The
Earth's
____________________
heat
source
provides
the
energy
for
our
dynamic
planet
,
supplying
it
with
the
driving
force
for
plate
-
tectonic
motion
,
and
for
on
-
going
____________________
events
such
as
earthquakes
and
volcanic
eruptions
.
Major
____________________
that
have
Contributed
to
Earth
?
s
Internal
Heat
.
First
,
the
____________________
Impact
which
proto
-
planet
Earth
would
have
grown
over
time
from
a
barrage
of
extraterrestrial
impacts
,
increasing
its
mass
with
each
bombardment
.
As
the
proto
-
planet
grew
in
size
its
____________________
gravitational
field
would
have
attracted
even
more
objects
its
surface
.
Second
,
____________________
contraction
which
in
the
early
stages
of
planetary
accretion
,
the
earth
was
much
less
compact
than
it
is
today
.
The
____________________
process
led
to
an
increasingly
greater
gravitational
attraction
,
forcing
the
Earth
to
contract
into
a
smaller
volume
.
Increased
compaction
resulted
in
the
conversion
of
gravitational
energy
into
heat
energy
,
much
like
a
bicycle
pump
heats
up
due
to
the
compression
of
air
inside
it
.
Heat
conducts
very
slowly
through
rock
,
so
that
the
rapid
build
-
up
of
this
heat
source
within
the
Earth
was
not
accommodated
by
an
equally
rapid
loss
of
heat
through
the
surface
.
Third
,
____________________
decay
which
refers
to
the
spontaneous
process
of
breaking
down
an
atomic
nucleus
by
releasing
particles
and
energy
.
Example
:
unstable
isotope
Uranium
-
238
,
for
example
,
will
slowly
decay
to
____________________
-
206
.
Last
,
the
internal
heat
____________________
which
drives
most
of
the
geological
processes
on
Earth
.
This
measures
the
flow
of
thermal
energy
coming
from
the
core
,
passing
through
the
mantle
,
and
up
to
the
atmosphere
,
which
is
mainly
due
to
the
mantle
convection
.
This
,
however
,
is
counteracted
by
the
solar
radiation
.
