Download EarthScience_Topic 9-Properties of Earths Interior

Properties of Earth’s Interior
• Lithosphere: the solid outermost part of Earth
– Includes the crust and upper mantle
• Crust: the rocky solid shell that floats on the
asthenosphere
– Includes the ocean floor
• Asthenosphere: plastic mantle- made of
molten material
• Mantle: Located between the crust and the
core
• Outer core: liquid- moves around the solid
inner core creating a magnetic field
• Inner core: solid
ESRT page 10
• MOHO: the boundary between Earth’s crust
and upper mantle
Earthquakes
• An earthquake is the shaking of Earth’s crust
caused by a release of energy
Possible causes?
• Interaction between lithospheric plates
Damages
• Buildings and bridges collapse, fires, water
shortage
Focus
• Located underground where the movement or
break began
Epicenter
• Located on Earth’s surface directly above the
focus
Occur at..
• Plate boundaries
P waves
•
•
•
•
Compression/ primary waves
1) back and forth motion
2) travel fastest- arrive first at a location
3) travel through solid, liquid, or gas
S waves
• 1) travel perpendicular to the direction of
movement
• 2) about half the speed of P waves
• 3) travel only through solids
• As seismic waves travel through earth’s
interior they bend (science word).
• Creates a Shadow zone: areas that get no
earthquake waves
Locating the epicenter
• Minimum of 3 seismic stations needed.
Measuring Earthquakes
• Richter scale: measures an earthquake’s
magnitude.
– Uses seismographs
– Each successive number is 10x greater than the
previous
Measuring Earthquakes
• Mercalli Scale: measures earthquakes
intensity based on reports from people who
experienced the event
How to Use the Earthquake Travel Time Graph (Page 11
of the Earth Science Reference Tables).
The graph on page 11 can be used to find several different variables. Let’s
start with:
A seismic station is 3000 kilometers from the epicenter of an
earthquake. How long will it take P waves from this earthquake to reach the
seismic station?
We know the distance (3000km)
We’re trying to find the time it will
take P waves to travel this distance.
Find 3 (really 3000km) on the bottom
axis and, using a straight edge
(a ruler) draw a line up to meet the
P wave line.
Now, using the ruler, draw a line over
to the left axis and read the travel
time: 5 minutes and 40 seconds.
Notice that each small box is worth
20 seconds.
Now try the same problem in reverse:
It takes P waves 5 minutes and 40
seconds to travel from an epicenter
to a seismic station. How far is the
seismic station from the epicenter?
We know the P wave travel time:
5 minutes and 40 seconds.
We are trying to find the distance
between the epicenter and the
seismic station.
Using a ruler, draw a line from the
time (5:40) to the P wave line.
Now draw a line straight down to
the bottom axis and read the
distance.............
3000 kilometers!
All of these problems are the same.
Given the time, find the distance.
Given the distance, find the time.
And it doesn’t matter whether you
are given P or S wave travel time
as long as you are careful to use
the correct line and, most important:
TAKE THE TIME TO BE CAREFUL AND ACCURATE!
It takes P waves 7:20 (7 minutes and 20 seconds) to travel from an earthquake
epicenter to a seismic station. How long will it take S waves from the same
earthquake to reach the seismic station?
We have to divide the problem into 2 parts. First, we do just what we did in
the previous problem. We use the P wave travel time to find the distance
to the seismic station and then.................................
We use the distance to the seismic station to find how long it took S waves
to travel that same distance.
Let’s see how it’s done....................................
We know P wave travel time is 7:20
so, using a ruler, draw a line from
7:20 over to the P wave line.
Now draw a line straight down to find
the distance from the epicenter.........
The distance is 4200 km. Notice that
each small box on the bottom axis is
200 km.
Now we can get on with the second
half of the problem. If the seismic
station is 4200 km from the quake
epicenter, how long (time) did it take
the S waves to travel that same
distance?
Draw a line from 4200 km straight up
to the S wave line.
Now draw a line over to the vertical
axis and read the time..........
13 minutes exactly (13:00)
And now for something completely different.......................
