Earth Processes: Tectonic Plates

Question 1

What did Alfred Wegener theorize in 1912?

Answer 1

In 1912, Alfred Wegener theorized that all of the continents had been apart of one big supercontinent, which he called Pangaea

Question 2

What did Wegener observe in fossils of the same species and mountain chains that supported his theory of Pangaea?

Answer 2

As Wegener did his research, he remembered reading about fossils of the same species being found on different continents. In his mind, the only way this would make since was if the continents were once connected. Wegener also observed similar land formations found across different continents. For example, mountain chains seemed to line up as if the continents were once together

Question 3

How was Wegener’s theory of Pangaea viewed around 1950? Despite this, which of his ideas was not accepted?

Answer 3

Around 1950, Wegener’s theory of Pangaea was widely accepted, but his explanation on how the continents moved was not. The study of the magnetic properties of different-aged rock, or paleomagnetism, showed a changing north-south orientation. Since this was true of rock from the ocean floor, it suggests that the continents alone were not in motion. A new theory suggested that the continents were part of larger crustal plates. Plate tectonics is the idea that Earth’s outer shell is divided into many plates that glide, or float, over the mantle. Convection in Earth’s mantle causes the plates to move slowly. The plates are pushing and spreading apart in some places, while pulling and moving downward in other places. Thus, the theory of plate tectonics evolved from the theory of Continental drift

Question 4

What was Wegener’s Theory of Continental Drift? How was it accepted?

Answer 4

Wegener believed the continents slowly drifted apart, or slid across the ocean floor, due to the rotation of the Earth over millions of years. He called this theory, the “Theory of Continental Drift”. Wegener’s ideas were controversial, and not everyone supported him. Many scientists objected to Wegener’s explanation that the continents moved apart.

Question 5

What were the four pieces of evidence for Wegener’s theory of Continental Drift?

Answer 5

Jigsaw fit, geological fit, glacial deposits, and fossil evidence

Question 6

How did Wegener’s evidence of a Jigsaw Fit support his theory? What is it?

Answer 6

The eastern coastline of South America and the western coastline of Africa showed the best fit. If matched at a depth of 1,000 meters below sea level, they fit even better. Still, gaps and overlaps existed, which scientists explained as changes that had happened since the split of Pangaea by:
• coastal erosion
• coastal deposition
• rises in sea level
• changes in land level

Question 7

How did Wegener’s evidence of Geological Fit support his theory? What was it?

Answer 7

Wegener found evidence that geological features, such as cratons, were continuous between continents. A craton is the stable interior portion of Earth’s crust that forms the nuclear mass of a continent. Other features, such as rocks, were also continuous across continents. For example, a belt of ancient rocks in Brazil matches a belt of ancient rocks in West Africa.

Question 8

How did Wegener’s evidence of Glacial Deposits support his theory? What was it?

Answer 8

A few hundred million years ago, glaciers were found across the southern continents. Wegener believed that Antarctica, southern South America, and southern Africa, India, and Australia were all joined around the south polar region. Glacial deposits found on these continents supported this idea.

Question 9

How did Wegener’s Fossil Evidence support his theory? What was it?

Answer 9

Fossils of the same species have been found on different continents. This was more evidence for Wegener’s theory. He believed the animals could not have found their way from one continent to another over water.

Question 10

Theory of Continental Drift Definition

Answer 10

The slow drift of continents apart from each other

Question 11

What had Wegener thought moved the continents? How was this disproven?

Answer 11

Wegener thought that the rotation of Earth created a centrifugal force that caused Pangaea to break apart and move away from the poles. Scientists, however, did some calculations, and most agreed that this centrifugal force would not be enough to move continents

Question 12

What other proposal did Wegener make to support his theory that the continents drifted across the ocean floor, after being disproven? How was he disproven again? What were some other theories scientists had to explain the evidence for Continental drift?

Answer 12

Wegener also proposed that the gravitational pull of the moon could cause the continents to move. This, too, was shown by scientists to be impossible. Scientists also had other ideas about how to explain some of the evidence for continental drift. Some suggested that animals had moved from one continent to another by land bridges that had since disappeared

Question 13

What are some examples of new evidence supporting Continental drift?

