The cool, rigid, outer layer of the earth, the lithosphere, is broken into massive plates.
There are about a dozen major plates and many smaller plates in continuous motion as they
collide with, slide under, or move past each other in a process known as plate tectonics.
A plate may be entirely oceanic lithosphere, like the Pacific Plate, or,
like the NorthAmerican plate can be part oceanic and part continental lithosphere.
We will return to lithosphere types in a minute, but first, let’s look inside the EArth to
clarify the layers. The mantle is a 2900 km-thick rock layer between the crust and core. The
composition is high-magnesium silicate. The uppermost part, the lithospheric mantle, is
cooler and more rigid than the deeper mantle. It lies above a hotter and more-ductile layer
of similar composition called the asthenosphere. As an analogy to how a rock can be either
brittle or ductile, consider a taffy bar, like the Big Hunk. When force is applied it
is brittle when cold, and ductile when warm. Zooming in to a tectonic cross section we
see that the lithospheric plates are composed of crust on top of the outermost rigid part
of the mantle. Although the mantle has a different composition and higher density than the crust,
these two layers migrate as a single mechanical layer called a lithospheric, or tectonic plate.
This is where earthquakes occur as brittle rock fractures and breaks. The hotter and
more-ductile asthenospheric rock does not fracture to produce earthquakes.
Continental lithosphere, with continental crust above lithospheric mantle, typically
ranges from 150 to 200 km thick. Continental crust stands above sea level because it has
an average thickness of 40 km and is mostly more buoyant silica-rich low-density granitic,
sedimentary and metamorphic rocks which form the continents.
OCEANIC lithosphere, formed at spreading ridges, is typically 50–140 km thick. Oceanic crust
is only around eight kilometers thick and is denser than continental crust because it
contains less low-density silica, and more high-density iron and magnesium. Thus, it
forms the ocean floor with its top surface below sea level.
Relative motion between plates can be broadly grouped into three main categories: Transform,
Divergent, and Convergent. RETURNING to the world map view, we show.
1) Transform, or strike-slip, boundaries: where plates move horizontally against each other,
2) Divergent, or constructive, boundaries: where plates move apart from one another, and
3) Places were plates press into one another are called convergent, or destructive, boundaries.
Lithosphere is neither created nor destroyed along transform boundaries which connect segments
of spreading oceanic ridges and other plate boundaries. Transform boundaries can also
cut across continents as the San Andreas Fault Zone does in California where it connects the East
Pacific Rise to the Cascadia subduction zone. Shallow earthquakes on long transform
boundaries that cut continental crust, can approach magnitude 8 whereas those on oceanic
transform boundaries tend to be smaller. At divergent boundaries, oceanic crust forms
at spreading ridges where plates pull away from each other. A small volume of the mantle
melts to create the crust. The hot, buoyant upwelling mantle supports the oceanic ridges
that forms Earth’s longest mountain systems. Because temperature increases rapidly with
depth at divergent boundaries, there is only a thin layer of brittle rock to fracture in
earthquakes. Most earthquakes occur within the upper ten kilometers and have magnitudes
that are generally less than six. More than 75% of all earthquakes occur on
or near convergent boundaries. Here, an oceanic plate is forced beneath a continental
in a process called subduction. Indeed, the world’s largest earthquakes occur near the
shallow edge of the boundary where magnitude 9’s have been recorded.. At this location
stress builds up over 10’s to 100’s of years until it releases like a spring. and
can produce tsunamis. A broad zone of shallow earthquakes occurs within the overriding plate
due to compressive forces near the convergent boundary. earthquakes can reach depths of
700 km within the subducting plates because the oceanic plate can remain cold and brittle
as it dives into the deeper mantle. Similar processes occur when an oceanic plate
subducts beneath another oceanic plate. Here, an ocean trench marks the location where the
plate is pushed down into the mantle. In this case, the line of volcanoes that grows on
the upper oceanic plate is an island arc. Not all convergent boundaries involve subduction.
When the continental parts of converging plates come together, neither can subduct. Instead
the two continents collide, producing horizontal deformation and uplift of continental crust
to build mountains and plateaus. Frequent shallow earthquakes in continental collision
zones can exceed magnitude 8 and are generally less than 40 km deep.
In addition to these three boundaries, there are also diffuse boundary zones in which deformation
occurs over a wide region. Although less common than earthquakes along the plate boundaries,
earthquakes in the interior part of plates, called intraplate earthquakes, do occur.
Nevertheless, tectonic activity and earthquakes are chiefly concentrated at, or near plate
boundaries where many geological features including volcanoes, mountains, trenches occur.