Introduction: Plates and Boundaries.
The surface of the Earth is divided into approximately six large 'plates', plus a number of smaller ones. Where they underlie the World's oceans, these plates are composed of the same material as the fluid mantle, but are supercooled into solid rock, and are typically around 6 to 10 miles (10-16 km) thick according to current estimates. (Where the plates contain continental material, their thickness can vary considerably, as will be discussed in a later chapter.)
The thickness of oceanic plates is defined by the insulating properties of this 'frozen' mantle material. The surface of the plates is maintained at 39°F (+4°C) - the temperature of deep ocean water, while the underside is right at the melting point of the mantle material, currently estimated at around 1100°F (~600°C). Thus a thermal gradient exists vertically across the plates, much like the ice on the surface of a frozen pond: the colder the surface temperatures, the thicker the ice. (Ice is not, other than this particular example, a good analogy for the behavior of frozen mantle material. Most solids, including mantle material, are heavier than and will sink into their liquid counterpart; ice is less dense than water, and floats.) As the surface temperature of oceanic plates and, in most cases, the mantle temperature beneath them are held constant, it stands to reason that plates are generally of a uniform thickness. (Exceptions where elevated undersurface temperatures occur will be discussed later.)
Continents and islands are made up of a mixture of lighter materials imbedded within this frozen mantle. As the plates move, any land masses that are imbedded in them are carried passively along.
The plates are bounded by an interconnected network of ridges, transform faults, and trenches. Ridges, also called spreading centers, occur where two plates are moving away from each other. As the plates separate, hot molten mantle material flows up to fill the void. The increased heat resulting from this flow reduces the density of the plates, causing them to float higher in the molten mantle, thus elevating the boundaries by many thousands of feet above the colder surrounding sea floor. Ridges on the ocean floor form the longest continuous ranges of mountains on the planet, but only in a very few places on the Earth do these 'mountains' rise above the ocean surface.
Ridge segments invariably form perpendicular to the relative motion of the two separating plates. However, the general trend of the boundary between two spreading plates need not be, and in fact usually is not, perpendicular to the motion of the plates. Ridges therefore tend to be made up of small spreading segments offset from each other and connected by transform faults, forming a zig-zag line, where the 'zigs' are perpendicular to the 'zags.' Along transform faults, the two plates are sliding past each other going in opposite directions. (See figure at right, above. Arrows indicate direction of plate movement.)
New sea floor is constantly being created along spreading centers. Obviously somewhere else old sea floor must be going away. This occurs in trenches, also called subduction zones. Trenches occur along the boundary between two plates that are moving towards each other. Where this occurs, one plate is bent downwards, typically at about a 40° angle, and plunges under the other plate's leading edge, eventually to melt back into the liquid mantle below. Where the subducting plate bends downward, there is left a gap in the ocean floor running along the plate boundary - thus the common term 'trench.' The Marianas Trench off Guam in the western Pacific plunges to a depth of over 36,000' or 11,000 m (compared to an average ocean depth of 13,000' or 3950 m), and is the lowest point in the ocean floor.
As the subducting plate is heated back up to mantle temperatures, certain minerals in the plate melt sooner than others. Minerals that melt at lower temperatures and are lighter than the surrounding material tend to rise, melting their way up through the overriding plate to erupt as volcanoes on the ocean floor. As these volcanoes grow, they rise above the ocean surface to form lines of islands along the leading edge of the overriding plate. Numerous islands of Micronesia and Melanesia in the western Pacific were created in this way.
When a continent occupies the leading edge of an overriding plate, the differentially melted minerals from the subducting plate will melt their way through the continental mass to erupt on its surface, forming ranges of volcanic mountains. The entire Pacific margin of the Americas provides abundant examples of this process. When continents occupy the leading edges of both converging plates, neither continent can be subducted; continental material is too light, too buoyant, to sink into the mantle (see "Buoyancy and Floating Continents" in the next lesson). What happens instead is that one or both continents are deformed and thickened, causing them to float higher above the molten mantle to form a range of mountains. (Which continent deforms and the nature of the deformation will be discussed in a subsequent lesson.) After two continents have been in collision for a while, it appears as if a range of mountains has formed in the interior of a continent. The Himalayan Mountains, where the Indian Plate is pushing up into the Eurasian Plate, are an example of this process in action.
It should be noted that all of the mountain and volcano producing events described in the preceding paragraphs occur only on or near the margins of plates. One additional process is basic to the theory of plate tectonics and is not confined to plate boundaries; that process is the action of plumes. Plumes are thought to be columns of exceptionally hot material rising from deep within the Earth's mantle, perhaps even from its bottom, where the mantle contacts the Earth's liquid core. Plumes are hot enough to melt their way through the ocean floor or through a continent, whichever happens to be above them. They are thought to be (relatively) stationary within the Earth, while plates and continents move above them. Thus a volcano which forms on a moving plate above a plume will eventually move away from the rising column, which will then melt through at a new location and form another volcano, while the old volcano becomes extinct (i.e. no longer capable of erupting).
The Hawaiian Islands represent a chain of volcanic islands formed in the middle of the Pacific Plate from the actions of such a plume. Only the big island of Hawaii harbors active volcanoes; the volcanoes which formed the other islands, now moved away from their source of molten material, are all extinct. Yellowstone National Park in Wyoming is a caldera formed by the catastrophic explosion of a volcano which formed over the Yellowstone Plume. The Snake River Basalts of southern Idaho are lavas marking previous eruptions of the same plume as North America slowly passed over it. Iceland is a volcanic island formed by a plume which happens to exactly straddle a ridge or spreading center: the Mid-Atlantic Ridge.
Earthquakes tend to be concentrated along (but not exclusively confined to) the margins of plates. Deep seated earthquakes, often severe in magnitude, generally occur beneath subduction zones, as one plate is forced deep into the mantle beneath another. Shallower, but equally severe earthquakes occur along transform faults where one plate is dragged sideways against another. Transform fault earthquakes are most noticeable where the fault passes through a continent, rather than the sea floor. The San Andreas Fault of California is a prime example.
Earthquakes along spreading centers tend to be very mild. One exception can occur where a new spreading center forms beneath the interior of a continent. Such events tend to be short-lived, in geological terms, for as the spreading continues, a rift is formed that will eventually fill with sea water while the spreading center creates new sea floor beneath it. The Atlantic Ocean was created in this way between 50 and 150 million years ago. The Red Sea is in a transitional stage, already filled with sea water but not fully separated, while the Rift Valley of East Africa is in the early stages of what will ultimately be the complete separation of east Africa from the rest of the continent (assuming the process continues as it now is).
The Red Sea and the Rift Valley of Africa are the only documented examples of spreading centers beneath continents today (technically, the San Andreas Fault is a stairstep of very short spreading center segments connected by very long transform faults, all a portion of the East Pacific Rise), but geology and oceanology tell of numerous such occurrences in the past.
Thus is the floor of the ocean continually being recycled; no sea floor is known to be older than around 200 million years (compared to 3 1/2 BILLION years for portions of some continents). Continent to continent collisions along subduction zones make continents larger, while rifts along spreading centers break them apart again. Volcanoes and compressive deformation during collisions along subduction zones makes them thicker (and hence higher), while erosion wears them down, making them broader but flatter; all in a never ending cycle of aging and rejuvenation. The continents we know today are but a frozen snapshot of an unending global dance. The job of plate tectonics is to unravel the clues in that snapshot, to learn the dance steps, to determine where the dancers have been, and, for a while, where they are going.