Answer 13

Seafloor spreading, hotspots, transform faults, and mantle convection

Question 14

Seafloor Spreading Definition

Answer 14

The formation of new oceanic crust at mid-ocean ridges through the upwelling of magma

Question 15

Hotspot Definition

Answer 15

A place on Earth’s surface which is fed by underlying mantle that is hotter than surrounding mantle

Question 16

Transform Fault Definition

Answer 16

A tectonic plate boundary where plates slip past each other

Question 17

Mantle Convection Definition

Answer 17

Movement in the mantle in which cooler parts move down and warmer parts move up

Question 18

What is the evidence of Seafloor spreading and how does it support the theory of Continental Drift?

Answer 18

In the early 1960s, Harry Hammond Hess proposed the theory of seafloor spreading to explain how the continents moved. Sonar allowed the ocean floor to be mapped. Undersea mountain ranges called mid- ocean ridges were discovered. Some are as tall as 1.5 km above the seafloor.
Instead of continents floating on Earth’s surface, Hess thought that the continents were part of different tectonic plates and that the plates moved. At the mid-ocean ridges, two tectonic plates move apart. Magma moves up through the mid-ocean ridges and flows out, creating new crust. Old crust is destroyed at the same time new crust is made. The old crust sinks into ocean trenches and melts.

Question 19

What is the evidence of hotspots, and how does it support the theory of Continental drift?

Answer 19

There were still some unanswered questions about plate tectonics. One major question was why volcanoes could be found far away from plate boundaries. Geophysicist John Tuzo-Wilson proposed the idea of hotspots to explain this in 1963. Hotspots are fixed spots in Earth’s mantle that the tectonic plates move over. These hotspots can form volcanic island chains, such as the Hawaiian Islands.

Question 20

What is the evidence of transform faults, and how does it support the theory of Continental drift?

Answer 20

Tuzo-Wilson discovered a new type of plate boundary in 1965. Transform faults are also called conservative plate boundaries. At these boundaries, plates slip past each other. These faults connect oceanic ridges to ocean trenches and allow plates to move without oceanic crust being created or destroyed.

Question 21

What is the evidence of mantle convection, and how does it support the theory of Continental drift?

Answer 21

The most widely accepted cause of plate movement is mantle convection. The mantle is the layer of mostly solid Earth that lies between its inner core and its outer layer, the crust. Movement of heat and material in the mantle is what scientists believe drives the movement of plates. Convection is movement in a gas or liquid in which the cooler parts move down and the warmer parts move up, like how hot magma moves up through mid-ocean ridges. Convection in the mantle is similar to convection of hot water in a teakettle.

Question 22

What are the three types of plate boundaries and the interactions that occur at them?

Answer 22

1. Plates move past each other at transform boundaries
2. Plates move away from each other at divergent boundaries
3. Plates push against each other at convergent boundaries

Question 23

What does the theory of plate tectonics describe?

Answer 23

The theory of plate tectonics provides an explanation for the movement of continents and how mountains and volcanoes form. It also explains earthquakes, which happen at transform boundaries, and describes how Earth’s surface has changed over time. This theory developed mostly in the 1900s, as new technologies allowed scientists to collect data that supported many of the parts of the theory of continental drift which led to the theory of plate tectonics

Question 24

What did Alfred Wegener do in 1915?

Answer 24

In 1915, Alfred Wegener wrote a book about his theory of continental drift

Question 25

Fossil Definition

Answer 25

Traces or remains of past plant and animals life

Question 26

What are the different types of fossils?

