Download Van Allen radiation belt

9) Solid Earth. The student knows Earth’s interior is
differentiated chemically, physically, and thermally.
The student is expected to:
c. Explain how scientists use geophysical methods such as seismic
wave analysis, gravity, and magnetism to interpret Earth’s
structure; and
d. Describe the formation and structure of Earth’s magnetic field,
including its interaction with charged solar particles to form the
Van Allen belts and auroras.
Paleomagnetism
Polar wandering
Magnetic reversals and mechanism of sea-floor spreading
Vocabulary: Mohorovicic discontinuity (Moho); Shadow Zone, PWaves; S-Waves; Magnetosphere; Solar Wind; Aurora
Borealis; Aurora Australis
We’ve spent a couple of lessons already on the internal
structure of the Earth. Just how do we know what lies
beneath our feet?
Do we have direct evidence from these depths?
• 1989 Kola Superdeep Borehole in Russia: 12,262 M (40,230 ft)
• 2008 Al Shaheen Oil Well in Qatar: 12,289 M (40,318 ft)
(40, 502 ft)
• 2011 Sakhalin-I Odoptu Well: Devastation
12,345after
M 1906
Earthquake in
San Francisco, which killed 700 people
Anyway you look at it, we haven’t come close to drilling
through even the crust, or outer surface of the Earth yet…so,
how do wePersian
know?Gulf
Scientists have used indirect measurements and seismic waves from
1989
earthquakes to determine the internal structure
of the Earth. What
exactly is a seismic wave?
• Seismic waves are those which are produced by movements and
motions of the Earth’s crust along its tectonic boundaries…What types
of boundaries are there again? Convergent, divergent, and transform
Analysis of the Earth’s internal structure is made
possible because earthquakes produce vibrations
called seismic waves. These waves travel through the
interior of the Earth and can be measured with
sensitive detectors called seismographs. Scientists
have seismographs set up all over the world to track
movement of the Earth’s crust.
While we may not have
practical experience at
these depths, we do
have ways of
determining Earth’s
interior through
extrapolation based
upon behavior of waves
through various media.
1. What are the three types of tectonic boundaries?
2. Describe each of the following:
• Convergent
• Divergent
• Transform
3. What is a seismic wave?
Seismic waves are divided into two types: Body waves and
surface waves. Body waves include P and S waves, and these
are the two types of waves that are used to determine the
internal structure of the Earth.
There are other types of seismic waves, but they relate to
surface damage and only travel through the Earth’s crust, not
the entire Earth.
Measuring seismic waves
traveling through the Earth
allowed seismologists to
determine that the core begins
at a depth of 2900 km, or in
other words, the mantle
extends to 2900 km; its
composition is probably fluid
rock. This is based on the
velocity of the waves, mass of
the Earth and other lines of
evidence.
As humans have studied, and know how these various body
waves travel through different substances (ie solids,
fluids…etc), we can extrapolate about the structures found
deep within the Earth by determining how these waves act as
they move through it. (ie, the speed and direction of travel…etc)
The shadow zone is the area of the
Earth from angular distances of 104
to 140 degrees from a given
earthquake epicenter that does not
receive any direct P waves.
The shadow zone results from S
waves being stopped entirely by the
liquid core and P waves being bent
(refracted) by the liquid outer core.
Through measuring how P and S waves travel through the
Earth and out the other side, this seismic wave shadow zone
was discovered in 1910.
From the lack of S waves and a great slowing of the P wave
velocity (by about 40%) it was deduced that the outer core is
made of liquid. The shadow zone also defined the diameter of
the core.
Of Earth’s Interior…there is some!
One type of direct evidence is a type of rock called "Xenoliths." Xenoliths
are foreign or alien rocks. The word "xeno" means stranger.
Xenoliths are basically small pieces of small fragments of rock
usually enclosed in volcanic rocks that are thought to be pieces
of the Earth's mantle, which are brought up along with the
lava during the volcanic eruption.
Another type of direct
a type
of rock
structure called
Theevidence
ophiolite is
mantle
material
looks
"Ophiolites".
somewhat darker than the gabbro above it
The ophiolites are thought to be pieces of the mantle
connected to pieces of the crust, which have been uplifted and
embedded by tectonic processes in the crust of the Earth.
They have a particular structure, which involves a transition
from one rock type to another.
4. How do scientists use seismic waves to determine
what the Earth’s interior must be like?
• Mantle
• Core
5. What tool is used to measure the intensity of an
Earthquake?
6. Describe each of the following body waves:
• P Waves
• S Waves
7. What are xenoliths and ophiolites?
8. How are xenoliths and ophiolites direct evidence of
Earth’s interior?
You can use a spring scale to measure gravity. Notice that
when this is hanging without moving, the spring is stretched by
a certain amount. If the mass is pulled down, it causes the
spring to stretch further. A machine called a gravimeter can
be built based on this principle to measure Earth's gravity or
differences in Earth's gravity between one place and another.
