Unit 2 - The Earth
The geometric shape of the Earth is oblate spheroid
Equatorial radius is about 13 miles greater than the polar radius
This means objects/people at the equator are moving faster than those near the poles.
Liquid Outer Core
(1800 miles)
Crust
(12 – 43 miles)
Mantle
(1865 miles)
Solid Inner Core
(Radius of 758)
The most common elements in the Earth’s crust are oxygen and silicon
Rocks with a combination of oxygen and silicon are call silicates.
Density: a measure of how much material is packed into a given volume (M/V)
See Table 6.1 on Page 149
The average density of the Earth is 5.5 gm/cm3
We believe the Earth has gone through differentiation to form layers.
Differentiation of mixtures with various densities can occur if they are in their melted forms, where the more dense materials can sink and lighter materials will rise to the surface.
We can infer that the Earth was at one time almost entirely melted, probably in its infancy.
A larger celestial body will lose heat more slowly, retaining heat longer due to a smaller surface area compared to their volume
A smaller celestial body will lose heat more quickly.
Celestial bodies lose heat slowly through radiation
The speed of heat lose depends on multiple factors, such as size, composition, and surface properties, as well as internal and external heat sources.
The outer core is a liquid.
The inner core is solid because of the great pressures that it is under.
The intense pressures within the inner core force the atoms so close together, that despite the hot temperatures, the material is solidified.
Liquid
How do we know the Earth has layers?
Seismic waves (earthquake waves) help us to determine the layers and composition of the Earth
The speed at which seismic waves travel depends on the properties of the material they travel through.
Earthquakes produce 2 main types of seismic waves
Body Waves – Travel through a medium
Surface waves – Travel along the surface
Love waves
Rayleigh waves
P waves – primary waves
S waves – secondary or shear waves
P waves = Primary or compression waves
These are the fastest waves and the first to be detected after an earthquake
Can travel through solids, liquids, and gases.
The more rigid the material the faster they move
S Waves = Secondary or Shear waves
These are the second-fastest seismic waves and arrive after the P waves
Causes rocks to move side-to-side
S waves move perpendicular to the direction the waves are traveling
S waves can travel only through solid materials
Surface waves are the most destructive.
Surface waves rolling through rocky basins are usually long and slow-rolling waves. If strong enough, they are mostly a threat to larger structures, such as bridges and high-rises. Smaller structures are able to ride out the wave and remain intact.
Seismic waves are used to study earth’s interior.
Shadow zone – locations on Earth where no body waves can be detected
Body waves are affected to due the material they are traveling through
S wave shadow zone
P wave shadow zone
Seismograph – an instrument used to measure ground vibrations
Seismogram
Triangulations are used to determine the location of an earthquake.
Scientists measure the time between the arrival of P-waves and the arrival of S-waves.
The larger the lag time between waves, the farther away the earthquake is.
The distance to the earthquake then becomes the radius of the circle.
Why are three points needed for triangulation?
Why is the core so hot?
1. Accretion process
Gravity brought smaller bodies of material together. As each smaller body of material collided with the Earth, its gravitational potential energy heated up the Earth’s core.
2. Radioactive Decay
All rocks contain trace amounts of radioactive elements. As these radioactive elements break down they will release energy in the form of heat. This heat is trapped inside by the outer rocky layers.
Smaller bodies lose heat more quickly than larger bodies.
In essence, smaller celestial bodies will have their interiors cool and freeze faster than larger celestial bodies.
Radioactive Decay: The process in which a radioactive element ejects a subatomic particle from its nucleus.
Daughter Atoms: The new atoms produced when a radioactive substance breaks down
Radioactive Half-life: The amount of time it takes for half of the radioactive atoms of a specific substance to decay.
Cobalt 60 = 5.3 years
Carbon 14 = 5,730 years
Potassium Argon = 1.25 billion years
Radioactive dating have shown that the oldest rocks on Earth are over 4 billion years old.
Rocks from the Moon, asteroids, and samples from other bodies in the Solar System put the age of the Solar System is a little over 4.5 billion years old.
Continental Drift: The idea that the continents moved and were at one time joined into a super-continent called Pangaea.
First proposed by Alfred Wegener
Sea floor spreading – Rifting under the ocean at plate boundaries
Iron filings aligned with the poles in the ocean floor recorded the when the pole has reversed in the past.
Earth’s internal heat drives the movement of plates on the Earth’s crust
Convection currents cause hot mantle rock to rise and is responsible for the movement of tectonic plates.
Rifting is where two plates are separating, or a plate is being pulled apart.
Subduction zones – Where on plate dives below another plate. (Usually, a denser oceanic plate will dive below a lighter continental plate)
The subducting plate will usually have a higher water content so it will melt more quickly, and a chain of volcanoes are a common occurrence near subduction zones. (Ex. Cascade Mountains)
Magnetic field
The field is generated by electric currents flowing in the molten iron core.
It is believed the currents develop from a combination of convection and the rotational motion of the Earth.
This combination of the rotational motion and convection call this process the magnetic dynamo.
On average the magnetic poles reversed about every 250,000 years
The magnetic field protects us from electrically charged particles emitted by the Sun and other cosmic rays.
Many of these particles are energetic enough to damage living cells.
Van Allen radiation belts: Two donut-shaped regions around the Earth that trap some of the harmful charged particles.
The particles can penetrate spacecraft, damage sensitive electronic equipment, and damage the genetic material and other tissues of living passengers.
Charged particles are deflected in a spiraling motion, causing many to flow toward the polar regions.
The charged particles excite the electrons in the nitrogen and oxygen gas, causing them to move to higher orbital.
As electrons drop back to lower orbitals, they emit colorful lights known as the aurora borealis.
Atmosphere
There are 5 layers in the atmosphere:
Troposphere: Extends up to about 12 kilometers
This layer contains our weather and is where most airplanes are found
Because of the compression of overlying layers, three quarters of the mass (75%) of the mass of the atmosphere is within this layer.
Stratosphere: The layer just above the troposphere (12 km – 50 km)
This layer contains the ozone layer, which protects us from much of the ultra-violet rays of the sun.
Located at an altitude of 25 km or 80,000 feet
THE GREENHOUSE EFFECT
The earth is insulated by the atmosphere.
The Earth’s atmosphere is more transparent to visible radiation but not as transparent to infrared wavelengths.
When the visible radiation passes through our atmosphere, it hits the surface and is converted into infrared wavelengths (heat).
Carbon dioxide, methane, and water vapor are gases which trap heat energy.
Without these gases and the atmosphere, the earth would be about 30o Celsius or 86o Fahrenheit cooler.
Coriolis Effect
When ocean and air currents moving across a spinning planet are deflected from their original direction of motion.
Currents in the Northern Hemisphere are deflected to the right.
Currents in the Southern Hemisphere are deflected to the left.
The Coriolis Effect is what causes storm systems, hurricanes, and tornadoes to spin.
The Coriolis effect also establishes the direction of the jet streams.
Jet stream: Narrow bands of rapid, high-altitude winds.