Let’s take this one step further. A
seismic station is 3000 km from the
epicenter of an earthquake. If P
waves from that quake arrive at the
station at 4:25:40 PM (4 hours, 25
minutes and 40 seconds), at what
time did the earthquake occur?
We do exactly the same thing we
did before, we find the travel time
which is 5:40 (5 minutes, 40 sec.)
Now some math. If the waves arrived
at 4:25:40 and they’ve been traveling
for 5:40, when did they start out?
Subtract: 4:25:40
5:40
-----------4:20:00
The quake occurred at 4:20:00 PM
Determining Arrival Time Differences
• 1. find the distance on the bottom of the chart
• 2. place a scrap paper vertically for that
distance
• 3. Mark the scrap paper where the s wave and
p wave touch it.
• 4. move the paper to the left of the chart.
Place one mark at zero and read the time
using the mark above it.
Determining Distance using differences
in time
• 1. Place a scrap paper on the left side of the
chart
• 2. Mark the zero and the arrival time
difference on the paper
• 3. keep the paper vertical and move it to the
right until the top mark is touching the s wave
line and the zero mark is touching the p wave.
• 4. follow the paper down and read the chart
to determine distance
A seismic station is 3000 km from
the epicenter of an earthquake.
How long after P waves arrive will
S waves from the same earthquake
arrive at the station?
This time place your paper on the
3000 km mark taking care to keep
the paper straight and to cover
both the P and S wave lines.
Make small, thin, and accurate
marks where your paper crosses
the P line and the S line.
Then..............................
Another type of question involves the
“difference in travel time between P
waves and S waves”.
Whenever you are given the difference
in arrival times OR you are given both
times so that you can subtract and find
the difference, you should immediately
realize that you will be dealing with the
shaded area (yellow) between the two
lines.
Here’s a sample problem:
P waves arrive at a seismic station
4:30 (4 minutes, 30 seconds) before
the arrival of S waves from the same
earthquake. How far from the epicenter
is the seismic station?
Here’s what you know: P & S waves
arrive at a seismic station 4 minutes and
30 seconds (4:30) apart. You are asked
to find the distance to the epicenter.
Here’s how:
1) Take a sheet of paper and line up
the left edge with the vertical axis
(time). Be sure that most of the paper
is hanging down below the graph. This
is important.
2) Make a small, thin, and accurate mark
on the paper at 0 time. Make another
small, thin, and accurate, mark at 4:30
(4 minutes, 30 seconds).
Your 2 marks are now 4:30 apart!
Now slide your paper to the right
until one of your marks is exactly
on the S wave line and the other
is exactly on the P wave line. It is
very important to be sure your
paper is straight (vertical).
Now look to see where the bottom
of your paper crosses the lower
(epicenter distance) axis. In this
case it crosses at exactly 3000 km
which is the answer.
When P & S waves arrive 4 minutes
and 30 seconds apart it means that
the seismic station is exactly 3000
km from the epicenter of the quake.
Of course we can do the same
problem in reverse!
One last kind of problem to become familiar with. Sometimes, instead of giving you
the difference in arrival times, you will be given a seismogram (a record made by a
seismograph) instead.
Using this seismogram
find when the P waves
and the S waves arrive.
Do this by carefully
making a mark at the
P wave and S wave
arrival times.
Now count the number of
minutes between the
arrival of P and S waves at
the station.
From this point on it’s just like the previous problem: If the difference in arrival times
between P and S waves is 6 minutes, how far is the seismic station from the
epicenter of the earthquake?
One last type of problem: Find the
‘time of origin of the earthquake’.
In other words, use the information
given to find out when the quake
occurred.
Here’s a sample problem: A seismic
station is 4000km from the epicenter
of an earthquake. P waves arrive
at the station at 2:48:00 PM. At what
time did the earthquake occur?
First, use the distance to find the P
wave travel time.
OK, the P waves took 7 minutes to
travel the 4000km distance.
If they arrived at 2:48:00 and the
trip took 7 minutes, they must have
started out 7 minutes before 2:48 PM
So subtract. 2:48:00
- 7:00
--------------Origin time: 2:41:00 PM
Let’s try another one: P waves arrive
at a seismic station at 4:22:10 AM.
S waves from the same earthquake
arrive at 4:28:50 AM. What is the time
of origin of the earthquake?