Earth’s rotation causes the polar regions to
flatten down slightly, making Earth an ellipsoid
instead of a sphere. As a result, the force of
gravity is slightly stronger at the poles
compared to the equator (generally).
Smaller variations in gravity across the Earth’s
surface are caused by differences in the
thickness and rock density of Earth’s crust, as
well as density differences deep in Earth’s
interior.
• The overall density of the Earth is much higher than the density of the
rocks we find in the crust. This tells us that the inside must be made of
something much denser than rock.
• Meteorites (created at the same time as the Earth, 4.6 billion years ago)
have been analyzed. The most common type is called a chondrite and they
contain iron, silicon, magnesium and oxygen (Others contain iron and nickel).
A meteorite has roughly the same density as the whole Earth. A meteorite
minus its iron has a density roughly the same as Mantle rock.
• Iron and Nickel are both dense and magnetic. Earth has a Magnetic Field.
Convection currents in the liquid outer core force the inner core to spin.
The inner core spins faster than the rest of Earth. This creates a
magnetic field through something called the dynamo effect.
It is Earth’s rotating, convecting, electrically conducting
metallic fluid outer core that enables this dynamo, in turn
energizing the magnetic field, protecting us from the Sun.
9. Would the force of gravity be stronger at the poles, or
equator…and why?
10. Where would gravity be stronger, on the continents or oceanic
floor…and why?
11. How can the composition of meteorites suggest the makeup of
the Earth’s interior?
12. What is the dynamo effect and how does it contribute to the
formation of the magnetic field on Earth?
13. Why is Earth’s magnetic field so important?
The Van Allen radiation belt is a ring of
energetically charged particles (plasma)
around Earth, which is held in place by
Earth's magnetic field. It is believed that
most of the particles that form the belts
originated from solar wind and cosmic rays.
The Van Allen
Belt is located in
the inner region
of the Earth's
magnetosphere.
It is split into
two distinct
belts, with
energetic
electrons forming
the outer belt
and a combination
of protons and
electrons forming
the inner belts
The auroras, both surrounding the
north magnetic pole
• Aurora borealis
and south magnetic pole
• Aurora australis
occur when highly charged electrons
from the solar wind interact with
elements in the Earth's atmosphere.
Different gases give off different colors when they are
excited. Oxygen at about 60 miles up gives off the familiar
yellow-green color, Oxygen at higher altitudes (about 200
miles above us) gives the all red auroras. Ionic Nitrogen
produces the blue light and neutral Nitrogen gives off the
red-purple and the rippled edges.
14. What is the Van Allen Belt, and where do the particles that
compose it come from?
15. What composes the outer Van Allen Belt?
16. What composes the inner Van Allen Belt?
17. Why does Earth have a Van Allen Belt?
18. Which auroras surround the north magnetic pole?
South magnetic pole?
19. Why are auroras composed of different colors?
Paleomagnetism is the
study of the magnetic
properties of rocks over
time.
Paleomagnetism aids in
our understanding of
plate tectonics,
minerology, petrogenesis,
geochronology, and the
history of the Earth's
magnetic field.
Paleomagnetism is
possible because some
magnetic minerals
common in igneous rocks,
such as magnetite, and
hematite, will "acquire"
the magnetic field of
their surroundings as the
rock cools. As it cools,
the surrounding magnetic
field becomes "frozen,"
in the mineral, so at the
time of formation, these
rocks preserve the
direction of the magnetic
field.
Pink= Normal magnetism
White= Magnetic reversal
During World War II, geologists employed by the military
carried out studies of the sea floor, in order to understand
the topography of the sea floor to find hiding places for both
allied and enemy submarines.
The topographic studies involved measuring the depth to the
sea floor. These studies revealed the presence of two
important topographic features of the ocean floor:
•Oceanic Ridges - long ridges that occupy the middle of
the Atlantic Ocean and the eastern part of the Pacific
Ocean.
• Oceanic Trenches - deep trenches along the margins of
continents, particularly surrounding the Pacific Ocean.
Polar Wandering is basically the apparent migration of the
magnetic poles of the Earth through geologic time. Scientific
evidence indicates that the magnetic poles have slowly and
erratically wandered across the surface of the Earth. Today,
magnetic north is here, and True geographic north is here.
Calculations of pole locations
over the past 20 million years
show that the magnetic pole
hasn’t moved greatly.
Go back 30 million years,
however, and substantial
deviations occur.
Calculations of polar wandering
formed one of the first
important pieces of evidence for
continental drift.
Scientists have determined that the magnetic poles do not
actually wander that much; they generally stay close to the
geographic poles. Therefore the concept of apparent polar
wander is very useful in plate tectonics, since it can retrace
the relative motion of continents, as well as the formation and
break-up of supercontinents.
As the image on the
right shows, the paths
of polar wandering would
be different if the pole
were fixed, compared to
the path if the continent
was fixed (on the left,
image a ).
20. What is paleomagnetism, and how do we use it?
21. What two important oceanic features were discovered during
WWII?
22. What is polar wandering, and what did its discovery help to
prove?