Saturday, May 30, 2009

The Vortex2 Fiasco

For the past three weeks, a team of meteorologists and storm chasers has been zigzagging across the Great Plains in pursuit of severe thunderstorms and tornadoes; with 2 days to go in May, they've hardly come close to a thunderstorm and haven't seen a single tornado. Christened "Vortex2," the project was designed to study the formation and dynamics of these dangerous storms, with the goal of improving forecasts in the future.

This expensive project, while established to collect valuable scientific information, was, unfortunately, wed to entertainment programming on the Weather Channel. This dual commitment has kept much of the team together, driving long distances in a futile attempt to intercept the storms. Had the project been purely scientific, I imagine that various sub-teams would have been stationed across the Plains, at various latitudes; since the forecasting of these storms remains far from accurate, the sub-teams would have been within much shorter range, allowing them to position their equipment when essential conditions developed.

Of course, bad luck with regional weather patterns has been part of their problem, forcing the media participants to report on daily routines, minor injuries, historical markers, geographic scenery and other non-weather stories. My recommendation: position the team in Weld County, Colorado, during the month of June; plenty of supercell storms are almost guaranteed to pass nearby!

Friday, May 29, 2009

The Shadow Bird

Last evening, a pair of gray catbirds moved through the undergrowth of our woodland border. Like their cousins, mockingbirds and thrashers, these birds are known for their "songs" of varied phrases and notes but are named for the cat-like mews that they deliver from their secluded haunts. Unlike their cousins, gray catbirds stalk the shadows, preferring dense shrubbery, tangles and the lower branches of trees; their slate-gray plumage, marked only by a black cap and a cinnamon patch under the base of their tail, suggests a fondness for shady habitat.

Arriving in the Midwest by late April or May, gray catbirds pair off and soon build a bulky nest of sticks among the shrubs or thickets. They will feed on insects throughout the nesting season but, by late summer, develop a fondness for ripening berries. Come October, they head for wintering grounds which stretch along the Southeast Coast, from the Carolinas to Florida, and along the Gulf Coast, from Florida to Central America.

Thursday, May 28, 2009

Maps, Streams and Topography

Since my pre-teen years, I have been fascinated by maps. I can still recall how that first geography text, with its simplified images of the Nile and Amazon, instilled a sense of adventure in my soul. At about the same age, I began to draw maps of our neighborhood forest, with its trails, cliffs and creeks, giving me an early appreciation for regional topography. In later years, when I set out for nature preserves and wilderness areas, I graduated to the use of topographic maps and, in the production of our outdoor guides, took special delight in sketching maps for each location.

Those of us who appreciate the varied landscapes of this planet know that a map of rivers and streams provides the best clue to a region's topography. A dendritic pattern suggests a dissected plateau while parallel, meandering streams indicate a flat plain. Creeks that end in ephemeral lakes descend the walls of a basin while those that abruptly disappear suggest karst topography with sinkholes, caves and underground streams. Finally, mirrored, branching networks flank a divide, revealing the presence of a ridge or mountain range. Always seeking low ground, streams guide our eyes across the landscape, from lofty summits to the vast, level sea.

Wednesday, May 27, 2009

Spring Fades to Summer

By late May, spring is fading to summer. The trees are now in full leaf and our multi-colored spring landscape has given way to various shades of green. Hot, humid weather is settling into the Midwest and, in concert, wild creatures adopt their summer behaviors.

Birdsong will become more subdued as most species, now limited to summer and permanent residents, retreat from the mid-day heat. Snakes, lizards and turtles are now more active (and easier to find) while their mammalian neighbors have become increasingly nocturnal. Even humans, equipped with air-conditioned homes and offices, avoid the hot, afternoon sun. Only the insects, energized by summer weather, are oblivious to the heat and humidity.

This summer pattern will build through July and last into September. Throughout the period, early morning and evening hikes will be far more productive for wildlife viewing (for reasons mentioned above) and we will long to be rescued by those Canadian fronts that usher cool, dry air into our Midwestern steam bath.

Monday, May 25, 2009

May at the Marsh

A mild and showery afternoon discouraged human visitors at our local wetland preserve but, if anything, seemed to increase the activity of its natural residents. Cricket frogs chirped from the shallows, painted turtles lounged on half-submerged logs and green-backed herons moved about the marsh, searching for good fishing spots. A bluebird struggled with a large caterpillar on the graveled pathway, eastern kingbirds chased one another across the wet meadows and great crested flycatchers called from the woodland border.

