceanic plate descends at a shallower angle and moves farther east before melting.
        • Volcanic activity shifts from W to E across present Great Basin region.
    3. 40-10 mybp: northern Mexico and southern California override the East Pacific Ridge.
      1. Rotation of the continent centered around the Blue Mountains (northeastern Oregon).
      2. Rate of subduction slows.
        • Oceanic plate descends at steeper angle.
        • Volcanic activity largely shifts from E to W across present Great Basin region.
      3. Uplift and extension cause faulting that creates generally N-S ranges from Sierra Nevada in California through Wasatch Mountains in Utah and south into Arizona and New Mexico
    4. <10 mybp: Miocene-Pliocene Uplift elevates western USA and Rocky Mountains.
    5. <2 mybp: Glaciers advance and retreat through Washington; mountain glaciers throughout the region also expand and retreat.

CLIMATE, OCEAN CURRENTS, and TIDES

General Weather Patterns
    Hadley Cells (see handout)

      In the tropics (about 0° latitude), air is heated and rises, which causes it to cool and release precipitation (cool air holds less water than warm air). At about 10 km in altitude, this rising air diverges to the north and south. It is pushed from below by air moving into the area of low pressure created at the surface as the air rises.

      In the subtropics (about 30° latitude), the cool air descends, and as it does, it becomes warmer and drier (warmer air can hold more water, so the relative humidity drops). This creates an area of high pressure (e.g., North Pacific High, discussed below) near the surface. This air reaching the surface diverges away from the area of high pressure to the south (back to tropics) and to the north. This circulation is complemented by a second area of rising air at about 60° latitude, which creates a northern area of lower pressure (e.g., Aleutian Low, discussed below).

    Coriolis Effect and Major Pacific High & Low Pressure Areas (see handout)

      The speed of the air rotating with the Earth is fastest near the equator and progressively decreases at higher latitudes, because air at higher latitudes covers less distance than air near the equator during the same period of time. The following explanation of the Coriolis effect applies to the Northern Hemisphere.

      As air shifts to the north, its relative speed as it circles the planet is greater than the rotational speed of the Earth at the more northerly latitude, so the faster air moving north is displaced to the right (to the east). As air shifts to the south, its relative speed as it circles the planet is less than the rotational speed of the Earth at the more southerly latitude; thus, the slower air moving south is also displaced to right (this time to the west). In the Northern Hemisphere, air moving north or south is displaced to the right.

      Therefore, air moving out from areas of high pressure is displaced to the right, which creates a clockwise flow around the center of the high pressure area. Likewise, air moving into areas of low pressure is also displaced to the right, which creates a counterclockwise flow around the center of the low pressure area.

      The North Pacific High (generally west of California) expands during the summer, as the sun and warmer temperatures shift to the north, and it contracts during the winter, as the sun shifts to the south. The clockwise rotation around the North Pacific High brings winds generally from the northwest to the coast of the Pacific Northwest during the summer.

      Conversely, the Aleutian Low to the north expands during the winter, as the sun shifts to the south, and it contracts during the summer, as the sun shifts to the north. The counterclockwise rotation around the Aleutian Low brings winds generally out of the southwest along the coast of the Pacific Northwest during the winter.

    Relative Temperatures and Precipitation

      Along the Coast: Ocean temperatures are relatively stable compared with temperatures on land, which fluctuate with changes in the amount of solar radiation during the seasons. Thus, the temperature of the air moving onto land is moderated by the ocean. During the winter, air moving onto land from the ocean is relatively warmer than the air over the land, and during the summer, air moving onto the land from the ocean is relatively cooler. Thus, the coast experiences warmer winters and cooler summers than areas farther inland (in other words, the highest summer temperature and lowest winter temperatures along the coast do not differ as much as they do at inland sites).

      In the Mountains: Temperatures at higher elevations are relatively cooler than at lower elevations nearby. Air pressure decreases as altitude increases; thus, heat is lost as air rises over the mountains, producing cooler temperatures at higher elevations.

      Image of Clouds over Mountains
      Rain Clouds Developing as Air Rises over the Magdalena Mountains, Central New Mexico
      Photograph by Mark Eberle, August 2008

      As moisture-laden air moves to the east and rises over mountains, it cools, which causes its capacity to hold water to decrease, so precipitation falls as the air moves up the western slopes of mountains in the Northern Hemisphere. As the now drier air moves back down the eastern slope of the mountain, it warms as it descends, which increases its capacity to hold water (i.e., decreases its relative humidity), so conditions are more arid on eastern slopes. Additionally (in the Northern Hemisphere), solar radiation is greater on south-facing slopes, which increases evapotranspiration and creates drier conditions than on the north-facing slopes that receive less solar radiation. Thus, at the same elevations, conditions are generally drier on the south-facing slope than on the north-facing slope, and conditions are drier on the east-facing slope than on the west-facing slope.