Answer 26

Mold—An impression in rock created when an organism leaves its shape in mud or other soft material, which then hardens

Cast—The three-dimensional shape of an organism that forms when a mold fills with minerals that later harden

Body Fossil—A part of the organism—bones, teeth, skin—left behind and preserved Petrification—The transformation of an organism into stone when minerals infiltrate and eventually replace its organic material

Amber Encasement—Preservation of a living organism, such as an insect, when it is trapped inside a substance such as tree resin, which later hardens

Carbonization—A thin, dark carbon impression of an organism, such as a leaf, left behind when it is trapped and squeezed between layers of rock

Trace—Preservation of something the animal left behind, such as a footprint or tooth mark

Question 27

How did fossil evidence from the Mesosaurus fossils support Wegener’s theory?

Answer 27

Mesosaurus was a small reptile that lived in fresh water. Mesosaurus fossils were found in two places—Southern Africa and eastern South America. Because Mesosaurus is a freshwater animal, it does not have the ability to swim across a large body of salt water such as the Atlantic Ocean. Wegener believed the location of the Mesosaurus fossils proved that Africa and South America were once joined. His theory was strengthened by the coastlines of the two continents, which seem to fit together. If that were true, the continents must have moved to their present positions—an ocean apart

Question 28

Why do the coastlines of Africa and South America not fit exactly?

Answer 28

The coastlines of Africa and South America do not fit exactly. The forces of costal weathering and erosion over millions of years could be part of the cause. The shape of Continental shelves could also affect the way the two continents fit together

Question 29

How do Continental shelves support Wegener’s theory?

Answer 29

Continental shelves are the areas around continents where the sea is fairly shallow compared to the depths of the open ocean. The edges of Continental shelves extend beyond the coastline. Where the coastline does not fit exactly, the Continental shelves actually fit well, further supporting Wegener’s ideas

Question 30

How does the fossil evidence from Cynognathus fossils supporting Wegener’s theory?

Answer 30

Like Mesosaurus, fossils of the Cynognathus were also found in South America and Africa. The Cynognathus was a land reptile about three feet in length that lived during the Triassic Period. The areas where fossils of this reptile are found are now separated by miles of ocean

Question 31

How did fossil evidence from Lystrosaurus fossils support Wegener’s theory?

Answer 31

The Lystrosaurus was another land reptile common during the Triassic period. Fossils from this extinct animal have been found in India, Africa, and even in Antarctica. Aside from being separated by vast oceans today, these areas have very different climates . Wegener reasoned that it is unlikely that Lystrosaurus could live in such different climate zones

Question 32

How did the fossils of the Svalbard islands, north of Norway, support Wegener’s theory?

Answer 32

Wegener studied fossils found on the Svalbard islands North of mainland Norway. The islands have a harsh, cold climate during much of the year. Interestingly, the fossils Wegener located were of plants native to a much warmer climate. How could fossils of such plants be found North of the Arctic Circle? Wegener’s answer: the Svalbard islands have not always been in their current polar positions. Rather, they were once located where the climate was warmer

Question 33

How did the fossils of Glossopteris support Wegener’s theory?

Answer 33

The Glossopteris was a fern that grew in subarctic climates.inverstiagtaions show that the fossil left behind by the plant during the Paleozoic era were found in what are now Africa, Australia, India, and South America. Later, an expedition to the South Pole returned with Glossopteris fossils found in Antarctica. The areas where the fossil ferns were found have diverse climates today—not all them hospitable for the plant. Wegener therefore believed these lands were grouped in a common climate region in the distant past

Question 34

What do the ages of rock along mid-oceanic ridges reveal? What is magnetic normal? What is magnetic reverse? How do the magnetic fields of rock compare?

Answer 34

The ages of rock along mid-oceanic ridges reveal that the sea floor spreads outward from the ridge. As scientists investigated more rocks from the ocean floor, they identified another important source of evidence. The rocks could be placed into categories according to their magnetic field direction. Some rocks had a north-south direction, similar to Earth’s current magnetic field (magnetic normal). Other rocks had magnetic fields oriented in the opposite direction (magnetic reverse). They also discovered magnetic patterns in lava rock found on land

Question 35

What element in rocks determine their magnetic alignment? How does this work?

Answer 35

As tectonic plates move apart, lava spews out. The iron in lava is magnetic, so its molecules align with Earth’s magnetic field. Over millions of years, layers have been forming as plates diverge, lava disperses, and the lava cools.

