Unit 6: Earth Shaped
from the Outside in
Cycle 18: Volcanism & earthquakes
Cycle 19: Plate Tectonics
Cycle 20: Weathering & Erosion
Connection to last unit…
 To deeply understand the processes that
create rocks and minerals such as
volcanism, weathering, pressue, etc. need
more information about where in the world
these things happen
Connections beyond…
 Why learn it?
 So inherently cool!
 So dangerous, need to be prepared
 So relevent, in the news eery day impacting
people around the world
Class Opener
 Do mapping activity
 Are all the earthquakes
and volcanoes evenly
spaced randomly across
earth’s surface?
 Why do earthquakes and
volcanoes occur where
they do?
Volcanoes &Volcanism
 Volcanoes=the cone structure
volcanism=movement of magma out of the
earth
 Volcanism is caused by solid rock melting.
Liquid rock is less dense and therefor rises
 Review: kinetic molecular theory of matter
What 3 things cause rock
to melt and rise?
 Increasing temperature
 Adding water
 Decreasing pressure
Convergent Subduction Plate
Boundary Volcanism
Divergent Plate
Boundary Volcanism
 Along plate boundaries & intra-plate areas
 Ring of Fire
 Mid-Ocean Ridge
 Hot Spots
 Volcanic Arcs
Intra-Plate
Volcanism (hotspots)
•Convergent Subduction Volcanism
–Plates are colliding with each
other and subducting. lithosphere
is being melted
–Volcanism due to increased
temps, water
•Divergent Volcanism
–Plates are ripping apart
–Volcanism due to
melting of
asthenosphere due to
less pressure from
above
•Intraplate Hotspot
Volcanism
–In middle of plates
–Hot spot in mantle
–plume rises towards the
surface due to density
difference
Volcanic Structures
 As eruptive materials pile up at an opening a
“volcano” forms
 Shield
 Composite
 Cinder
Shield Volcano
Shield Volcanco
-Low angle slopes of 110
Largest volcano
-Hawaii, Iceland
-Generally on ocean
floor
-”quiet” eruption
-made of basalt lava
flows, no ash or
pyroclastics
-basaltic magma, low
gas trapping
Composite Cone/Stratovolcano
Steeper slopes 10-25, taller
than cinder cones. Smaller
in height and width than
shield volcanoes
Cascades (Mt. St. Helens),
Andes
Volcanoes on continents over
ocean-land subduction
zones
Alternating Explosive/quiet
eruptions
Built up by alternating layers of
lava and pyroclastics
Built over tens to hundreds of
thousands of years
Andesitic (or Granitic)
composition magma, gas
Cinder Cones
Paricutin, Mexico, cinder cone soon
after its birth in 1943 in a Mexican
cornfield.
Smallest volcanic features have large craters
with steep slopes of 30-40
Paricutin, Mexico
Generally forms on continents
Explosive eruptions
Built of pyroclastics (tephra/cinders) not lava
flows.
Generally short lived eruptions - weeks to a
few years until the magma is degassed, then
it solidifies in the pipe and flows form from
the base
What determines the
shape, eruption type…

What determines what kind of volcano forms?


What determines what kind of eruptive materials
occur?


Type of eruption (explosive vs. quiet)
What determines what kind of eruption occurs?


The eruptive materials (lava vs. pyroclastics)
The amount of pressure build up from trapped gas
especially
What determines the gas content and composition
of the magma?

What type of rocks are being melted, ocean floor,
continental, both, asthenosphere, LOCATION
Warning signs of
impending eruption
 Earthquakes increase in
frequency and strength
 Ash and gas increases
 Bulging of volcano side
Dangers
 Pyroclastics: breathing, hit by it
 Lahars/mudflows: concrete flood
 Poisenous hot gases: can’t breath
 Pyroclastic flows: instant incineration

700km/hr (450mph)
Deaths 1600-1982
Earth’s Layers

Formed during early precambrian when earth was liquid rock
all over

3 layers based on differences in composition


5 layers based on differences in “stiffness/fluidity” of material


Crust, mantle, core
Lithosphere, asthenosphere, lower mantle, outer core, inner core
How do we know?

Earthquakes!

Seismic waves travel faster in denser materials.

S-waves only travel through solids (both rigid and plastic)

Inferences based on overall density calculations for earth,
observations of surface materials, lava, and meteorites
What is an Earthquake?
 Definition: sudden, strong shaking of the earth’s
crust caused by release of energy
 Usually happens along faults

Movement of rigid
solid crust causes
stress in rocks

Rocks
bend/stretch/com
press/distort their
shape

Breaking point
reached and rocks
release energy
and move along
fault

Go back to
original “unbent”
shape

Can only occur in
rigid solids of crust
What are seismic
waves?
Body waves
P waves
S waves
 Primary wave
 Secondary
 Focus
 Focus
 Faster
 Arrive first
 Push-pull waves
(compress/stretch rock,
no shape change)
 Move through solids,
liquids and gases
 Slower
 Arrive Second
 Up and down waves
(change shape of rock)
 Only move through solids
How do we measure
earthquakes?
Locating the epicenter of
an earthquake
 The farther the
station is from the
epicenter, the
greater the time is
between the arrival
of primary and
secondary waves
 Need 3 stations
Travel Time Graph
1.
How long does it take the S & P
wave each to travel 1000 mi?
2000 mi?
2.
How much sooner will the P
wave arrive at a seisograph
station 1000 mi away from the
earthquake location? 2000mi?
3.
What is the general relationship
between difference in arrival
time between P & S wave and
distance?
4.
This graph can help geologists
identify distance to earthquake
epicenter but not…

Direction!
Earthquake dangers
 Falling objects
 Collapsing buildings
 Collapsing bridges, tunnels, etc.
 Fires (caused by broken gas lines etc.)
 Tsunamis near coast
 Interuption in medical and other emergency
services due to debris and road closures
Safety Precautions
 Inside:
Drop, cover, hold on! Protect your head from falling
objects, windows etc.
 Don’t try to get outside during shaking
 Be prepared for aftershocks

 Preparing your home for EQ safety



Bolt bookcases etc. to wall
No heavy objects above bed
Know how to turn off gas & water
 Caught at the coast:
Did the tide go out quickly? If you are seeing more of the
bottom of the ocean than you should…
 Get to higher ground!

Earthquake frequency
and cause
 Earthquakes in New England?
 What causes earthquakes
 Most frequently at plate boundaries but can
happen anywhere in plate, especially if
there are faults already in rock