PLATE TECTONICS
Alfred Wegener (1915) proposed that in the past there had been only one supercontinent, Pangaea. Three hundred million years ago (or so), Pangaea broke up and the pieces began to drift apart. Strong support for this hypothesis comes from fossils found in South America and Africa which indicate that the regions had similar lifeforms and from fossils which indicate that tropical lifeforms existed in Antarctica around 200 million years ago. Further evidence is provided by paleomagnetism [the study of the Earth's magnetic field.]
The continents currently drift apart at a rate of 2 to 4 cm per year. Today, we can determine this rate using Global Positioning Satellites (GPS). In the past, methods based on paleomagnetism were often used and other methods also looking at places where plates moved apart (divergent plate boundaries) were used.

One of the reasons that this idea, which seems so compelling, was not initially accepted was that no one could imagine how things like continents could move around (or more precisely, what could push continents around). There also was no direct evidence that continents were moving (paleomagnetism did not take hold until the 1960s). Today, we have a reasonable idea for the mechanism that drives planet tectonics.
Recall that the interior of the Earth is hot and that it can be divided into the core, mantle, and crust, based upon chemical differences but that from a mechanical standpoint, it was better to consider the crust and the outer part of the mantle as one unit, the lithosphere and the plastic layer underneath as one unit, the asthenosphere.
Some properties of the lithosphere are as follows:

The lithosphere is less than ~ 100 km in thickness

The lithosphere is composed of two types of material distinct in composition and structure; the oceanic crust (55 %) and the continental crust (45 %).

The oceanic crust is thin (< 6 km) and is composed of basalts, similar to those found in lunar maria. The oceanic crust is denser than the continental crust.

The continental crust is thicker (~ 20 - 70 km) and is composed of granites. Granites (as are the basalts) are igneuous rocks, however, granites were formed under high pressure below the surface of the Earth. The continental crust is less dense than the oceanic crust.

The lithosphere is not one unit, it is segmented. It is composed of 7 major plates and around two dozen smaller ones. The places where the plates come together are well-marked as regions of enhanced geological activity. There are three types of plate boundaries: (1) divergent plate boundaries where plates move apart; (2) convergent plate boundaries where plates collide; and (3) transform plate boundaries where plates slide along each other.

The lithospheric plates are moved around by convective motions (e.g., Oatmeal convection, horizontal motion ). Because the Earth is hottest at its center and cools as one moves outward an outward flow of heat is set up. The heat is carried by conduction in the lithosphere. The heat is carried by convection in the asthenosphere.

The large scale circulations (motions) in the asthenosphere move the overlying lithospheric plates on the surface of the Earth leading to the continental drift observed today.

Lithospheric plates separate near rifts (divergent plate boundaries), places where new crust is produced. There is a large rift in the mid-Atlantic stretching from Iceland to Antarctica, the mid-Atlantic Ridge. Overall, there are around 60,000 km of active rifts on the surface of the Earth. Most rifts are in the oceans but some are on land, e.g., the Great Rift Valley in Africa. New crust is created continuously.

PLATE BOUNDARIES
Because the Earth is not growing in size while crust is created continuously, crustal material must also be destroyed continuously. To see where and how this occurs look at the interactions where plates meet.

Divergent Plate Boundaries

rift zones or spreading centers occur where plates separate (see the Mid-Atlantic Ridge). The upswelling convective motions in the asthenosphere push the plates apart and the upwelling lava cools forming basaltic rock. In addition to this new crust, heat and minerals are also deposited. The oceanic crust varies in age from very young near a rift to roughly 100 million years or so far away. (The oldest oceanic crust found has been around 180 million years old.) The age of oceanic crust is thus much smaller than the age of the Earth which is around 4.5 billion years.
Convergent Plate Boundaries

subduction zones:

Subduction zones occur where continental plates meet oceanic plates (note that the oceanic basin spreading from the mid-Atlantic ridge pushes the North American and South American plates, there is no subduction zone at the eastern coasts of North and South America). Because the oceanic plates are denser and thinner than are the continental plates, they are forced inward (into the Earth). The oceanic plates are forced downward to the regions of high temperature where the rocks melt (around 200 - 300 km below the surface). Some of the released material returns to the surface via volcanos while most is recycled into the mantle coming out in rift zones.
Near subduction zones you find oceanic trenches, mountain ranges, volcanism, and earthquakes.

Of particular interest to us is the Juan de Fuca plate which forms a shallow angle subduction zone. Shallow angle subduction zones lead to violent activity, such as earthquakes and volcanism. In Oregon, we get large earthquakes every 500 years or so. The last happened at 9 pm on January 26, 1700, the Cascadia earthquake, magnitude 8.7-9.2. This is large; a magnitude 6 earthquake has 1 Megaton of seismic energy. A magnitude 7 earthquake has 32 Megatons of seismic energy. A magnitude 8 earthquake has 1,000 Megatons of seismic energy. A magnitude 9 earthquake has 32,000 Megatons of seismic energy! We are roughly due for a large earthquake.

When two continental plates collide, because there is no strong tendency for one plate to slide under the other, the plates tend to crumple or crease and we get mountain range formation. An example of this is the Himalayan mountain range.

Transform Plate Boundaries

transform faults occur where two plates slide parallel to each other. An example of this is the San Andreas fault in California. The Pacific plate is forced northward at a rate of a few centimeters per year with respect to most of the North American plate (map). At this rate, Los Angeles will be next to San Francisco in about 20 million years. Earthquakes occur near fault lines because the plates do not slide smoothly. There is friction between the plates which causes the motion to go in fits and spurts. The San Andreas fault lets go every century or so. The last big earthquake occured in 1906 in San Francisco where the plates slipped by around 6 m (the 1989 earthquake relieved less than 3 m of stress). The southern section moves about 7 m in a large earthquake and hasn't really let go since the great Fort Tejon earthquake of 1857. This is a concern.

Comment:

The Hawaiian Islands represent a different kind of volcanism (compared to the volcanos found, e.g., in the Cascades). They do not occur near plate boundaries. They are formed near a hot-spot in the mantle of the Earth. Through so-called plumes, magma rises to the surface of the Earth. The magma oozes to the surface forming what are known as shield volcanos (as opposed to stratovolcanos). Shield volcanos have gradual slopes and are produced by lava flows building on each other. The Hawaiian islands are the largest volcanos on the Earth having bases with diameters of ~ 200 km (120 mi) and heights of 9 km (the volcano on the Big Island, Mauna Loa, is the largest active volcano on the Earth). The reason that there is an island chain is due to the fact that the Pacific plate is moving (at a rate of a few cm per year). In one or two million years the Pacific plate moves a distance equal to the average distance the islands. The Hawaiian island chain thus represents a time sequence with the Big Island being the youngest. This process continues to today and to ancient times. There is another Hawaiian island forming, Loihi, about 20 km southeast of the Big Island. Loihi is currently about a kilometer below the surface of the Pacific. It is expected to make its appearance in 50,000 years or so.