Question 36

How did the differences in the polarity of lava support the theory of plate tectonics?

Answer 36

Through paleomagnetism, the study of ancient magnetic fields, scientists discovered a uniqueness in the polarity of lava. They found that the molecules of newer lava were aligned with the current North and South Poles, while molecules in older layers of lava were skewed. This finding left scientists with two hypotheses to test. Have Earth’s poles drifted? Or have the continents shifted? The hypothesis of Earth’s poles drifting was proven inaccurate because same-age lava found on different continents was not magnetically aligned in the same ways. Through this same finding, scientists determined that the hypothesis that the continents have drifted was accurate. The older layers of lava that appeared skewed were at one time aligned with Earth’s magnetic field. These findings about paleomagnetism are the basis for our understanding of plate tectonics.

Question 37

What does the magnetic direction of rock depend on?

Answer 37

The magnetic direction of the rocks depend on the direction of Earth’s magnetic field when the rocks formed. Magnetic iron pieces within lava align with the Earth’s magnetic field when the lava cools to form solid rock

Question 38

How do scientists use the magnetic direction of ancient rocks to explain that the continents have moved over time?

Answer 38

Layers within lava rock have slightly skewed magnetic directions. Scientists conclude that the directions are skewed because the plate on which each rock layer formed was moving relative to Earth’s magnetic field, which remains relatively stable for long time periods. If the plates move, the continents atop them move, too

Question 39

Remember: Rocks on the ocean floor that record large reversals in Earth’s magnetic field also provide valuable paleomagnetic evidence that supports the tectonic plate theory

Answer 39

Rocks on the ocean floor that record large reversals in Earth’s magnetic field also provide valuable paleomagnetic evidence that supports the tectonic plate theory

Question 40

Paleomagnetism Definition

Answer 40

The study of ancient magnetic fields

Question 41

How do maps of the magnetic direction of rocks found at different distances from mid-oceanic ridges help scientists draw conclusions?

Answer 41

When scientists construct maps showing the magnetic direction of rocks found at different distances from mid-oceanic ridges, they find patterns of irregular, alternating magnetic stripes. The stripes are mirror images on either side of the ridge. Scientists use the pattern to determine how fast new rock is forming at the ridge, as well as how fast landforms supported by the plate move along Earth’s surface

Question 42

What is the difference between the magnetic North Pole and the geographic North Pole?

Answer 42

The geographic North Pole and the magnetic North Pole are not in the same place. The geographic North Pole is a fixed location on the planet. The magnetic North Pole is the place where a compass points towards north. Recently, the magnetic North Pole has been moving rapidly toward Siberia at a rate of more than 48 km (30 mi) a year. Scientists keep track of the location in the World Magnet Model which is used to correct GPS systems used by navigators

Question 43

How can scientists use each magnetic stripe along the ocean floor to measure time?

Answer 43

Magnetic stripping patterns help scientists measure the rate of tectonic plate growth and motion. The approximate dates when Earth’s magnetic poles reversed can be determined by using other geologic evidence. As a result, scientists can use each magnetic stripe along the floor as a measure of time

Question 44

Approximately, how much time passed between the last pole reversal and the one before it. What does the time required to form the magnetic stripes that correspond to these pole reversals equate to?

Answer 44

Scientists know that the last pole reversal was approximately 780,000 years ago. They also know that 120,000 years passed between that pole reversal and the one before. The tie. Required to form the magnetic stripes that corresponded to these polereversals equals approximately 900,000 years

Question 45

What tools are needed to analyze magnetic striping patterns?

Answer 45

Analysis of magnetic striping patterns on the ocean floor can reveal the direction plates move and how fast, as well as record large- scale geologic events that took place in the past. The tools needed to analyze magnetic striping patterns are a geomagnetic time scale and a geomagnetic map.

Question 46

What is a geomagnetic time scale? What is a geomagnetic map? How do they relate? What do they show?

