AClimate Change Part 2
Is It Really? Politics and power control spread fear works always lets find out
Tectonic Plates
Earth’s tectonic plates are in constant, albeit slow, motion. . This movement is driven by conveection current’s in the earth’s mantle where heat fromthe core causesmaga to rise , cool and sinkcreating a circular motion that drug sthe plates along. The plates can move apart, collide, or slide past each other, leading to various geological phenomena like earthquakes, volcanic eruptions, and the formation of mountains.
The earth’s crust is broken into separate pieces called tectonic plates (Fig. 7.14). Recall that the crust is the solid, rocky, outer shell of the planet. It is composed of two distinctly different types of material: the less-dense continental crust and the more-dense oceanic crust. Both types of crust rest atop solid, upper mantle material.
The upper mantle, in turn, floats on a denser layer of lower mantle that is much like thick molten tar.
Each tectonic plate is free-floating and can move independently. Earthquakes and volcanoes are the direct result of the movement of tectonic plates at fault lines. The term fault is used to describe the boundary between tectonic plates. Most of the earthquakes and volcanoes around the Pacific ocean basin—a pattern known as the “ring of fire”—are due to the movement of tectonic plates in this region. Other observable results of short-term plate movement include the gradual widening of the Great Rift lakes in eastern Africa and the rising of the Himalayan Mountain range. The motion of plates can be described in four general patterns:
- Collision: when two continental plates are shoved together
- Subduction: when one plate plunges beneath another (Fig. 7.15)
- Spreading: when two plates are pushed apart (Fig. 7.15)
- Transform faulting: when two plates slide past each other (Fig. 7.15)
The rise of the Himalayan Mountain range is due to an ongoing collision of the Indian plate with the Eurasian plate. Earthquakes in California are due to transform fault motion.
Evidence for the Movement of Continents
The shapes of the continents provide clues about the past movement of the continents. The edges of the continents on the map seem to fit together like a jigsaw puzzle.
For example, on the west coast of Africa, there is an indentation into which the bulge along the east coast of South America fits. The shapes of the continental shelves—the submerged landmass around continents—shows that the fit between continents is even more striking
Some fossils provide evidence that continents were once located nearer to one another than they are today.
Fossils of a marine reptile called Mesosaurus (Fig. 7.20 A) and a land reptile called Cynognathus (Fig. 7.20 B) have been found in South America and South Africa. Another example is the fossil plant called Glossopteris, which is found in India, Australia, and Antarctica (Fig. 7.20 C).
The presence of identical fossils in continents that are now widely separated is one of the main pieces of evidence that led to the initial idea that the continents had moved over geological history.
Evidence for continental drift is also found in the types of rocks on continents. There are belts of rock in Africa and South America that match when the ends of the continents are joined.
Mountains of comparable age and structure are found in the northeastern part of North America (Appalachian Mountains) and across the British Isles into Norway (Caledonian Mountains). These landmasses can be reassembled so that the mountains form a continuous chain.
Paleoclimatologists (paleo = ancient; climate = long term temperature and weather patterns) study evidence of prehistoric climates.
Evidence from glacial striations in rocks, the deep grooves in the land left by the movement of glaciers, shows that 300 mya there were large sheets of ice covering parts of South America, Africa, India, and Australia.
These striations indicate that the direction of glacial movement in Africa was toward the Atlantic ocean basin and in South America was from the Atlantic ocean basin. This evidence suggests that South America and Africa were once connected, and that glaciers moved across Africa and South America.
There is no glacial evidence for continental movement in North America, because there was no ice covering the continent 300 million years ago. North America may have been nearer the equator where warm temperatures prevented ice sheet formation.
Hot Spots
Recall that some volcanoes form near plate boundaries, particularly near subduction zones where oceanic crust moves underneath continental crust.
However, some volcanoes form over hot spots in the middle of tectonic plates far away from subduction zones (Fig. 7.25). A hot spot is a place where magma rises up from the earth’s mantle toward the surface crust.
When magma erupts and flows at the surface, it is called lava. The basalt lava commonly found at hot spots flows like hot, thick syrup and gradually forms shield volcanoes.
A shield volcano is shaped like a dome with gently sloping sides. These volcanoes are much less explosive than the composite volcanoes formed at subduction zones.
Some shield volcanoes, such as the islands in the Hawaiian archipelago, began forming on the ocean floor over a hot spot. Each shield volcano grows slowly with repeated eruptions until it reaches the surface of the water to form an island (Fig. 7.25). The highest peak on the island of Hawai‘i reaches 4.2 km above sea level.
