Earth Science: Chapter 3

Earth Science- Chapter 3

Earthquakes- the shaking or rapid motion of Earth’s soled layer or crust (aka tectonic plates).

There are about 1,000 earthquakes daily. 30,000 of them are felt yearly. Only 75 of them are significant and cause damage.

How does an earthquake happen? A fault is a zone of weakness or a crack in the crust where previous motion has already happened. This stress builds up in the crust until there is too much stress for the crust to resist. The crust breaks and energy is released.

3 types of stress that build up under the ground:
1. Tension- when there is this kind of stress, two pieces of the earth move apart.
2. Compression- when two pieces of the earth are pushed together and they overlap.
3. Shearing- the earth rubs against each other. At the fault, there will be tension.

The exact place inside the earth where the earthquake occurs is called the focus. Directly above the focus on the surface of the earth is the epicenter. Energy is released from these two areas.

2 ways to measure an earthquake:
1. Intensity scale- measures the effects of the earthquake- how much damage the earthquake did based on what people said. We use the Mercalli Scale- based on Roman numerals from 1 (least)- 12 (most). Depending on how close a person is, that’s how he’ll feel it, so the Mercalli’s scale isn’t that good.
2. Magnitude scale- Richter scale-a seismologist measures the strength of an earthquake using seismometers. The most accurate seismometer is a seismograph. A seismograph measures the vibrations of the earth. There is a heavy weight with a pen. The pen writes on a piece of paper that is attached to the scale. Based on the different movements, there will be different writings. It records the movements of the earthquakes’ waves. Every number on the Richter scale is 10 times the number before it.

Earthquake waves- there are 3 types of waves:
1. P- waves- Primary waves- they travel the fastest and they move in a push- pull motion.
2. S- waves- Secondary waves- they are slower than the P-Waves. They only move through solid material in the ground. They move from side-to-side.
3. Surface waves- includes waves that move in a push-pull and side-to-side manner. These waves cause the most damage to the surface of earth.
Each earthquake has all 3 types of waves.

Locating the epicenter-
We have to record the speed of the P and S waves. In order to locate the epicenter, we see how long it took for the P and S waves to reach the seismograph.
Origin time- time the earthquake began. The origin time can be figured out based on P- waves:
1. The arrival time of the P-waves at the seismograph.
2. Have to know the travel time of the P-waves.
a. What time they got there
b. How long it took
3. Arrival time- travel time = origin time.

Layers of the earth-
A layer is anything under the surface. We live on the surface of Earth.

Layers:
1. Crust- the outermost layer. It’s located right under the surface. There are two parts to the crust:
a. Continental crust- under the continents.
b. Oceanic crust- under the oceans.
Crusts vary in depth from 5-60 km. We know what the crust is made up of based on:
a. Surface rock- same as the crust rock because the surface and the crust are so close together.
b. If you dig down to the curst and see.
The majority of continental crust is made up of granite. The majority of oceanic crust and deep continental crust is made up of rock similar to basil.
2. Mantle- the boundary or interface between the crust and the mantle is called the Moho. The mantle is 2,900 km deep. The earthquake waves move faster through the mantle than through the crust. The mantle contains most of the volume of earth- it’s the densest place. Most of earth is concentrated in the mantle.
Magma (lava that comes out of a volcano) is stored in the mantle. Magma is made up of dense or mafic rocks. The mantle is not only made up of dense mafic rocks, but also olivine and pyroxene.
3. Core- the middle of the earth. The core is made up of the:
a. Inner core- it’s very dense. It’s made up of solid iron and nickel. The inner core is solid because of the intense pressure.
b. Outer core- is very dense. It’s made up of melted iron and nickel. The outer core is liquid and the S-waves can’t pass through it.

Earthquakes and volcanoes usually take place along the Ring of Fire, located in the Pacific Ocean.

Earthquakes are aka seismic.

