so now that we have the basics down for
plate tectonics what it is how it works
and how we know about it so what
how does plate tectonics affect life in
California or anywhere else for that
matter to answer that let's look at
California's current plate tectonics
setting as this map shows we have a
number of plate boundaries within or
near the state notice at the northern
part of California we have a divergent
plate boundary or an offshore seafloor
spreading center where new oceanic crust
forms and then is pushed away here along
the coast that crust collides with and
then sinks under Northern California
that produces a convergent plate
boundary where plate with an oceanic
crust leading-edge collides with a plate
with continental crust on the edge
result subduction a trench earthquakes
and inland an active chain of volcanoes
this volcanic chain is known as the
Cascade Mountains and in California is
expressed with the active volcanoes of
Mount lassen and Mount Shasta
the most recent eruption in that area
was the explosive eruption depicted in
this image of Mount lassen on May 22nd
1915 what happens to the seafloor
spreading Center often northern
California like all such centers it is
offset at regular intervals by transform
faults if we move along the ridge north
we hit this transform boundary that then
connects to the continuation of the
spreading centers a bit west if we go
back and follow the ridge south we hit
another transform this one offsets the
ridge eastward where all the way south
to the Sea of Cortez where it connects
with the seafloor spreading centers that
are separating the Baja Peninsula from
the rest of Mexico this long transform
boundary that connects these two
sections of ocean ridge is known as the
San Andreas Fault system those of us who
live along its path are familiar with
its associated earthquake hazards the
Loma Prieta earthquake in 1989 was one
of the most recent large earthquakes the
magnitude six point nine earthquake
caused significant shaking and damage
from Santa Cruz to San Francisco as
released built-up stress related to the
transform plate motion why is the
transform so long what made it this way
and why are the seafloor spreading
centers so close to the continent and
not in the middle of the ocean this
animation from the University of Santa
Barbara shows what the Pacific Ocean
looked like 80 million years ago when
the mid-ocean ridge seafloor spreading
Center with all its offset transforms
was actually close to the middle of the
ocean notice what has happened over the
past 80 million years as the Atlantic
Ocean spread north america was pushed
westward the western continental margin
of north america was a large long
subduction zone with associated
volcanoes and terrain accretion but
about 25 million years ago the spreading
center began to be overrun its abducted
under the north american continent and
the san andreas fault system began to
form pause now
subduction now over the volcanism of the
Sierra Nevada also ended but what
happened to that spreading Center we can
image the crust under North America and
see evidence of the old plates still
subducting and right above the spot
where we suspect the spreading center
would now be we have an area of tension
where the continental crust is being
pulled apart similar to the breakup of
Pangaea the crustal stretching has
created sub parallel mountains and
valleys collectively known as the Basin
and Range along the westernmost edge the
Sierra Nevada mountains are being pulled
up out of the ground causing high rates
of erosion an exposure of deep granite
rock bodies that are the now frozen
remnants of the magma chambers that used
to feed the active subduction volcanoes
so walking in glacially carved yosemite
granite is actually walking along the
remnants of old volcanic cores what else
results from the stretching of the Basin
and Range in addition to exposing the
eroded underbelly of ancient Sierra and
volcanism the rifting activity is also
producing a source of new volcanism to
the area the resulting volcanoes are
completely different in character and
cause to subduction zone volcanoes both
by being more voluminous in their magma
supply and more explosive in their
eruptive style mammoth mountain is one
volcanic structure found in the middle
of this region known collectively as
long valley in this region of rifting
and thin crust mantle rock is melting
and rising beneath the surface just like
what happens at seafloor spreading
divergent plate boundaries mantle melts
combined with large amounts of crustal
melt collecting a giant magma chamber
just under the surface the Long Valley
caldera is a giant crater left behind
after one of the largest volcanic
eruptions to happen in North America
emptied the magma chamber and the
surrounding crust collapsed the last
large eruption about seven hundred and
sixty thousand years ago left bits of
ash and pumice strewn across most of the
western United States there's still an
active magma chamber under the caldera
which releases carbon dioxide
side gases and hot water at numerous
locations around the rim it has been
classified as one of the largest
volcanic centers in North America the
most recent eruptions were those that
produced panem crater and the Inyo
craters 500 to 600 years ago let's leave
this unique volcanic region and return
to the remnants of the ancient
subduction zone volcanoes now exposed as
granite in the central and western
Sierras if you want to experience some
of this granite closer to home the San
Andreas Fault has done a great job of
moving rocks around in the state as a
result of the transform motion sections
of the southern Sierra Nevada granite
were separated from their origination
spot and carried northwards
we see them today as far north as Point
raised they also make up the rock of
Montero Mountain and the rock of the
Farallon Islands pause now
what about the rest of the rock around
us where did that come from
while some of the rock that makes up the
santa cruz mountains in the hills of san
francisco comes from recent deposition
of coastal sediments during changing sea
levels and ice ages the majority of it
consists of individual blocks of
terrains that were created during the
past periods of subduction prior to the
collision of a mid-ocean ridge with the
subduction zones there were hundreds of
millions of years of active subduction
during that time ocean crust was carried
into the subduction zones and portions
of what it carried were scraped off and
attached to the edge of the continent
extending the continent westward in a
paper mache style sequence of addition
or accretion accreted material can
include ocean sediment that got scraped
off the top of the subducting plate
continental sediment that would have
poured off the continental shelf into
the trench ocean sea mounts or islands
that would have gotten stuck in the
trench and scraped off sections of
entire oceanic crust and metamorphic
rocks produced during collision and
subduction
as you can see plate tectonics has had a
huge impact on the past and present
landscape of California it produces many
of our major geologic hazards and it has
rafted in and combined the very land
masses that now make up our state take a
field trip to one of these nearby
locations where you can see the evidence
and walk on the old surfaces yourself
pause now
for more information and more detail
continue on to the next video in this
series
you