Download Unit F Plate Tectonics

18.2
A volcano marks a crack in the lithosphere where
magma (molten rock) and gases reach Earth’s
surface. Once magma reaches the surface, it is
called lava.
Magma forms deep underground when the
asthenosphere experiences at least one of three
things, each of which results in increased melting
of the rock:
drop in pressure (e.g., at a crack in the crust);
change in composition (e.g., in the subduction zone); or
increase in temperature (e.g., above a hot spot or
convection upwelling).
Hot spots are small regions of very hot mantle,
thought to be heated by a concentration of
radioactive substances near Earth’s core.
This heat creates very hot columns of rising
mantle, which cause the lithosphere to thin out
and crack as it bulges up. Eventually, magma
bursts through the weakened lithosphere over
the hot spot, forming a volcano
A volcanic belt, or chain of inland volcanoes, is
created on an overriding continent parallel to a
convergent boundary (Figure 4).
A volcanic island arc, or line of volcanic
islands, is created on an overriding oceanic
plate parallel to an oceanic–oceanic boundary.
Boundaries between Earth’s tectonic plates are the
source of many earthquakes, vibrations through
Earth’s crust caused by volcanoes and movement
along plate boundaries (tectonic activity).
The Shuddering Slide
The movement of tectonic plates along boundaries
is not smooth; the plates are enormous, made of
solid rock, and subjected to tremendous
compression and stretch. The edges of plates are
not even, and friction causes them to deform,
bend, lift, and crack.
Energy is stored in the deforming plates, like the
energy in a compressed spring. Eventually, the
tectonic forces are strong enough to overcome the
friction, and the plates lurch ahead releasing the
stored energy as earthquakes.
A fault is a displacement of the lithosphere
(vertically, horizontally, or both) created by the
movement of tectonic plates.
The source of an earthquake within the
lithosphere is called the focus.
The epicentre is found on the surface,
directly above the focus.
Most earthquakes occur along tectonic plate
boundaries, due to the movement of the
plates themselves
Earthquakes transmit mechanical energy in the
form of seismic waves (mechanical waves or
vibrations) in Earth.
There are two main types of seismic waves: body
waves and surface waves.
1) Body waves travel through a medium, such as
sound waves travelling through air.
Primary waves (P–waves) are compression waves:
the particles in the medium vibrate forward and
backward along the path of the wave. P–waves can
travel through solids, liquids, or gases; they pass
through all the layers of Earth.
Secondary waves (S–waves) are shear
waves: the particles in the medium vibrate
perpendicular to the direction of the wave.
S–waves travel more slowly and can
only travel through solid rock, not
through Earth’s liquid core.!!!
Measuring the changes
in a wave’s speed
and direction
has provided
evidence about the
internal structure of
the planet
Surface waves travel along the outside of Earth,
causing the most destruction.
They are created when body waves reach the
surface. These waves travel more slowly than
body waves, and their range is limited to a few
hundred kilometres.
Scientists use devices called seismographs to
detect earthquake waves.
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Summary; 534+535!!!!!!!!!