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.