Settled in for the summer, tree swallows guarded their nest boxes or strafed the seasonal lake, feasting on insects. A great blue heron stalked the shallows, indigo buntings flashed among the thickets and a quintet of turkey vultures soared overhead. But the highlight of today's visit was a mass exodus of tiny tree frogs, making their way from their wetland hatcheries to moist recesses of the surrounding woods; thousands were crossing the graveled trail, potential victims of unwary hikers or opportunistic birds and snakes.

Regardless of the season, wetlands never fail to provide plenty of entertainment for the naturalist. And for those of us who appreciate solitude, foul conditions can make the visit even more enjoyable.

Sunday, May 24, 2009

Prairie Shorebirds

Shorebirds include a diverse group of species that generally winter on southern coasts; they include sandpipers, plovers, phalaropes, godwits, curlews, stilts and avocets. Since most Americans encounter them on vacations to the beach, they associate these birds with coastal ecosystems. In fact, most summer and breed on the Great Plains and Arctic tundra and are common migrants through the Heartland; a number of species are specifically associated with the vast prairie grasslands of North America.

Upland sandpipers are common on the tallgrass prairie of the Central Plains and Upper Midwest, often seen perched on fence posts. Killdeer, the most common and widespread inland shorebirds in North America, are noisy plovers that inhabit farmlands, airports and urban parks; they are especially common on mudflats that line our reservoirs. Near streams and lakes, they are often joined by spotted sandpipers, which bob their way along the waterline and escape to cover on fluttering, stiffly-held wings.

The shortgrass prairies of the western Mountain States are home to long-billed curlews, mountain plovers, snowy plovers and American avocets; the latter favor salt flats and ephemeral pools, where they sweep the shallows with their long, up-curved bill. Wetlands of the western prairies also attract willets, black-necked stilts and Wilson's phalaropes. Finally, marbled godwits, among our largest shorebirds, inhabit the grasslands of the Dakotas, Montana and Canada's Prairie Provinces.

Saturday, May 23, 2009

A Cap on the Plains

As I left Denver this morning, a weak upslope flow produced a layer of low, gray clouds above the city. Banners of virga dipped below the ceiling but only occasional showers reached the ground. Beyond the Palmer Divide, the sky cleared and sunny conditions prevailed across eastern Colorado.

From the Kansas line to western Missouri, white, "popcorn" cloud formations dotted the blue sky, increasing in number and size as I continued to the east; however, there were no signs of thunderstorm development. Such conditions are typical of a "capped atmosphere," when overlying high pressure and warm air aloft prevent the upward expansion of storm clouds. Since there was no front or upper level low to provide lift, the warm, humid air was trapped in the lower atmosphere and only a few showers appeared across the Plains.

Southeast of Kansas City, a few storms had clearly broken through the cap, as evidenced by the development of anvil clouds, produced by strong winds at higher altitudes. Once punctured, the cap begins to break down (at least locally) as lift is provided by outflow from these initial storms.

Friday, May 22, 2009

The Natural History of Crust

The Earth formed about 4.6 billion years ago and the age of its crust varies greatly across the globe. The basement rock of the Continents is Precambrian rock (igneous and metamorphic), more than a billion years old; indeed, the oldest discovered bedrock is on Greenland, dating back 3.8 billion years. Most of this ancient crust has since been covered by sedimentary and volcanic rocks of more recent eras but it is still visible under three conditions: where it has been uplifted to form mountains, where rivers have cut deep canyons through the overlying sediments or where glaciers have scoured away the more recent deposits. Eventually, even the ancient continental bedrock is recycled, caught in an endless sequence of rifting, collision, uplift and erosion.

Oceanic crust, on the other hand, is far younger. After forming at mid oceanic ridges, this crust moves outward and is eventually recycled at subduction zones, where it dips into oceanic trenches. The oldest oceanic crust on Earth is about 180 million years old, having formed along the East Pacific Rise (in the southeast Pacific) and soon to disappear beneath the North American Plate off eastern Siberia. Since the Continents have been pushed about and bisected by the opening and closing of oceans, the age of the ocean crust, determined by deep sea drilling, provides evidence regarding the history of continental drift. For example, the Atlantic began to open about 160 million years ago, rifting North America from Africa; we know the timing of this event by studying the age of the ocean crust off the opposing coasts of the two continents (it is, indeed, of Jurassic age). Proceeding inward (away from the continents) toward the Mid Atlantic Ridge, the ocean crust is gradually younger and, at the Ridge, new crust is forming today. In the case of Australia and Antarctica, their opposing, offshore oceanic crust is Eocene in age, confirming that their separation, via the Southeast Indian Ridge, began about 50 million years ago.