Answer 46

A geomagnetic time scale is a record of the Earth’s magnetic polarity through time. It begins with the current period of magnetic normal, then shows alternating time periods of magnetic reverse (white bands) and magnetic normal (dark bands) that took place in the past. Ages on the scale shown are in millions of years (Ma). A geomagnetic time scale can be used to interpret a geomagnetic map, which is a map that shows the magnetic striping patterns on a particular region of Earth’s surface.

Question 47

What does paleomagnetic evidence show about the Juan de Fuca plate?

Answer 47

The Juan de Fula plate is located off the western coast of North America. As it moves northeast at 4 cm per year, it subducts below the North American plate. Paleomagnetic evidence reveals some of the history of the Juan de Fula plate. The broke alignment of the magnetic stripes indicates that the plate fractured along a transform fault during a large-scale seismic event that likely involved a series of earthquakes. Scientists can use the striping pattern to deter,Jen approximately when the earthquakes took place

Question 48

Where do plate interactions leave their traces?

Answer 48

Plate interactions leave their traces in the rock record. Geologists study this rock record to piece together Earth’s history

Question 49

Why is part of the rock record erased in some areas?

Answer 49

As tectonic plates gradually move, they cause mountains to form and erode. In the process, they deposit layers upon layers of sediment. Over time, crustal material is often uplifted and exposed to long periods of erosion. Part of the rock record is, thus, erased in some areas

Question 50

Where can some of the oldest rocks on Earth be found?

Answer 50

Oceanic Crust tends to move away from mid-oceanic ridges. When tectonic activity causes a plate with oceanic crust to collide with another plate, the denser oceanic crust sinks beneath the other plate’s edge. In this way, rock that makes up oceanic crust is recycled over time. In contrast, Continental crust tends to remain intact for longer periods. Rock material in the center of continents is often far from tectonic activity that would cause it to be drawn back into the mantle. As a result, material in the continental cores can contain some of the oldest rock on Earth

Question 51

How do mountain ranges and cratons support the idea that South America and Africa were once together? What was the landmass of Gondwana?

Answer 51

When geologists mapped the rock layers in South America and Africa, they noted the locations of mountain chains and the position of cratons that were dated as over 2 billion years old. The positions of these patterns in the rocks match across the edges of these two continents. This suggests that they had been one continuous landmass for an extremely long time. Geologists hypothesize that these continents broke apart around 180 million years ago. The super-continent formed by South America and Africa also consisted of present day India, Antarctica, and Australia. Geologists call this landmass “Gondwana”, after a region in India

Question 52

Craton Definition

Answer 52

Stable area of Earth’s crust that forms the core of a continent

Question 53

What was the landmass of Laurasia? How did the collision that formed this landmass contribute to the formation of the Appalachian mountains and other ranges?

Answer 53

The rock formations that make up the Appalachian Mountains in the eastern United States are thought to be the product of four stages of mountain-building events. Identical processes also resulted in similar mountains of the small edges in Europe and Africa. Geologists have concluded the 1st of these mountain-building events occurred over 400 million years ago. The landmasses that would later become North America, Greenland, United Kingdom, and Scandinavia collided. They formed one continent, which geologists call Laurasia. The edge where these landmasses came together resulted in a long chain of mountains

Question 54

What landmass did the supercontinents of Laurasia and Gondwana form when they collided? How did this collision contribute to the formation of the Appalachian mountains and other identical mountain ranges in Scandinavia, the United Kingdom, and East Africa?

Answer 54

Over time, the combined supercontinent of Laurasia collided with Gondwana. The result was one global continent that scientists called Pangaea. The force of the collision folded and buckled the edges of the continents so severely that a long chain of mountains rose. They are believed to be higher than the present-day Himalayan mountains. The remnants of these mountains, long since eroded, are what makes up the Appalachians and the nearly identically eroded mountains found in Scandinavia, the United Kingdom, and east Africa

Question 55

Geologists have found clues about ancient climates in the rock record. These clues make sense only when analyzed in the light of plate tectonics. How does coal contribute to these clues?