However, the base of this volcanic island lies almost 7 km below the water surface, making Hawai‘i’s peaks some of the tallest mountains on Earth—much higher than Mount Everest.
Almost all of the mid-Pacific and mid-Atlantic ocean basin islands formed in a similar fashion over volcanic hot spots. Over millions of years as the tectonic plate moves, a volcano that was over the hot spot moves away, ceases to erupt, and becomes extinct (Fig. 7.25).
Erosion and subsidence (sinking of the earth’s crust) eventually causes older islands to sink below sea level. Islands can erode through natural processes such as wind and water flow. Reefs continue to grow around the eroded land mass and form fringing reefs, as seen on Kauaʻi in the main Hawaiian Islands
Eventually all that remains of the island is a ring of coral reef. An atoll is a ring-shaped coral reef or group of coral islets that has grown around the rim of an extinct submerged volcano forming a central lagoon (Fig. 7.27).
Atoll formation is dependent on erosion of land and growth of coral reefs around the island. Coral reef atolls can only occur in tropical regions that are optimal for coral growth.
The main Hawaiian Islands will all likely become coral atolls millions of years into the future. The older Northwestern Hawaiian Islands, many of which are now atolls, were formed by the same volcanic hot spot as the younger main Hawaiian Islands.
Can scientists predict earthquakes?
No, and it is unlikely they will ever be able to predict them. Scientists have tried many different ways of predicting earthquakes, but none have been successful. On any particular fault, scientists know there will be another earthquake sometime in the future, but they have no way of telling when it will happen.
Tsunamis
Tsunamis are just long waves — really long waves. But what is a wave? Sound waves, radio waves, even “the wave” in a stadium all have something in common with the waves that move across oceans. It takes an external force to start a wave, like dropping a rock into a pond or waves blowing across the sea. In the case of tsunamis, the forces involved are large — and their effects can be correspondingly massive.
What is a tsunami?
A tsunami is a series of extremely long waves caused by a large and sudden displacement of the ocean, usually the result of an earthquake below or near the ocean floor. This force creates waves that radiate outward in all directions away from their source, sometimes crossing entire ocean basins. Unlike wind-driven waves, which only travel through the topmost layer of the ocean, tsunamis move through the entire water column, from the ocean floor to the ocean surface.
What causes tsunamis?
Most tsunamis are caused by earthquakes on converging tectonic plate boundaries. According to the Global Historical Tsunami Database, since 1900, over 80% of likely tsunamis were generated by earthquakes. However, tsunamis can also be caused by landslides, volcanic activity, certain types of weather, and—possibly—near-earth objects (e.g., asteroids, comets) colliding with or exploding above the ocean.
Tsunami movement
Once a tsunami forms, its speed depends on the depth of the ocean. In the deep ocean, a tsunami can move as fast as a jet plane, over 500 mph, and its wavelength, the distance from crest to crest, may be hundreds of miles. Mariners at sea will not normally notice a tsunami as it passes beneath them; in deep water, the top of the wave rarely reaches more than three feet higher than the ocean swell. NOAA Deep-ocean Assessment and Reporting of Tsunmi (DART) systems, located in the deep ocean, are able to detect small changes in sea-level height and transmit this information to tsunami warning centers.
Geology
Geologists have hypothesized that the movement of tectonic plates is related to convection currents in the earth’s mantle. Convection currents describe the rising, spread, and sinking of gas, liquid, or molten material caused by the application of heat.
An example of convection current is shown in Fig. 7.16. Inside a beaker, hot water rises at the point where heat is applied. The hot water moves to the surface, then spreads out and cools. Cooler water sinks to the bottom.
Earth’s solid crust acts as a heat insulator for the hot interior of the planet. Magma is the molten rock below the crust, in the mantle. Tremendous heat and pressure within the earth cause the hot magma to flow in convection currents. These currents cause the movement of the tectonic plates that make up the earth’s crust.
The earth has changed in many ways since it first formed 4.5 billion years ago. The locations of Earth’s major landmasses today are very different from their locations in the past (Fig. 7.18). They have gradually moved over the course of hundreds of millions of years—alternately combining into supercontinents and pulling apart in a process known as continental drift.
The supercontinent of Pangaea formed as the landmasses gradually combined roughly between 300 and 100 mya. The planet’s landmasses eventually moved to their current positions and will continue to move into the future.
Plate tectonics is the scientific theory explaining the movement of the earth’s crust. It is widely accepted by scientists today. Recall that both continental landmasses and the ocean floor are part of the earth’s crust, and that the crust is broken into individual pieces called tectonic plates (Fig. 7.14).