Thursday, May 21, 2009

The Antarctic Plate

When the Tethys Sea opened, 200 million years ago (MYA), Pangea was split into Laurasia (the northern Continents) and Gondwanaland (the southern Continents); future Antarctica, as one might expect, was a component of Gondwanaland. Africa split from its neighbors as the South Atlantic opened, India broke free 80 MYA and Australia drifted away about 55 MYA. Finally, losing its connection with South America, Antarctica settled over the South Pole some 20 MYA.

All of this rifting has resulted from the opening of oceans and seaways (a process that continues today). The rift zones, known as oceanic ridges are not continuously active and, today, the Antarctic Plate is only rifting from the Pacific Plate (along the Pacific-Antarctic Ridge) and from the Australian Plate (along the Southeast Indian Ridge); its borders with the Nazca, South American and African Plates are characterized by a complex of compression faults, transverse faults and subduction zones. Since these margins lie deep beneath the Southern Ocean, they are not yet fully mapped and understood.

For much of the post-Pangea period, Antarctica remained in the Subtropical and Temperate Zones; it was thus home to a wide variety of plants and animals that inhabited these climatic regions. Then, as it drifted to the South Pole, Antarctica's fauna changed and, today, fossils of its warmer past lie entombed beneath its thick coat of ice and snow.

Wednesday, May 20, 2009

Planting Trees

There are few activities more productive than planting a tree. These long-lived plants beautify our property, provide food and shelter for wildlife, consume carbon dioxide, release oxygen and, eventually, offer shade from the summer sun.

Since most species take many years to mature, some might question the effort, pointing out that we will be long gone before the tree is full sized. But planting a tree is an expression of faith and an act of generosity. Like raising a child, it is the nurturing that counts and we get the most enjoyment from its growth and development.

So keep planting those trees. They are better for the environment than that manicured lawn and require a lot less work. Besides, they are gifts to future generations of humans and wildlife.

Tuesday, May 19, 2009

Evening Light Shows

Our Littleton, Colorado, farm sits on the west wall of the South Platte Valley and commands a broad view to the southeast. On almost every warm day, from May through August, we are treated to an evening light show, as thunderstorms move above the Palmer Divide and adjacent High Plains.

Through the night, cool air descends from the Front Range, pooling along the valley floors of the South Platte River (north of the Palmer Divide) and Fountain Creek (to its south). By late morning, as the sun begins to warm this air, it flows upward across the terrain, picking up moisture from Piedmont lakes and mountain forests. Cooling and condensing as it rises, this flow ignites afternoon thunderstorms above the Front Range that drift eastward as they build into the prevailing, westerly winds.

Since the South Platte and Fountain Creek Valleys point toward the west end of the Palmer Divide and adjacent foothills, the Tarryall and Platte River Mountains are often a focal point for thunderstorm generation and the late-day parade of storms is generally most abundant along and near the broad ridge of the Divide. Our view is thus perfect for watching the evening displays of lightening as these storms drift eastward into Elbert and Lincoln Counties.

Monday, May 18, 2009

Grace on the Wing

Natives of North America and Eurasia, barn swallows are among the most beloved of songbirds, admired for their attractive plumage, graceful flight and voracious appetite for insects. Common above fields and wetlands, they fly low to the ground, feeding on the wing, and are easily identified by their navy-blue back, rusty throat, buff-colored chest, tapered wings and deeply-forked tail.

Before human settlers arrived, barn swallows nested on rock ledges or on thick tree limbs, constructing a cup of mud and grass. Today, they favor man-made structures, building their nest on wooden beams in the corner of barns or under bridges. Less colonial than most swallows, these attractive birds are usually seen alone or in small flocks and only a few pair generally nest in the same structure.

Arriving in late April, barn swallows raise 3-6 young and, by early October, depart for Central and South America (some migrate as far as Argentina); Eurasian birds winter in Africa or southern Asia. Traveling by day, they feast on insects as they go, stopping at night to rest. One can only begin to imagine the massive number of potentially harmful and destructive insects that these agile hunters consume in the course of a year!

Saturday, May 16, 2009

Thunderstorm Highway

Late yesterday afternoon, a series of severe thunderstorms had lined up along I-70, from Kansas City to St. Louis, reflecting a cold front that stretched across the State. The first storm hit Columbia about 5:30 PM and it looked as if we were in for recurrent episodes throughout the evening.