Answer 55

Coal is one of the fossil fuels, along with natural gas, crude oil, and oil shale. Coal was made from swamp organisms that were buried and exposed to long periods of heat. Over time, about three meters of dead plants can eventually be compressed into about 30 cm (1 ft.) of coal. When geologists examine a bed of coal, which is often filled with imprints of leaves and bits of the plant matter that formed it, they know that the area was a swamp millions of years ago. The three largest coal reserves are in the United States, Russia, and China, many in areas far from tropical swampy areas. Some of the coal beds in these reserves are as thick as 30 m (98 ft.). When these coal deposits are put into the context of plate tectonics, their locations make sense. Present-day coal reserves were formed along vast coastal areas that were near the equator around 300 million years ago. At that time, Laurasia was approaching Gondwana, causing mountain ranges to rise and begin eroding. Their weight had bowed down Earth’s crust in that area, submerging coastal water into the seas.

Question 56

How has glacial evidence contributed to our understanding of Gondwana and the Earth?

Answer 56

Geologists have noted that glaciers have covered Earth’s surface during many periods in the past. Glaciers consists of rocks and pebbles in a matrix of ice. They can be as much as a mile high. Because of their mass and size, glaciers leave clues that show where they have been. For example, they can carve grooves in bedrock where sheets of ice have scraped across. Glaciers also act like giant bulldozers, leaving behind enormous hills of sediment where their outer edge once was. Geologists have identified signs of glaciation that occurred about 300 million years ago in parts of the world where one would not expect to find glaciers. Geologists might expect to find signs of glaciers around the entire Earth during periods when the whole planet was in an ice age. But geologic evidence indicated that during that era, Earth was not in a deep ice age. The glaciers left their mark on large areas of South America, Africa, India, Antarctica, and Australia. Geologists concluded that either there was a massive ice sheet extending across the oceans or that these continents were once connected as one supercontinent during that period. When geologists consider the movement of plate tectonics, the evidence suggests that the supercontinent Gondwana was centered over the South Pole.

Question 57

What makes Australia’s assortment of animals unique?

Answer 57

It is well known that each present-day continent has a unique assortment of animals. For example, North America is home to large mammals, such as buffalo and bears. South America is home to llamas and some large rodents, and Africa has elephants and zebras. Australia’s animal life is particularly unique. It has a wide variety of marsupials, such as kangaroos and koalas, and very few placental mammals.

Question 58

Marsupial Definition

Answer 58

Fur-covered animal that produces milk and whose offspring develop within the mother’s pouch

Question 59

How was the distribution of marsupial before?

Answer 59

This assortment of animals was not always this way. Discoveries of ancient fossils have shown that millions of years ago, marsupials across the southern continents were much more similar. In fact, fossils of marsupials found in New Zealand, Australia, Antarctica, and South America dating back to 60 million years ago were the same. After that time, the fossils of marsupials in Australia began to grow more distinct

Question 60

When did Pangea start to break up? When did it break up?

Answer 60

The rock record suggests that Pangaea slowly began to break up around 200 million years ago. South America, Antarctica, and Australia were the last three continents to remain connected. They were all finally split up around 65 million years ago.

Question 61

True or False: Coal is organic, meaning it is not made from minerals but from organic matter, from organisms.

Answer 61

True

Question 62

True or False: the slower magma cools, the larger the crystals in rock

Answer 62

True

Question 63

Where can thermal convection take place?

Answer 63

Thermal convection can take plac​e within sma​ll or large fluid systems, which include liquids or gases. The systems may be open or closed. There are convection cycles above a cup of hot cocoa, within a closed kitchen oven, and in air currents along the ocean shore.

Question 64

What is a volcano?

Answer 64

A volcano is a rupture in Earth’s crust where lava and gases can burst out onto the surface

Question 65

What is the mountain formed by a volcanic eruption called?

Answer 65

The mountain formed by a volcanic eruption is also known as a volcano

Question 66

How is a volcano classified as active, dormant, or extinct?

