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Observations of these alternating regions, found to be symmetrical on both sides of ocean floor ridges, have given scientists strong evidence of a key mechanism for creating continental drift. In studies funded by the U. Navy to improve submarine warfare, researchers began mapping the ocean floor, collecting data to explain the magnetic anomalies that were affecting sonar readings. From this data, scientists confirmed that the sea floor was indeed spreading apart.

From miles above the Earth, Geosat used radar to create an incredibly accurate map of the sea floor.

Book: The Dynamic Earth (Kious and Tilling) - Geosciences LibreTexts

Within a few short years, scientists studying this new data found conclusive evidence of continental drift and discovered additional evidence to validate the theory of plate tectonics. The margins of where these plates spread, collide, and slide past each other are where earthquakes, volcanoes, and mountains are concentrated. From the massive Pacific and European plates, to the smaller Caribbean and Juan de Fuca plates, each plate can react differently when coming into contact with another plate.

Converging boundaries occur when two plates collide. The area where one plate slides over another is called the subduction zone. Transverse boundaries occur when two plates simply slide past one another, which occurs less frequently. The stress and strain that results from the sliding of plates results in faults. As the stress becomes too great for the plates to bear, earthquakes can occur. A spreading boundary is characterized by two tectonic plates pulling apart from one another.

Dynamic Planet/Earthquakes and Volcanoes

A wide variety of geographic formations are explained by from plate activity. The collusion of the Juan de Fuca and Gorda plates created the Cascade Mountain range and was responsible for the eruption of Mount St. The subduction of the Pacific plate under the Eurasian plate created the Japanese islands, and, in South America, the South American plate collided with the Nazca plate to create the Andes Mountains as well as both the Cotopaxi and Azul volcanoes.

Scientists have a fairly good understanding of the movements of plates and their interaction at plate boundaries, but there are still unanswered questions. And, while the plate tectonic theory helps to explain phenomenon such as volcanoes and earthquakes, it does not yet offer an explanation why the continents were originally joined or why they broke apart.

Shield volcanoes are broad, domed volcanoes that resemble shields hence the name.

These volcanoes are formed from basaltic lava flows, which have a low viscosity. Shield volcanoes' wide shape is caused by the fluid lava, which travels farther out than the high viscosity lava of composite volcanoes.


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The way the lava of these volcanoes erupts and flows out in many different directions to form a shield shape is called an effusive eruption. Cinder cone volcanoes, also called scoria cones, are the conical volcanoes that we usually think about when it comes to volcanoes. Eruptions can spew large amount of gasses into the atmosphere, which contribute to a concept know as global cooling.

This occurs where the gas particles reflect sunlight before it even reaches Earth's surface. Global cooling can help offset Global Warming, and has been know to induce periods of cooling. Negative factors of this include a major change in solar radiation, and changes in El Nino and Southern Oscillation, which have great impact on global weather patterns.

Tsunamis are generated when sea floor abruptly changes. When it approaches shore, the tsunami wave slows, grows, and erodes the shoreline, thus causing significant damage at landfall. Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth.

Earthquakes can be generated by bomb blasts, volcanic eruptions, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake prone areas, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth. An earthquake itself is a vibration of the earth produced by rapid release of energy.

The focus is where the rock breaks creating an earthquake, and energy moves outward from the focus in seismic waves. Most natural earthquakes are caused by sudden slippage along a fault zone. The elastic rebound theory suggests that if slippage along a fault is hindered such that elastic strain energy builds up in the deforming rocks on either side of the fault, when the slippage does occur, the energy released causes an earthquake.

This theory was discovered by making measurements at a number of points across a fault. Prior to an earthquake it was noted that the rocks adjacent to the fault were bending.

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These bends disappeared after an earthquake suggesting that the energy stored in bending the rocks was suddenly released during the earthquake. Rock breakage is typically caused by stress AKA pressure acting on a rock. Types of stresses include:. When an earthquake occurs, the elastic energy is released and sends out vibrations that travel throughout the Earth. These vibrations are called seismic waves. The study of how seismic waves behave in the Earth is called seismology. The source of an earthquake is called the focus , which is an exact location within the Earth were seismic waves are generated by sudden release of stored elastic energy.

The epicenter is the point on the surface of the Earth directly above the focus. Sometimes the media get these two terms confused.

Plate tectonics: investigating and visualising our dynamic earth

Seismic waves emanating from the focus can travel in several ways, and thus there are several different kinds of seismic waves. Body Waves emanate from the focus and travel in all directions through the body of the Earth. There are two types of body waves. P-waves are Primary waves. They travel with a velocity that depends on the elastic properties of the rock through which they travel.

Dynamic Structure of the Earth

P-waves are the same thing as sound waves. They move through the material by compressing it, but after it has been compressed it expands, so that the wave moves by compressing and expanding the material as it travels. Thus the velocity of the P-wave depends on how easily the material can be compressed the how incompressible it is , how rigid the material is the rigidity , and the density of the material. P-waves have the highest velocity of all seismic waves and thus will reach all seismographs first.

S-waves are Secondary waves, also called shear waves. S-waves travel through material by shearing it or changing its shape in the direction perpendicular to the direction of travel. The resistance to shearing of a material is the property called the rigidity. It is notable that liquids have no rigidity, so that the velocity of an S-wave is zero in a liquid.

This point will become important later. Note that S-waves travel slower than P-waves, so they will reach a seismograph after the P-wave. Surface waves differ from body waves in that they do not travel through the Earth, but instead travel along paths nearly parallel to the surface of the Earth. Surface waves behave like S-waves in that they cause up and down and side to side movement as they pass, but they travel slower than S-waves and do not travel through the body of the Earth.

Structure of the Earth

Seismic waves travel through the Earth as vibrations. A seismometer is an instrument used to record these vibrations and the resulting graph that shows the vibrations is called a seismograph.

Geography - The Earth & Tectonic Plates: Geography Exam Tips

The seismometer must be able to move with the vibrations, yet part of it must remain nearly stationary. This is accomplished by isolating the recording device like a pen from the rest of the Earth using the principal of inertia. For example, if the pen is attached to a large mass suspended by a spring, the spring and the large mass move less than the paper which is attached to the Earth, and on which the record of the vibrations is made. Houfeng Didong Yi was the name given to the first seismoscope.

According to the Book of the Later Han a compilation of Chinese Historical works by Fan Ye Houfeng Didong Yi was bronze, had a radius of approximately 1 meter, and had 8 dragons spaced on its top, each holding bronze balls.

The dragon who's ball fell out would indicate the direction from which the earthquake came.