This initial storm was, indeed, severe, with hail, strong winds and heavy rain. But, as the front inched southward and other storms arrived from the west, their intensity gradually waned; intermittent, torrential rain and frequent lightening were the only threats.

This scenario is typical of clustered, training thunderstorms. The leading storm, feeding on the warm, humid, unstable air ahead of the front, is most severe. However, trailing storms encounter air that has been cooled (and stabilized) by outflow and precipitation from the initial thunderstorm. As a result, they weaken and are less likely to produce large hail and tornadoes. The latter are most often encountered in isolated "supercell" thunderstorms; like compact, terrestrial hurricanes, they feed on the hot, humid air that surrounds them and are not subject to the cooling effect of nearby storms.

Friday, May 15, 2009

Nature's Beauty

For most humans, the beauty of nature is represented by spectacular sunsets, magnificent vistas, colorful foliage and graceful wildlife. But her true beauty lies in her diversity and in both the interrelationship and interdependence of her physical forces, chemical processes, biologic systems and countless life forms.

Nature is both evolution and extinction, life and death, growth and decay. She is a mosaic of heat and cold, light and darkness, fragility and awesome power. Nature offers sunshine and hurricanes, swans and maggots, rain forests and pond scum, whales and bacteria, distant galaxies and the molecules of life.

Nature is neither good nor bad, benevolent nor judgemental, sentimental nor discriminatory. We enjoy her gifts and are subject to her whims. We are, after all, a component of her beauty, just like the golden eagle and the dung beetle.

Wednesday, May 13, 2009

The Middle Age Syndrome

Middle age is a difficult period of life for many (if not most) humans. Youth has slipped away and most of life's mysteries have been answered; how and when we will die is about the only one left. Passion, diversity and freedom have given way to boredom, routine and nagging responsibilities. We dwell on the past, dredging up lost opportunities and second guessing our decisions. In many cases, this wave of loss and regret leads to impulsive and destructive behavior: marital problems, affairs, alcohol abuse, poor job performance and self-absorption.

Since we cannot restore our youth or undo the past, the remedy for this middle age syndrome is to take on new challenges and involve ourselves in diverse activities that inject enthusiasm, socialization and a sense of purpose into our lives. Athletic activities, volunteer programs, educational courses (foreign languages, art, music, writing or other areas of interest), travel, teaching and a variety of new hobbies can reinvigorate our mid-life doldrums. While some may profess no time for such diversions, most soon realize that enthusiasm creates its own opportunity and that wasted time was part of their problem from the start.

As far as we know, we only have one life to live. We might as well make the most of that brief span, middle age included.

Tuesday, May 12, 2009

Tulip Tree Aviary

The large tulip tree (yellow-poplar) in our backyard has been blooming for the past two weeks and its showy yellow and orange flowers (and the insects that zoom among them) attracted a large assembly of birds last evening. Northern and orchard orioles dominated the scene, joined by warbling vireos, a pair of great crested flycatchers, several ruby-throated hummingbirds, blue-gray gnatcatchers and a few warblers (Tennessee and Blackburnian). All of the noise and activity also enticed more common residents to the scene, including cardinals, house wrens and gray catbirds. Though absent last evening, cedar waxings often feast on the petals of these tulip tree flowers and will likely visit in the coming weeks.

While especially spectacular last evening, such bird congregations are not unusual. Drawn to favorite food sources (nectar, berries, insects), certain species descend on a site and, like those noisy gulls on the beach, soon attract the attention of other birds. These large, mixed feeding groups benefit from the discoveries of one another and offer a certain degree of protection from predators (more eyes provide a better warning system). Then again, they might just enjoy the company!

Sunday, May 10, 2009

Maternal Devotion

In the mammalian world, there is nothing stronger than the maternal bond. A product of genetics, gestational interaction and childhood nurturing, it is especially apparent in humans, disrupted only by certain psycho-social disorders or premature death. By comparison, the father-child and spousal bonds are distant competitors, lacking significant components of the mother-child relationship.

Today we celebrate the love and achievements of mothers across the globe, acknowledging their vital role in our present lives and throughout the course of human history. Few would argue that motherhood is both the most rewarding and the most difficult challenge that any human will ever experience. A mother's devotion, or lack thereof, determines the trajectory of our lives and, in a broader sense, the welfare of mankind.

Happy Mother's Day to all who accepted this awesome responsibility!

