PLATE TECTONICS,
EARTHQUAKES, & VOLCANOES
Rolesville Earth Science 2016
THEORY OF PLATE TECTONICS
 1. The theory proposes that Earth’s outer shell consists of
individual plates that interact in various ways and thereby
produce earthquakes, volcanoes, mountains, and the crust itself
 2. Lithosphere – rigid outer layer of Earth, including the crust
and upper mantle
a. divided into segments called plates seven major
plates
b. largest = Pacific plate
 3. Move about 5 cm/yr (about as fast as your fingernail)
PLATE BOUNDARIES
PLATE TECTONICS & GEOLOGICAL TIME
 Alfred Wegner
1. Continental drift hypothesis
a. single supercontinent called
Pangaea
b. 200 million years ago Pangaea
(all land) began to break
up and started drifting to
their present
positions
PLATE TECTONICS & GEOLOGICAL TIME
2. Evidences
a. Matching fossils - Mesosaurus fossils limited to South America and
southern Africa
b. Rock Types and Structures - several mountain belts that end at one
coastline, reappear on a land mass across the ocean
3. Hypothesis rejected
a. Wegener could not describe a
mechanism that was capable of
moving the continents across
the globe
LITHOSPHERIC PLATE MOVEMENT
 What drives the movement of the
lithospheric plates?
 1. Ridge Push- Magma comes up
through the mid-ocean ridge heating
the rock around it.
 As it heats
(asthenosphere/lithosphere), it
expands and elevates, sloping away
from the ridge.
 As the magma cools and becomes new
rock, the older lithospheric rock slides
away making room for the new rock.
LITHOSPHERIC PLATE MOVEMENT
 2. Slab Pull (Gravity Pull) Occurs at a subduction
boundary
 One plate is denser than the
other plate causing the
denser plate to subduct (go
below) the less dense plate.
 The subducting plate is
colder and heavier so it
sinks down below the other
plate.
LITHOSPHERIC PLATE MOVEMENT
 3. Mantle Convection Very slow movement of the
mantle caused by convection
currents that come from the
heat of the interior of the Earth
to the surface.
 Can lead to the subduction of
plates.
 Not as powerful a force in plate
movement as once thought.
TYPES OF PLATE BOUNDARIES
 1. Divergent Boundaries- occurs when 2 plates move apart
 Results in the upwelling of material from the mantle to create new seafloor.
 A. Ocean Ridge- seafloor that is elevated along well developed divergent
boundaries
 B. Rift Valleys- deep faulted structures found along the axes of some
segments
 C. Seafloor Spreading- The process in which the ocean floor is extended
when two plates move apart, forming a crack where magma can rise to the
surface, cooling and forming new crust. Ex: East African Rift valley
TYPES OF PLATE BOUNDARIES
 Upwelling
 Rift valley
 Seafloor spreading
 Oceanic ridge
TYPES OF PLATE BOUNDARIES
 2. Convergent Boundaries- when 2 plates move together
 A. Results in a subduction zone- when 1 oceanic plate is forced down into the
mantle beneath a second plate
 B. Creates an oceanic trench
 C. Continental-Continental – when 2 plates collided and form mountains (Ex.
Himalayas)
TYPES OF PLATE BOUNDARIES
 3. Transform Fault- 2 plates grind past each other without
the production or destruction of the lithosphere
 Can cause earthquakes
 Ex. San Andres Fault
MAGMA VS. LAVA
 Magma- molten rock found underground
 Contains fragments of unmelted rock, crystals,
dissolved gases
 Contains metals like aluminum, iron,
magnesium, calcium, sodium, potassium
 Lava- molten rock and gases that break through
the Earth’s surface
 Red hot when it emerges from vent /volcano
 Cools and becomes grayish/black (sometimes
dark red)
VOLCANOES AND PLATE BOUNDARIES
 Volcano- an opening in the earth’s crust through which molten lava, ash, and
gases are ejected.
1. At divergent boundaries, spreading center volcanism occurs. Refer to slide 6
(ridge push)
A. most magma is produced along the oceanic ridges during seafloor
spreading
B. mantle rises upward to fill in the rift where the plates have separated
C. rock rises  pressure decreases  rock melts produces large
amounts of magma
a. magma is less dense than mantle rock so it rises
VOLCANOES AND PLATE BOUNDARIES
2. Convergent Plate Boundary
A. plate motions provide the mechanisms by which mantle rocks
melt to generate magma
B. slabs of crust are pushed down into the mantle
C. slab sinks deeper = increase in temperature and pressure drives
water from the crust
D. fluids eventually reduce melting point of hot mantle rock enough
for melting to begin
5. magma formed slowly migrates upward forming volcanoes
VOLCANOES AND PLATE BOUNDARIES
3. Transform Fault Boundaries
Do not form/cause volcanic activity because the magma sources needed are
not available.
Divergent Boundary
Convergent Boundary
OTHER ERUPTIVE MATERIAL
 Lahars – destructive mudflows occur when volcanic debris
becomes saturated with water and rapidly moves down steep
volcanic slopes
a. Can occur when a volcano is not erupting
b. Ash fall – resulting from eruption columns that form
eruption clouds
i. Heavy ash can collapse buildings
ii. Minor ash can damage crops,
electronics, and machinery
iii. Serious hazard to aviation
c. Volcanic gases
EARTHQUAKES
 What is an earthquake?
 vibration of Earth produced by the rapid release of energy
 Anatomy of an earthquake:
 1. Focus- The point within the Earth where the earthquake starts
 2. Epicenter- The point at the surface of the Earth directly above the
focus
 3. Fault- A fracture in the rocks that make up the Earth’s crust
 4. Plates- Large portions of the Earth’s surface whose movement
along faults trigger earthquakes
 5. Seismic waves- The waves that transmit energy released by an
earthquake
EARTHQUAKES
EARTHQUAKES & PLATE BOUNDARIES
1. Converging plate boundaries
 a. can produce deep-focus earthquakes that occur 180
miles or more below the Earth’s surface
2. Transform fault boundaries
 a. produce smaller, shallow-focus earthquakes that
occur 0-40 miles deep
 Ex. San Andreas Fault (California)
EARTHQUAKE ENERGY
Earthquake waves
1. Surface waves – seismic waves
that travel along Earth’s outer layer
a. travel along the ground and
cause the ground and anything
resting upon it to move
b. movement is like ocean waves
that toss a ship
c. up-and-down motion as well as
side-to-side motion
d. most destructive earthquake
waves!
EARTHQUAKE ENERGY
Body waves – other waves that travel through Earth’s interior
a. P waves – push (compress) and pull (expand) rocks in the direction the waves travel
i. also known as compression waves
ii. travel through solids, liquids, and gases
iii. have the greatest velocity of all earthquake waves
iv. move through the entire earth-crust, mantle, and core like a slinky
b. S waves – shake particles at right angles to the direction that they travel
i. also known as transverse waves
ii. travel only through solids
iii. slower velocity than P waves
iv. move through the ground (Earth’s outer layer) like a rope
EARTHQUAKE ENERGY
A seismogram shows all three types of seismic
waves
a. first P wave, then first S wave, and then
surface waves
hint: alphabetical order
magnitudes – a measure of the size of seismic waves
or the amount of energy released at the source of
the earthquake
i. quantitative measurements – rely on
calculations using seismograms
MAGNITUDE OF EARTHQUAKES