GWD7-Q22 to Q25:

Earth’s surface
consists of rigid plates that are constantly shifting and jostling one
another.
Plate movements are the
surface expressions of motions in the mantle—the thick shell of rock that lies
between Earth’s crust and its metallic core.
Although the hot rock of the mantle is a solid, under the
tremendous pressure of the crust and overlying rock of the mantle, it flows
like a viscous liquid.
The mantle’s motions, analogous to those in a pot
of boiling water, cool the mantle by carrying hot material to the surface and
returning cooler material to the depths.
When the edge of one plate bends under another and its cooler material
is consumed in the mantle, volcanic activity occurs as molten lava rises from
the downgoing plate and erupts through the overlying one.

Most volcanoes occur
at plate boundaries.
However,
certain “misplaced” volcanoes far from plate edges result from a second,
independent mechanism that cools the deep interior of Earth.
Because of its proximity to Earth’s
core, the rock at the base of the mantle is much hotter than rock in the upper
mantle.
The hotter the mantle rock
is, the less it resists flowing.
Reservoirs of this hot rock collect in the base of the mantle.
When a reservoir is sufficiently large,
a sphere of this hot rock forces its way up through the upper mantle to Earth’s
surface, creating a broad bulge in the topography.
The “mantle plume” thus formed, once established, continues
to channel hot material from the mantle base until the reservoir is emptied. The
surface mark of an established plume is a hot spot—an isolated region of
volcanoes and uplifted terrain located far from the edge of a surface plate.
Because
the source of a hot spot remains fixed while a surface plate moves over it,
over a long period of time an active plume creates a chain of volcanoes or
volcanic islands, a track marking the position of the plume relative to the
moving plate.
The natural history
of the Hawaiian island chain clearly shows the movement of the Pacific plate
over a fixed plume.