Dangers of earthquakes/ seismic hazards-
1. Because of the shaking, buildings or bridges may collapse or become unsafe for use.
2. Tsunami
3. Fires

Hazards of volcanoes- when a volcano explodes, 3 things come out:
1. Lava- burning rocks. It flows down the volcano and burns everything in its path;
2. Toxic gases- are very dangerous.
3. Ashes

Volcanoes are also very useful:
1. It produces fertile soil.
2. Created the Hawaiian Islands.
3. We get geothermal energy from volcanoes.

Shadow zones-
When an earthquake occurs, the waves travel all over. The S-waves can’t get to the opposite side of the earth because they can’t pass through the core. Therefore, there is a P-wave only zone.
In this P-wave zone, there are certain areas that have no waves at all. Why? Because when the P-waves go through the core, the rock is extremely hard and it retracts- bends the P-waves to the side. The result is an area without any P-waves either. This area with no waves is known as the shadow zone.
A shadow zone is 102˚- 143˚ from the epicenter of the earthquake. Every earthquake has its own shadow zone, located after the core.

Continental draft-

There is a theory that states that the continents are moving further apart from each other. Originally, they were all connected as one big planet called Pangaea. Proofs that there was once one big continent:
1. All the continents fit together like a puzzle.
2. The sides of the continent of South America and the continents that connect have the same rock formation.
3. The ocean floors- it appears that the ocean is constantly growing through the creation of mid-ocean ridges- ridges under the ground that show us that the age of the ocean crust increases as you get further away from the ridge.
How the mid-ocean ridge formed- under the ground, there is magma. This magma comes up from the ridge and spreads out over the ridge. While the magma is still hot and molten, the iron inside the magma aligns with the earth’s magnetic field. The ocean floor continues to expand because more magma is coming up and pushing the magma away in two opposite directions. (The new magma pushes away the old magma.) Now, the ocean floor is bigger. Earth’s magnetic field is constantly reversing its direction and therefore when the new layer of magma aligns itself it will be in the opposite direction of the old magma.

Plate tectonics- the plates that the earth is made up of. Earth is made up of 12 major plates and many smaller plates.
There are boundaries along the plates called plate boundaries. When there is movement along the boundaries, it causes volcanoes, earthquakes…

Different types of movement along the boundaries:
1. Convergent boundaries- the two boundaries have collided or crashed:
a. Continental crashing into continental
b. Continental crashing into oceanic- the oceanic, which is denser, will sink beneath the continental plate. When this happens, it’s called a subduction zone. These subduction zones have ocean trenches formed. By subduction zones, magma rises up and creates islands called Island arcs.
2. Transform boundary- when two plates slide past each other. This doesn’t cause such a tremendous earthquake.
3. Divergent boundary- plates move apart from each other. This is aka a rift zone.

What makes the tectonic plates move? Because there is a massive amount of heat in the earth. Heat moves from a high concentration to a low concentration, so the heat moves from the center to all the other parts of earth. The heat travels through convection through the earth. When something gets hot, it expands and gets less dense. The heat is traveling and spreading itself out through convection. Any area that receives this heat expands and becomes less dense. Once it’s less dense, it’ll rise and will cool off and eventually sink back down. This movement inside the mantel is called convection cell. (Hear moves in slow motion in the mantle.) Because of these convection cells inside the mantle, which causes the rocks inside to move, the rigid part of the curst (aka lithosphere) is moving as well. The plates are made up of the crust, so the moving in the mantle causes the plates to move.

Convection cells bring hot material up to the surface. If there is a crack in the crust, the hot material will get through.

Sometimes, there’s an area under the crust that’s a source of magma. This source of magma is called a hot spot. As the plates slowly move over this hot spot, the magma rises from beneath the surface and forms a volcano. As the plates move along, new volcanoes are formed.

I left out how to find the epicenter of an earthquake and I did the dangers of seismic hazards very briefly. Hatzlocha Rabba!