    Winter Rain and Summer Drought on the Pacific Northwest Coast

      During the winter, warm, subtropical moisture is pulled north into the low pressure area. It rises over colder, polar air, which causes the air to cool and lose its ability to hold moisture, generating rain. Additional moisture is lost as low pressure cells, moving generally to the east, rise over the Olympic Mountains and the Coast Range, which similarly causes the air to cool, resulting in precipitation. As much as 80% of the precipitation along the Pacific Northwest coast falls from October through March, when the Aleutian Low expands southward. In addition, air flowing into a low pressure cell accelerates (the circumference of rotation decreases), which creates the strong, gusty winds associated with winter storms.

      During the summer, warmer air holds more moisture, so only 5% of the annual precipitation on the Pacific Northwest coast falls during July and August. However, fog is produced along the coast during the summer. Air cools as it reaches the relatively cooler ocean. The temperature of water changes much more slowly than the temperature of air, which is why the ocean is relatively cooler than the air during the summer and relatively warmer during the winter. The water temperature is even cooler along the coast where summer upwelling occurs (described below). This cooling effect of the ocean on the air increases the relative humidity of the air and, at about 45°F, the moisture condenses as fog. This cool, moist air is relatively dense, so fog stays near the surface and spreads inland at night as the temperature over the land cools. This fog layer slowly dissipates as the air and land warm during the day, reducing the relative humidity. In addition, the speed of air flowing out of a high pressure zone is more constant than wind associated with low pressure, and this results in a generally lighter wind along the Pacific Northwest coast during the summer, allowing the morning fog to persist.

Ocean Currents (see handout)
    California Current and Summer Upwelling

      The Subarctic Current (North Pacific Drift) flows east across the northern Pacific Ocean and diverges north and south near the Northwest coast. The California Current is the south-flowing branch that moves down the coast of Washington, Oregon, and California. Because of the Coriolis effect, moving objects veer to the right relative to the Earth in the Northern Hemisphere (as with high and low pressure areas discussed above). Thus, during the summer, the prevailing winds and ocean currents from the north where they flow parrllel to the coast move the ocean water offshore (to the right), which produces lower pressure and lower sea-levels near the coast (within about 15 km). Deeper water moves up to fill this "void"; this is referred to as upwelling.

      Compared to surface water, the deeper water is cooler and contains more nutrients, both of which are important to coastal ecosystems. The cooler water helps to generate summer fog along the coast, which provides moisture to forest plants, in the form of "drip" from trees (see additional comments for the redwood forests in the Highlights from Previous Trips section of this website. The greater nutrient load in the upwelling supports an important phytoplankton bloom that, in turn, supports other components of the coastal marine ecosystem.

      In general, winds that generate upwelling are not as strong or reliable off the coast in the northwestern USA as they are in California, but the presence of deep, underwater canyons off the Northwest coast enhances the nutrients in the rising water (NOAA website). Variability in wind speed and direction and in physical features along the coastline affect the extent of the upwelling. The greatest upwelling where we will travel occurs in 3 areas: 1) off Tillamook in northern Oregon, 2) along the central Oregon coast between Florence and Newport (or between the northern part of the Oregon Dunes and our campsite at Beverly Beach), and 3) between Cape Blanco in southern Oregon and Cape Mendocino in northern California (including the northernmost redwood forests).

    Davidson Current

      Winds from the south during the winter create the north-flowing Davidson Current over the continental shelf. The California Current is pushed off-shore (and continues to flow south). The Davidson Current flowing from south to north helps to moderate winter temperatures along the Pacific Northwest coast.
Waves
    Waves are a vertical oscillation that gives the appearance of a horizontal movement of the water. The height of waves increases as wind speed, duration, and distance increase. The heights of swells along the Pacific Northwest coast are typically less than 2 m during the summer. As waves approach the shallower depths along the shore, their energy is dissipated. They become slower and steeper until they break, perhaps once or perhaps several times before they reach the shore, depending on the vertical contour of the sea floor. The main threat from waves to attached organisms along the shore is from the logs or other objects that can be hurled against them by the waves. The higher the wave, the greater the energy that can imparted to the log or other object.
Tides
    The gravity of the moon exerts the greatest pull on water in the oceans closest to it. The moon also pulls the Earth and pulls the oceans on the opposite side of the planet to progressively lesser degrees. Thus, bulges of high water levels are created in the oceans closest to and farthest away from moon = high tides. Depressions of low water levels are created on Earth at 90° angles to the position of the moon = low tides. The gravity of the sun also exerts gravitational pull on the water and the planet, but to a lesser degree than moon.

