Earthquakes and Volcanoes

Earthquakes and Volcanoes

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What is an earthquake?

Turn on the TV or read the newspapers and almost always there is something devastating happening somewhere as a result of sheer nature's power. Examples of such natural occurrences are hurricanes, tornados, wildfires, volcanic eruptions, flooding, earthquakes and tsunamis. These are usually not caused directly by humans, but their effects live with us for a long time. In this lesson we shall look at one of such natural occurrences...earthquakes!


What is an Earthquake?

Simply, earthquakes are the rumblings, shaking or rolling of the earth's surface. It is usually what happens when two blocks of the earth suddenly slip past one another, or break apart from each other as a result of tension caused by prolonged energy build up.
Earthquakes come in many forms. It can be felt as a shock under your feet, or may be very powerful and destructive enough to flatten an entire city. They can happen anywhere, land or sea.

Foreshocks, Mainshocks and Aftershocks:
Sometimes, there are smaller shocks that occur before (foreshock) and after (aftershock) a main earthquake (mainshock). Sometimes foreshocks are so big and scientists are unsure if it is the main shock. Foreshocks and aftershocks can occur for days, weeks and months of a main earthquake.

Earthquakes are also called temblors.

It is important to understand
the earth’s makeup to help understand earthquakes better.

In this diagram, you will notice that the inner and outer core of the earth (middle part) are liquid in nature, containing iron and nickel of extreme temperatures (5,500°C).

The Mantle is semi-molten rock, also called magma. The outer is the crust, which is the hard part of the earth that forms the surface. This outer crust includes the land on which we live, the oceans and ocean deeps and anything within 40km (approx) down the earth's surface.

Earthquakes are developed in the outer crust of the earth.

Important terms to know about earthquakes
Let us take a moment to learn about these terms to help us understand earthquakes better.Tectonic Plates:
These are huge layers that make up the earth’s upper layers. They continually stretch, move, slide, and collide against each other. Even though they are constantly moving, we do not feel it. Each plate is about 50 to 250 miles (80 to 400 km) thick.

Faults (or Fault plane or fault lines):
These are weak lines that can develop in the plates, usually on the surface of the earth. There are different types of faults and the major types include dip-slip normal, dip-slip reverse, strike-slip and oblique-slip.
The hypocenter is the location below the earth’s surface where the earthquake starts. The epicenter is the location directly above it on the surface of the earth.

Seismograph and The Richter Scale (RS):
The seismograph is a device that scientists use to measure the magnitude of an earthquake. The Richter scale on the other hand is a scale or measure that is used to compare earthquakes. It is calculated in levels of ten. Example, an earthquake measuring 4 on the RS is ten times more than a measurement of 3, and an earthquake measuring 8 on the RS is 10 times more than one that measured 7 on the RS. As a guide, an earthquake measuring 3-5 is considered minor, 5-7 is moderate, 7-8 is major, and 8 or more is considered great and usually very devastating.

Ring of Fire:


This is the coastal belt of the Pacific Ocean (see diagram) which is the home of many volcanic eruptions, plate movement and major fault lines. About 90% of the world's earthquakes and 81% of the world's largest earthquakes occur along the Ring of Fire. The Ring of Fire is a direct result of plate tectonics and the movement and collisions of lithospheric plates. http://en.wikipedia.org/wiki/Ring_of_Fire

How do earthquakes form? Let us imagine what goes on in the outer crust with the help of this diagram. Earthquakes develop in the crust of the earth. The crust involves the earth's surface, submarine levels, down to the ocean floors. The inner part of the earth contains massive energy. Some of this energy escapes through cracks and other volcanic activity, but the bulk of it is stored within the earth’s inner part, contained in the crust. The earth’s outer crust is held in place like a completed jigsaw puzzle, with rough edges and lines. The energy stored here causes the pieces to slide, glide, knock and move around each piece. These pieces best describe what we call ‘Tectonic plates’ (See illustration below) After a period of time, the built up energy and movement causes huge tension in the plates, and there is massive pressure on the fault lines. This intense pressure resulting from energy build up causes the fault lines give way, and plates move over, against or apart from each other. It is important to note that there is usually a very thick natural cover (earth and vegetation) which makes fault lines obscure. There is an earthquake at this point. In the form of seismic waves (like water ripples) the escaping energy radiates outward from the fault in all directions. The seismic waves shake the earth as they move through it. When the waves reach the earth’s surface, they shake the ground and anything on it, tearing down houses and structures. Earthquake Waves There are 2 types of earthquakes waves and the difference lies in the way the seismic waves are transmitted. To understand this better, let us see what a seismic wave is. These are waves of energy that travel through the earth's layers, and other elastic layers, often as a result of earthquakes. A wave, by general definition is the transfer of energy from one place to another without transferring solid, liquid or gas matter. Examples include light and sound waves. During an earthquake, the waves released may be “P” or “S” waves depending on the speed and ways in which they travel. P-Waves (Primary Waves) P-waves are longitudinal in nature. The vibrations are along the same direction as the direction of travel. It is also known as compressional waves. P-waves travel faster than S-waves. S-Waves (Secondary waves) Here the waves travel at right angles to the direction of travel. They are also known as transverse waves and example include water waves. With this in mind, you will notice that if you are close to the point where an earthquake struck, you will feel both P and S waves close within the same time frame. If you are further away, you will feel the P-wave first and then the S-wave a bit later. Both waves can be destructive, but their study helps us to know where the earthquake struck.