Saturday, May 9, 2009

Flight Delays

Perhaps it has been the cool, wet weather, or my aging sight and reflexes, or just plain bad timing, but the spring bird migration has been underwhelming this year. Despite my daily, evening vigil in the backyard, the number and variety of birds visiting our neighborhood have been well below the norm. Even our reliable summer residents have been under represented.

Checking with other birders in our area, I have found that their experience has been similar; the populations of migrant warblers and flycatchers seem to be most reduced. Perhaps their travels have been delayed by recent heavy storms across the south and, hopefully, sightings will pick up in the coming weeks. Today's spring Audubon Bird Count should help to clarify the issue.

While bird migration is triggered by the solar cycle, the actual journey may be disrupted by regional weather patterns, forcing these travellers to change course or prolong their rest stops. Nature does not ensure safe and timely arrivals.

Friday, May 8, 2009

First Taste of Soup

Banished to South Florida for much of the winter, it begins to creep northward with the lengthening days of February. By March, it has become entrenched across the southern States and is poised to sweep northward as Pacific storms race to the east. But here in the Midwest, it doesn't gain a foothold until the middle of May.

Yesterday brought the first hot, muggy air of the season, carrying in a flock of nighthawks and setting the stage for strong thunderstorms later in the evening. Building through May, the soupy atmosphere will peak in mid summer before the jet stream drifts southward once again. Until then, many of us will come to loathe our Midwest steam bath, though, just weeks ago, we couldn't wait to escape the chilly days of early spring.

But humans are like that, impatient for the future and embellishing our memories of the past. For now, we'll make the most of this hot, humid season, enjoying the light shows, the summer birds and the verdant growth of May and June.

Thursday, May 7, 2009

The Evolution of Plants

Spring is that time of year when we tend to focus on plants. After a long, cold, bleak winter, we appreciate the greenery, flowers, fragrance and unbridled growth of spring. What better time to consider the natural history of plants?

Life evolved in the sea, some 3.6 billion years ago; for most of that time, plants and animals remained in the oceans, not yet equipped to colonize the land. Then, during the Silurian Period (about 420 million years ago), plants ventured onto the continental shores, nourished by the waves and tides. Over the eons, as they evolved stems, root systems and leaves, plants diversified and spread across the landscape, their seeds dispersed by wind, water and primitive animals. Ferns and the first tree-like plants appeared in the Devonian Period (about 375 MYA) while conifers evolved during the Pennsylvanian Period, some 300 MYA.

Flowering plants graced the scene during the Jurassic Period, about 180 MYA, cross-pollinated by the wind or by flying insects that first appeared back in the Mississippian Period (325 MYA); social bees would not appear until the Cretaceous (some 80 MYA). Broad leaf trees also evolved during the Cretaceous (the reign of Tyrannosaurus rex) but grasses would not appear until the Eocene-Oligocene (about 40 MYA), drawing small, woodland mammals onto the nutritious plains and triggering the age of Tertiary megafauna.

Tuesday, May 5, 2009

Fair Weather Bird

Unlike many avian insectivores, common nighthawks seem to understand the fickle nature of a Midwestern spring. While tree swallows and eastern phoebes tempt fate in March and chimney swifts arrive by mid April, nighthawks do not turn up until early May, usually in concert with the month's first wave of warm, humid weather. After a winter in the tropics, they have no taste for the cold spells of early spring.

Once in town, they are best observed toward dusk, flapping and gliding in the evening sky with a halting, erratic style of flight. Larger and slower than swifts, they are easily identified by their white wing patches. During the day, they roost and nest on the ground (or on flat rooftops) but often take to the air after heavy rains produce clouds of insects. They are also drawn to lighted parking lots and sports fields, where bright lamps concentrate their prey.

By late August, common nighthawks begin to drift southward, often migrating in large flocks. Before the first autumn chill descends on the Midwest, most will be off to the tropics, feasting on insects and enjoying their perpetual summer.

Monday, May 4, 2009

Naturalists and Religion

Naturalists are interested in the natural sciences and all that they tell us about our Universe. We understand that humans, like all life forms, are the current products of an evolutionary sequence that began with the Big Bang and continues today. Governed by natural selection, species evolve, interact with other life forms and, eventually, become extinct. Equipped with intelligence, manual dexterity and articulate speech, we exert more control than other species but, in the end, are subject to the same natural processes. It is the quest to discover the diversity of life, and to understand the interdependence of its varied forms, that most intrigues a naturalist.