The movement of these tectonic plates is likely caused by convection currents in the molten rock in Earth’s mantle below the crust. Earthquakes and volcanoes are the short-term results of this tectonic movement. The long-term result of plate tectonics is the movement of entire continents over millions of years (Fig. 7.18).
The presence of the same type of fossils on continents that are now widely separated is evidence that continents have moved over geological history.
Seafloor Spreading at Mid-Ocean Ridges
Convection currents drive the movement of Earth’s rigid tectonic plates in the planet’s fluid molten mantle. In places where convection currents rise up towards the crust’s surface, tectonic plates move away from each other in a process known as seafloor spreading
Hot magma rises to the crust’s surface, cracks develop in the ocean floor, and the magma pushes up and out to form mid-ocean ridges. Mid-ocean ridges or spreading centers are fault lines where two tectonic plates are moving away from each other
Mid-ocean ridges are the largest continuous geological features on Earth. They are tens of thousands of kilometers long, running through and connecting most of the ocean basins. Oceanographic data reveal that seafloor spreading is slowly widening the Atlantic ocean basin, the Red Sea, and the Gulf of California
The gradual process of seafloor spreading slowly pushes tectonic plates apart while generating new rock from cooled magma.
Ocean floor rocks close to a mid-ocean ridge are not only younger than distant rocks, they also display consistent bands of magnetism based on their age (Fig. 7.22.1). Every few hundred thousand years the earth’s magnetic field reverses, in a process known as geomagnetic reversal.
Some bands of rock were produced during a time when the polarity of the earth’s magnetic field was the reverse of its current polarity. Geomagnetic reversal allows scientists to study the movement of ocean floors over time.
Paleomagnetism is the study of magnetism in ancient rocks. As molten rock cools and solidifies, particles within the rocks align themselves with the earth’s magnetic field.
In other words, the particles will point in the direction of the magnetic field present as the rock was cooling.
If the plate containing the rock drifts or rotates, then the particles in the rock will no longer be aligned with the earth’s magnetic field. Scientists can compare the directional magnetism of rock particles to the direction of the magnetic field in the rock’s current location and estimate where the plate was when the rock formed
Seafloor spreading gradually pushes tectonic plates apart at mid-ocean ridges. When this happens, the opposite edge of these plates push against other tectonic plates. Subduction occurs when two tectonic plates meet and one moves underneath the other .
Oceanic crust is primarily composed of basalt, which makes it slightly denser than continental crust, which is composed primarily of granite.
Because it is denser, when oceanic crust and continental crust meet, the oceanic crust slides below the continental crust. This collision of oceanic crust on one plate with the continental crust of a second plate can result in the formation of volcanoes (Fig. 7.23). As the oceanic crust enters the mantle, pressure breaks the crustal rock, heat from friction melts it, and a pool of magma develops.
This thick magma, called andesite lava, consists of a mixture of basalt from the oceanic crust and granite from the continental crust. Forced by tremendous pressure, it eventually flows along weaker crustal channels toward the surface.
The magma periodically breaks through the crust to form great, violently explosive composite volcanoes—steep-sided, cone-shaped mountains like those in the Andes at the margin of the South American Plateis a
What causes earthquakes and where do they happen?
The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet.
But this skin is not all in one piece – it is made up of many pieces like a puzzle covering the surface of the earth.
Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle pieces tectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults.
Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.
Why does the earth shake when there is an earthquake?
While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up.
When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond.
The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us!
How they recorded
Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram.
The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement.
The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.
Tsunami effects on humans
Large tsunamis are significant threats to human health, property, infrastructure, resources, and economies. Effects can be long-lasting, and felt far beyond the coastline. Tsunamis typically cause the most severe damage and casualties near their source, where there is little time for warning. But large tsunamis can also reach distant shorelines, causing widespread damage.
The 2004 Indian Ocean tsunami, for example, impacted 17 countries in Southeastern and Southern Asia and Eastern and Southern Africa.
Tsunami forecasting
Scientists cannot predict when and where the next tsunami will strike. But the tsunami warning centers know which earthquakes are likely to generate tsunamis and can issue messages when one is possible.
They monitor networks of deep-ocean and coastal sea-level observation systems designed to detect tsunamis and use information from these networks to forecast coastal impacts and guide local decisions about evacuation.
Tsunami warning capabilities have become dramatically better since the 2004 Indian Ocean tsunami. NOAA scientists are working to further improve warning center operations and to help communities be prepared to respond.