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Earthquakes

 

 

Plate Tectonics

 

The Earth's crust is like a big jigsaw puzzle of land pieces called plates. The plates move slowly because the layer below them is soft and flows.

 

A simplified world map with continents shown in light green and oceans in pale blue. The map is overlaid with bold turquoise lines marking tectonic plate boundaries. Each plate is labelled in black text. The North American Plate covers most of North America, while the Eurasian Plate spans Europe and much of northern Asia. The African Plate sits centrally over the African continent, with the South American Plate covering South America and the Antarctic Plate across Antarctica. The Pacific Plate is shown as a large region in the Pacific Ocean, bordered by looping boundary lines. Smaller plates are also labelled, including the Juan de Fuca Plate off the west coast of North America, the Cocos Plate west of Central America, the Nazca Plate west of South America, the Caribbean Plate between North and South America, the Arabian Plate in the Middle East, the Indian Plate south of Asia, the Philippine Plate east of Asia, the Anatolian Plate in the eastern Mediterranean area, and the Australian Plate covering Australia and surrounding ocean. The turquoise boundary lines weave across the oceans and continents, illustrating constructive, destructive, and conservative plate margins. The overall image is a clean, schematic reference map intended to show the location and extent of the world’s major tectonic plates.World map of major tectonic plates and plate boundaries

 

There are 3 types of plate movements.

 

  • Constructive - plates separating

  • Destructive - plates colliding

  • Conservative - plates sliding past each other.

 

Destructive plate movement - This occurs when plates move towards each other.  Different types of collision may take place at these boundaries.  Oceanic and Continental plates or two continental plates can collide with each other.

 

Oceanic and Continental plates colliding:  Since the oceanic plate is more dense, it is always forced underneath the continental plate.  This is called subduction (see animation below).  As the oceanic plate is pushed down a deep trench is formed. The plate melts and creates pressure in the surrounding area due to all the melting rock.  The resulting molten rock finds its way to the surface and volcanoes form.  Earthquakes also happen as the two plates slowly grind past each other.  The continental crust is not destroyed.  It is simply compressed, folded into anticlines and synclines and thickened to form a fold mountain range similar to the Andes in South America.

 

 

 

Two continental plates colliding:  When two continental plates collide head on, neither of them is subducted.  Instead the sediment layers laying between the two continent land masses get squeezed.  The effect is to form fold mountains similar to the Himalayas.  This type of fold mountain range has no volcanoes or deep focus earthquakes.  India, in fact detached it self from Africa and piled into the bottom of Asia.  It is still doing so, pushing the Himalayas up and up.  This means Mount Everest is getting taller by a few centimeters every year as India continues to push up into the continent of Asia.

 

India

 

Conservative plate movement:  When plates slide past each other this type of movement results.  Here, material is neither created or destroyed.  The best known example of this is the San Andres Fault in California (see animation below).  It marks the boundary between the Pacific plate and the North American plate.  These plate of rock don't glide smoothly past each other.  They catch on each other and as the forces build up they suddenly jerk.  This sudden jerking only lasts a few seconds, but brings devastation in a built and heavily populated area.  Building come tumbling down and many people get killed.  The city of San Francisco sits along side this fault line.  It was completely destroyed in 1906 and was again hit by a powerful tremor in 1991.  This could happen again any time.

 

 

 

These days in earthquake zones, developers try to build earthquake-proof buildings which are designed to withstand small amount of shaking.  In poorer countries, earthquakes usually cause much devastation where they have badly constructed properties, overcrowding and inadequate rescue services.

 

Useful Websites

Title

URL

BBC Earthquakes Page

WikiPedia Earthquakes Page

 

 

🌍 Knowledge Check: Earthquakes

Test your understanding of tectonic movements and seismic measurement based on the webpage information.

1. What is the name of the point on the Earth's surface directly above where an earthquake begins?

2. Which instrument do scientists use to detect and record the vibrations caused by an earthquake?

3. Earthquake energy travels through the Earth's crust in the form of:

4. According to the Richter scale, how much more ground shaking does a magnitude 5 earthquake have compared to a magnitude 4?

5. Why do most earthquakes occur at plate boundaries?

Click to Reveal Answers
1. The Epicentre (The focus is underground; the epicentre is on the surface).
2. A Seismometer (This is the device that records the movement).
3. Seismic waves (Energy is released and travels as waves through the crust).
4. 10 times more (Each number on the Richter scale represents a tenfold increase in shaking).
5. Tectonic plates push or slide past each other (The movement and buildup of pressure at boundaries cause the sudden snap).

 

Tags: Continental Drift, Plate Tectonics, Earthquakes, Volcano, continents moving, earthquake plates map, names of tectonic plates, plate tectonic evidence

 

 

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