Answer 66

A volcano is classified as active if it has erupted within the past 10,000 years and dormant if it is not erupting, but is expected to erupt again. An extinct volcano hasn’t erupted within the past 10,000 years, and is never expected to erupt again

Question 67

What are the three types of volcanoes and how are they classified?

Answer 67

The three main types of volcanoes are stratovolcanoes, shield volcanoes, and cinder cone volcanoes. The different types are distinguished by shape, whether they have explosive or quiet eruptions, and the type of lava that flows from them

Question 68

Why are extinct volcanoes not expected to erupt again?

Answer 68

Extinct volcanoes are not expected to erupt again

Question 69

How do stratovolcanoes, shield volcanoes, and composite volcanoes compare?

Answer 69

Stratovolcanoes are explosive, sending tons of lava particles, gas, and ash into the air. The explosion leaves behind a crater. Shield volcanoes are shallow-pitched volcanoes that look like a shield. The thinner lava they release travels down their sides and builds up over eruptions. Composite volcanoes can be explosive like stratovolcanoes and also channel magma through fissures, creating thinner lava flows like shield volcanoes.

Question 70

What are the characteristics of a stratovolcano?

Answer 70

More than half of the volcanoes on Earth are stratovolcanoes. These tall volcanoes have a gentle slope near the base, rising to steep sides and a narrow cone at the top. They may also have vents at various places along the sides of the volcano. Stratovolcanoes are also known as composite volcanoes because of their layers of lava, ash, and rock. The magma of a stratovolcano is viscous, causing it to build up underground with increasing pressure until it suddenly explodes with a dangerous eruption. The thick lava from a stratovolcano flows slowly and hardens quickly to produce the steep sides of the volcano.

Question 71

When did the most destructive volcanic eruption in United Staes history occur? Where?

Answer 71

The most destructive volcanic eruption in United States History occurred in 1980 at Mount St. Helens

Question 72

What are the characteristics of a shield volcano?

Answer 72

A shield volcano gets its name from the resemblance of its wide, dome shape similar to a knight’s shield lying on the ground. The magma that forms a shield volcano is less viscous, producing quieter eruptions. The lava flows long distances from the summit or vents along the sides before cooling into thin sheets. The layers of lava give a shield volcano its broad base with gentle slopes to the summit.

Question 73

What are the characteristics of a cinder cone volcano?

Answer 73

One of simplest types of volcanoes is a cinder cone, also known as a scoria cone. Compared to other types of volcanoes, cinder cones are usually much smaller and might form in just a few months or years. The name cinder cone refers to the single, symmetrical cone at the top of a steep hill or mountain and the chunks of lava, called cinder, that are ejected and quickly cool during an eruption.

Question 74

How complicated is it to predict a volcanic eruption? How do scientists predict eruption?

Answer 74

Predicting a volcanic eruption is complicated. Many of the clues that an eruption is imminent occur far below Earth’s surface where magma can pool. Forces of moving rock begin to push the magma toward Earth’s surface, and the pressure builds. Inactive volcanoes pose little danger to people, but the eruption of some volcanoes can occur quickly and can be catastrophic to the surrounding area. Scientists use a variety of technology to monitor volcanoes and give predictions that allow as much time as possible to evacuate the area.

Question 75

How do you predict a volcanic eruption with sensors?

Answer 75

Sensors placed near active volcanoes alert scientists that gases once dissolved in underground magma are beginning to escape through the surface, indicating that an eruption may be imminent.

Question 76

How do you predict a volcanic eruption with tiltmeters?

Answer 76

Tiltmeters placed in boreholes near volcanoes monitor ground movements that indicate shifting of underground rock. These shifts can increase the pressure on underground pools of magma. A strong, rapid shift can eject the molten rock from the volcano.

Question 77

How do you predict volcanic eruptions with satellites?

Answer 77

Satellites using Global Positioning System (GPS) technology measure the deformation of land around a volcano that indicates a build-up of magma and gases. Satellites can also monitor the emission of gases and flow of lava once an eruption has begun.

Question 78

How do you predict a volcanic eruption with ground-based cameras?

