Transcript Unit 4 - Dynamic Crust Earthquakes & Volcanoes

Plate Tectonics, Earthquakes, Volcanoes
--A theory that says the Earth’s lithosphere
(Remember what that is?...hint: think crust)
is divided into solid sections of rock called
“plates.” These plates move in relation to one
Another. Tectonics are the forces that cause
the Earth’s crust to continually move and create
new landforms such as mountains, mid-ocean
ridges, trenches, or faults.
Plate tectonics incorporates the idea of
“Continental Drift”----this was first proposed
by a scientist, Alfred
Wegener in 1915.
Wegener, 1880-1930
He noticed that the continents appeared to fit
together like pieces of a giant jig saw puzzle, which
led him to propose that the continents were once
connected together as a giant land mass (Pangaea).
Evidence for Plate Motion:
Similar rock types & fossils found along the
eastern coast of South America
match those along the western
coast of Africa
Mountain chains appear to be continuous
(Appalachians)—once connected.
Rock & Fossil Evidence:
Similar rocks and fossils
found on opposite continents
Ancient climates were
different than today
--Glacial deposits in tropical regions
--Coal deposits in the Arctic
Rocks of the ocean basins are MUCH
younger than continental rocks—youngest
rocks are located at mid-ocean ridges
Evidence for Sea Floor Spreading:
1. Igneous Ocean Rocks
Igneous rock of the mid-ocean ridges
are younger than the igneous rock
farther from the ridges
2. Reversal of Magnetic Polarity
Over thousands of years, the magnetic
poles of Earth reverse their polarities.
This is recorded in the rocks of the ocean
floor.
N
S
1. Convergent
Ocean-Continental
Collision
2. Divergent
3. Transform
Find 1 example of
each plate boundary type
on page 5 of your ESRT!
Mantle Convection Cells:
A stream of heated material moving due to
differences in density.
Evidence suggests convection cells exist within
the asthenosphere. (See ESRT page 10 for arrows
indicating convection movement under crust)
Heat left over from the formation of Earth PLUS
heat generated by radioactive decay
Hot spots are places on Earth’s surface with high
heat flow rising upward
--Usually occur along active plate margins
--Caused by plumes of magma rising up from the
mantle---produce active volcanism
Hawaiian islands
Produced as the
Pacific plate passes
over a hot spot..the
only active volcano
is the one directly
over the hot spot.
The Hawaiian
islands have moved
Northwest over time.
1. Shield Volcano
Non-explosive eruptions, gentle slopes, basaltic
lava flows (low in silica)
Example: Hawaiian volcanoes
2. Composite volcano
Repeated pattern of explosive & non-explosive
eruptions. Moderate slope; built from alternating layers
of lava & tephra
Example: Mt. St. Helens
3. Cinder Cone volcano
Explosive eruptions. Built from tephra (lava
blasted into the air then solidifies as it falls)
Steep slope
Example:Mt. Paricutin, Mexico
EARTHQUAKE:
- Any vibrating, shaking or rapid
motion of the earth’s crust.
- Most occur when stress builds
along a zone of weakness or break
in the rock known as a FAULT.
FOCUS: The earthquake’s point of
origin
EPICENTER:
- The location at the earth’s
surface, directly above the focus.
When an earthquake occurs, seismic waves are
generated & move out in all directions from the focus.
The waves are detected and registered on instruments
called seismographs.
TYPES OF ENERGY WAVES
RELEASED BY EARTHQUAKES
1)PRIMARY WAVES: “P-WAVES”
-The fastest & first to arrive at a
seismographic station
- Also known as “push-pull waves”
-Like the motion of a spring pulled
tight, then released
P-waves travel through solid, liquid
& gas
2. SECONDARY WAVES: S-WAVES
-Travel more slowly than P-waves
-Vibrate in all directions perpendicular
to the direction of travel.
-Travel only through solids
**P & S-waves
Are BODY
waves
Both P &
S-waves
create
“shadow
Zones”…
Where no
waves are
recorded
SURFACE WAVES: L-waves or
(Long period waves)
Are Rayleigh & Love waves that ripple the surface
of Earth
-Surface waves are the waves
that cause the damage associated
with earthquakes
We can determine the origin time of an
earthquake if we know the distance from the
epicenter (remember what that is?) & the
travel time of the S-waves & P-waves.
The epicenter
location is
determined by
calculating the
difference in
arrival time of
the P & S-waves,
& by using a
minimum of 3
seismograms
The epicenter is located where the 3 circles intersect:
The intensity of & energy of an earthquake
Is measured on 2 different scales:
1. Richter scale
2. Mercalli scale
The Richter scale measures the amount
Of energy released during an earthquake
**A measure of magnitude
Each step increase in magnitude on the Richter
scale represents an increase in wave amplitude
of 10 times.
But, the energy released by the earthquake
Increases about 32 times!
Example:
An earthquake of Richter magnitude 8.0 is 3
magnitude units higher than a 5.0 earthquake.
So, its wave amplitude is 1000 times greater
(10x10x10)
But, the energy released is about 30,000 times
greater (32x32x32)!
The modified Mercalli scale measures
earthquake intensity
The Mercalli scale is a measure of the effect
on people & buildings……measures damage