What is a volcano?

A volcano is an opening in the Earth's crust where hot liquid rock from deep within the Earth, called magma, erupts to the surface. 

Magma and Lava 

When rock beneath the Earth's surface gets really hot, it becomes molten or liquid. While it's still below the surface, it's called magma. Once the magma erupts to the surface through a volcano, it's called lava. The hotter and thinner the lava is, the farther it will flow. Lava can be very hot, sometimes as hot as 1000 degrees C. 

Eventually lava on the surface will stop flowing and cool and harden into rocks. Rocks formed from lava cooling are called igneous rocks. Some examples of igneous rocks include basalt and granite. 




Activity 

A volcano can be active, dormant, or extinct. An active volcano is one that has recently erupted or is currently erupting. A dormant volcano is one that hasn't erupted for a long time, but has the potential to still erupt. An extinct volcano is one that scientists think will never erupt again. 

Volcanic eruption with giant ash cloud

Explosive Volcanoes 

Some kinds of volcanoes just sort of ooze out lava over time. This is generally when the lava is thin. Other volcanoes have thicker lava that can plug up the vent of the of the volcano. When this happens pressure can build up over a long period of time. When the pressure gets high enough, the volcano can erupt with a huge explosion. Huge amounts of lava and ash can spew from the volcano.

Types of Volcanoes 

We usually think of volcanoes as tall mountains in the shape of a cone, however there are different types of volcanoes: 

Cinder cones - These are volcanoes formed from particles and blobs of lava ejected from a single vent at the top. They generally don't get taller than around 1,000 feet. 

Composite volcanoes - These volcanoes are also shaped like a cone, but are formed from layers of lava over many years. They can grow into huge mountains over 8,000 feet tall from their base. 

Shield volcanoes - They form from wide thin layers of lava that eventually are shaped like a shield. 

Lava domes - these are formed by thick lava that hardens right around the vent. They can sometimes form inside of other types of volcanoes. 

Volcano cloud as seen from a satellite

Interesting facts about Volcanoes

• The tallest volcano we know of in the Solar System is on Mars. It is called Olympus Mons and is 17 miles tall.
• A large volcano eruption can destroy an entire forest.
• The largest volcano on earth is Mauna Loa on the Hawaii Big Island. The tallest is Mauna Kea which is right next to it.
• The ash cloud from volcanoes can be dangerous. It can be harmful for people to breath and difficult for planes to fly through. A large ash cloud from Eyjafjallajökull, a volcano in Iceland, shut down most of the airports in Europe for several days in 2010.
• There are generally around 20 volcanoes erupting in the world at any given time.

Plate Tectonics, Earthquakes and Volcanoes

According to the theory of plate tectonics, Earth is an active planet. Its surface is composed of many individual plates that move and interact, constantly changing and reshaping Earth's outer layer. Volcanoes and earthquakes both result from the movement of tectonic plates.

Earthquake Facts

1. Natural events such as volcanic eruptions and meteor impacts can cause earthquakes, but the majority of naturally-occurring earthquakes are triggered by movement of the earth's plates.
2. The earth's surface consists of 20 constantly moving plates. The pressure increase from shifting plates can cause the crust to break. This break allows stress to be released as energy, which moves through the earth in the form of waves (aka earthquakes).
3. Normally, it's not the shaking ground itself that claims lives during an earthquake. It's the associated destruction of man-made structures and the instigation of other natural disasters such as tsunamis, avalanches and landslides. Not only is it important to have a plan for yourself in the case of an earthquake, but your pets need a disaster plan as well. Create an earthquake plan for the animals around so if disaster strikes, you’ll be prepared.
4. The National Earthquake Information Center (NEIC) records an average of 20,000 earthquakes every year (about 50 a day) around the world. There are, however, millions of earthquakes estimated to occur every year that are too weak to be recorded.
5. Each year the southern California area has about 10,000 earthquakes -- the majority of which go unnoticed. However, if there is a large earthquake the aftershock sequence will produce many more earthquakes of all magnitudes for months.

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