First subtract to get the difference in
arrival times:
4:28:50
- 4:22:10
-------------difference =
6:40 (6 min, 40 sec)
Now, as we did before, get a piece
of paper and carefully and
accurately make marks at time 0
and time 6:40
Slide the paper until one mark
is on the P wave line and the
other is on the S wave line.
Now read the distance from
the epicenter on the bottom:
4000 km.
Now that we know the distance
to the epicenter we can easily
tell how long it took P waves
to travel that distance.
P waves took 7 minutes to
travel the 4000 km and since
they arrived at 4:22:10 we
can subtract to find out when
they started.
4:22:10
- 7:00
-------------Origin time: 4:15:10
Plate Tectonics
• Continental Drift: The theory that Earth’s crust
is resting on a fluid which allows it to move
– The crust is broke up into pieces called plates that
move relative to each other
Evidence for Continental Drift
• Shape of the coastlines: continents fit
together
Evidence for continental drift
• Correlations: fossils match across ocean
basins
– Rocks match across ocean basins
Evidence for continental drift
• Climate changes: coal found in Antarctica.
– Glacial striations in Australia and Africa.
What happens to the age of oceanic
crust as distance increases from a
ridge?
Age of Oceanic Crust
Courtesy of www.ngdc.noaa.gov
Movement of Plates
• Convection currents within the asthensphere
move the plates that rest on the top
Convection currents
• Currents caused by differences in density
Rising currents
• Causes the plates to move apart- divergent
plate boundaries
Falling currents
• Cause the plates to move togetherconvergent plate boundaries
Divergent boundaries
• A location where two plates are moving away
from each other
• As plates separate, water fills low areas
• Formation of mid-ocean ridges
• Youngest rock located at center
Divergent plate boundaries
• Some igneous rocks contain minerals that are
magnetic
• Instruments measure small changes in
magnetism
• Bands of igneous rock on the ocean floor
show that Earth’s magnetic orientation has
been reversed in the past
Divergent plate boundaries
• Crust must have shifted
• Magnetic reversal supports sea floor
spreading
Evidence that plates are moving apart:
• Ocean floor is generally younger than the
continents
• Rocks continually form at mid ocean ridges.
• Heat flow is highest at the spreading center
• Magnetic reversal
Convergent boundaries
• Continental/ Continental NOT Collision
• Two plates collide
• Cause crust to be pushed upward, forming
mountain ranges
– Himalayan
– Andes
Continental/Continental
• Effects:
– intense folding and faulting,
– a broad folded mountain range,
– shallow earthquake activity,
Subduction boundaries
• One plate goes under (subducted) the other
plate
Ocean/Continental Plates
• Ocean plate is more dense than the
continental plate and is subducted
• Bordered by a mountain chain and volcanoes
on the continental plate
– Andes mountains
Ocean/Continental
• Effects:
– earthquakes
– ocean trench off shore of the continent,
– a line of volcanic eruptions inland from the
shoreline.
Ocean/ Ocean Plates
• one ocean plate is subducted under the other.
• Usually the older one
• Effects:
– deeper earthquakes
– an oceanic trench
– a chain of volcanic islands
Transform Boundaries
• Plates that slide past eachother
• Stress builds up between plates
• Major earthquakes
– San andres fault
Bench Marks
• Locations labeled with exact elevation
• Able to measure the change in elevation
Uplift of fossils
• Fish and marine fossils found on mountain
tops
Subsidence of fossils
• Shallow water fish buried deep in the ocean
floor
Looking at rock strata
• folded
•faulted
Looking at rock strata
• Undisturbed
• tilted
Hot Spot
• A place where magma is coming through
earth’s crust
– Hawaiian islands
Hot Spots
• Hot spots occur where rising material from
the lower mantle remains stationary for
millions of years.
• When a plate moves over the rising materials,
it melts.
• Become regions of intrusive and extrusive
volcanic activity that can build volcanoes and
lava flows.
• Can also push up regions to create mountains.
Hot Spots Cont.
• As the plates move over the rising material, a
series of volcanic mountains can form for
thousands of miles.
• These trails show the path of the plates past
movements.