Unlike religious philosophies, the naturalist view is that man is just another species in the web of life. Our physical and mental traits have evolved from other species and, if we don't destroy the planet first, we will evolve into "higher"forms. Naturalists reject the notion that we are the "chosen species" and that all other life is (or was) a prelude to our creation. We claim no special relationship with God (or gods) and refuse to allow mysticism to impair our search for truth.

Religions offer a simplified view of the Universe and, throughout human history, have attempted to derail scientific progress when it contradicts their dogma. In this respect, they are not merely a benign source of hope and comfort. While naturalists are interested in the human traits that led to (and sustain) religious philosophies, we abhor their impact on the welfare of this planet; hopefully, we will soon evolve beyond their influence.

Sunday, May 3, 2009

Alaska Heat and Southern Rain

During a typical spring, the jet stream undulates across North America, bringing a series of troughs and ridges to any given region; the former allow cool air to spill southward while the latter permit warmth to build northward. Cold and warm fronts separate these air masses, bringing intermittent storms and showers through the course of a week.

However, over the past week, a persistent ridge of high pressure, known as a "blocking high," developed over the Pacific Northwest. Shunting Pacific fronts to the north, this ridge also brought a steady, southerly flow into Alaska, producing temperatures well above the seasonal norm. At the same time, this blocking high allowed a broad trough to settle over much of the country, with its southern boundary stretching from Texas to the Mid Atlantic region. Upper air disturbances, moving eastward along this "stationary front," directed a series of storms across the same swath of country; such "training" results in heavy precipitation totals and eventual flooding.

Now that the Pacific high is breaking down, storm fronts will resume their typical march across the continent and this week's stationary front will be swept off the map. As a result, both Alaska and the Southeast should return to more seasonable weather patterns.

Saturday, May 2, 2009

Swine Flu and Mass Hysteria

Humanity is consumed by the Swine Flu saga. Since it first appeared in Mexico, the virus has been popping up across the planet, not terribly surprising in this era of global transportation. To date, the infection has not proved to be especially virulent, though, like many infections, it may cause fatalities in the very young, the elderly and in those with compromised immune systems; fortunately, this virus appears to be sensitive to our current influenza medications.

Nevertheless, its spread is receiving constant attention from vigilant news agencies, which provide hourly updates of new cases (many of which are merely the confirmation of previously suspected infection). Egypt, oblivious to scientific knowledge about the virus and its means of transmission, is slaughtering its entire swine herd and Vice President Biden, ever loose-lipped, has indirectly advised all of us to avoid mass transit and public gatherings. Photos of masked citizens, like ads for a doomsday movie, flash across our television and computer screens, giving the impression that the human population is in imminent peril.

While, as with all contagious diseases, efforts to prevent the transmission of Swine Flu are appropriate, the implication that a lethal cloud is circling the globe leads to unnecessary fear and inappropriate behaviour. Humans are prone to mass hysteria and, rather than providing a balanced perspective, the media tends to fan the flames.

Friday, May 1, 2009

The Fault and the Sea

Back in the Mesozoic Era, the oceanic Farallon Plate bordered the western edge of the Americas. Since then, as the Atlantic Ocean opened, the American Plates have been pushed westward and most of the Farallon has subducted beneath them. Today, remnants of the Farallon Plate include the Juan de Fuca Plate of the Pacific Northwest, the Cocos Plate off Mexico and Central America, and the Nazca Plate, off the west coast of South America. All three of these remnant plates continue to subduct, as evidenced by volcanism along and near the western edge of the Continents.

Between the Juan de Fuca and Cocos remnants, the Farallon has completely subducted and the edge of the Pacific and North American Plates meet; for millions of years, this was a rather passive margin. Then, about 5 million years ago, the southern end of this border began to rift; known as the East Pacific Rise, this rifting tore the Baja Peninsula from the edge of northern Mexico, opening the Sea of Cortez. Further north, the margin persists as the famous San Andreas Fault, a transform fault between the North American and Pacific Plates; in concert with the opening of the Sea of Cortez, the Pacific Plate has been sliding northwestward along the edge of North America.

This motion is gradually widening the Sea of Cortez and, intermit-tently, forcing Southern California, which sits on the Pacific Plate, to grind its way along the San Andreas Fault; pressure that builds between the Plates is released by earthquakes, common events throughout the region. Compression along the San Andreas has also forced up the Transverse Ranges and, eventually, the combined mass of Southern California and the Baja will separate from North America and become a long Pacific island.