    Generally there are 2 high tides and 2 low tides per day as the Earth rotates under the bulges and depressions, but the heights of the 2 low tides or the 2 high tides are not equal to each other. The orbital plane of the moon is not the same as the rotational plane of the earth, so the gravitational pull varies. As a result, the general sequence of the 2 high and 2 low tides during a 24-hour period is: 1) higher high tide, 2) lower low tide, 3) lower high tide, and 4) higher low tide. The lower low tides we will encounter will be early in the morning.

    "Spring Tides" have nothing to do with seasons; they are the highest and lowest tides that occur when the sun and moon form a roughly straight line with the Earth. During a new moon, the sun and moon are pulling from same side. During a full moon, the sun and moon are pulling from opposite sides. "Neap Tides" (moderate tidal changes) occur when the sun and moon are at right angles to the Earth (during quarter moons).

    The greatest differences in tides are during June and December, because these are when the Earth is closer to the sun during its elliptical orbit. Tides also vary with surface winds, atmospheric pressure, and other local phenomena (i.e., we want light winds and high atmospheric pressure when we visit the tidepools). Thus, tide tables offer only predicted tide levels. In the USA, tide heights are reported relative to the mean lower low tide (= 0 feet). The best low tides for viewing tidepools are below average levels (e.g., -1.6 feet).

COMMUNITIES

Montane Communities (see Highlights from Previous Trips for more information)
    Temperature-Precipitation Gradients

      As noted above, air temperature decreases as elevation increases, and more solar radiation reaches southern exposures of mountains in the northern hemisphere. Also, precipitation increases as elevation increases, but more precipitation falls on the western slopes than on the eastern slopes of mountains. Thus, temperature and precipitation form gradients along changes in elevation, which leads to changes in the community. However, these transitions can be obscured by local features, such as soil type. Dry conditions at low elevations limit tree growth in interior forests (excluding the coastal rain forest), and cold conditions (short growing season) limit tree growth at high elevations. Drier conditions on an eastern slope shift the communities higher than on a western slope. Similarly, as latitude decreases (i.e., as you move south), similar communities occur at progressively higher elevations. For example, treeline at Mount Rainier, Washington (47°N latitude) is about 1,800 m, but treeline at Mount Wheeler, Nevada (39°N latitude) is about 3,000 m.
    Fire

      Relatively frequent ground fires are important in the drier, interior forests, especially at lower elevations, where pines often dominate (see table below). Fire limits undergrowth and enhances the reproduction of some species, such as lodgepole pine and sequoia (see additional comments under "Rocky Mountains" and "Sierra Nevada Forests" in the Highlights from Previous Trips). Pines and other trees adapted to relatively frequent fires tend to lose their lower branches as they grow, which helps to prevent the fires from spreading to their crowns. Sequoias and redwoods also have thick, resin-free (less flammable) bark. Subalpine forests and coastal rain forests have lower frequencies of fires (see table below). Subalpine trees often retain their lower branches, which makes them susceptible to damage by fire. However, the pyramidal shape of subalpine trees is more effective than the rounded tops of pines for shedding snow.

      Fire-return intervals over several centuries for forest communities in Oregon and Washington from a literature summary by James K. Agee (1993. Fire ecology of Pacific Northwest forests. Island Press, Washington, D.C).
         

        Oregon


        Washington


        Forest type
        ha

        Fire cycle
        (years)
        ha burned
        per year
        ha

        Fire cycle
        (years)
        ha burned
        per year
        woodland
        1,001,000
          25
          40,000
        12,000
          25
             500
        ponderosa pine
        3,142,000
          15
        209,400
        1,438,000
          15
        95,800
        mixed conifer
        399,000
          30
          13,300
        504,000
          50
        10,100
        lodgepole pine
        757,000
          80
            9,400
        211,000
        110
          1,900
        Douglas-fir
        4,444,000
        150
          29,600
        3,068,000
        217
        14,100
        cedar/spruce/hemlock
        292,000
        400
               700
        1,291,000
        937
          1,400
        subalpine
        1,075,000
        800
            1,300
        935,000
        500
          1,900
    Wind

      Strong, gusty winds blow down trees during winter storms in temperate rain forests. The opening created by the fallen tree permits more sunlight to reach below the canopy to the forest floor. The downed tree also provides a growth platform ("nurse log") to support saplings and other organisms as decomposition slowly recycles the nutrients contained in the nurse log (see additional comments under "Olympic National Park/Coast Range" under the Highlights from Previous Trips). The spiral growth of a tree (visible on snags or fallen trees without bark) improves its flexibility and strength to resist wind damage.
    Nutrients: Relationships with other organisms that help plants to obtain nutrients.
    General Montane Communities
        (see Highlights from Previous Trips and handouts for more information)

Other Coastal Communities - Zonation
    (see Highlights from Previous Trips and handouts for more information)

Study Guide

Separation line
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