Answer 78

Ground-based cameras near volcanoes take images that reveal changes in the land or emission of gases or steam.

Question 79

How do you predict a volcanic eruption with thermal imagers?

Answer 79

Thermal imagers mounted on helicopters, drones, or satellites help identify active faults as well as cool spots where equipment can be placed.

Question 80

Remember: Information from every type of technology provides clues that a volcano may occur, and together the clues help scientists predict the timing and strength of a volcano.

Answer 80

Information from every type of technology provides clues that a volcano may occur, and together the clues help scientists predict the timing and strength of a volcano.

Question 81

How did the 1980 eruption of Mount St. Helens play out?

Answer 81

Mount St. Helens was quiet for more than 100 years. Then, in early 1980, it rumbled to life again. First, a series of earthquakes shook the mountain, and then blasts of steam issued from fissures in its sides. Finally, on May 18, 1980, Mount St. Helens erupted with a huge explosion that blasted away the top 400 m (1300 ft.) of the volcano. A pyroclastic flow pushed hot gases, rock, and ash out from the erupting volcano in a lateral blast moving at 480 km (300 mi.) per hour, toppling trees in the surrounding forests like matchsticks. The heat from the volcano melted snow and ice on the mountain, sending a lahar—a mix of mud and rock—rushing down its flanks and washing into streams. This torrent of mud, rock, and debris raced downstream sweeping away bridges and tearing up stream banks.

Question 82

Pyroclastic Flow Definition

Answer 82

Mass of hot gases, rock, and ash that moves down the sides of an erupting volcano

Question 83

What does the type of eruption depend on?

Answer 83

The type of eruption depends on where the volcano is located and, as a result, the composition of its magma

Question 84

Where are most volcanoes found?

Answer 84

Most volcanoes are found at plate boundaries, where tectonic plates either push together or pull apart. The plate movements cause processes that send plumes of magma to the surface to form volcanic mountains

Question 85

How do volcanic arcs form?

Answer 85

One common plate boundary feature is the volcanic arc. Often volcanic arcs form at the edges of continents, where an oceanic plate and a continental plate collide. Oceanic plates are denser than continental plates. As a result, subduction occurs. The denser oceanic plate slips under the lighter continental plate, causing uplift of the continental plate on top of it. The leading edge of the oceanic plate then undergoes partial melting when it meets intense heat about 97 km (60 mi.) down in the mantle. The melted crust forms hot magma, which is less dense than the solid crust above and surrounding it. Some of this magma rises through passageways toward the surface, where it bursts forth to form volcanic mountains.

Question 86

Volcanic Arc Definition

Answer 86

Line of volcanoes on land near a convergent plate boundary

Question 87

Subduction Definition

Answer 87

The forcing of one tectonic plate below another

Question 88

What volcanic arc is Mount St. Helens apart of? How did the arc form?

Answer 88

Mount St. Helens is part of a volcanic arc called the Cascade Range. The Cascades formed along the west coast of the United States where the Juan de Fuca Plate (oceanic) is subducted under the North American Plate (continental). The mountains of the Cascade Range are composite volcanoes, which often erupt explosively.

Question 89

How can trenches and troughs form at plate boundaries (where oceanic crust subducts underneath continental crust)?

Answer 89

Trenches can occur at these boundaries as well. A trough can form where one plate bends beneath the other.

Question 90

Where does the majority of volcanic activity occur? How do ridges and divergent plate boundaries contribute to this?

Answer 90

At divergent boundaries, when plates diverge, a crack is formed. Magma then seeps up from the earth’s mantle and fills in the gap between the plates, thus causing an elevated ridge when the magma cools. The majority of volcanic activity occurs along these ridges, with the largest being the Mid-Atlantic Ridge that runs through Iceland and the Atlantic Ocean

Question 91

What is the Pacific Ring of Fire and what formed it?

Answer 91

The Pacific Ring of Fire, a circular formation of volcanoes around the Pacific Ocean, was formed by subducting plates

Question 92

True or False: Volcanoes are also found at hotspots, which form islands

Answer 92

True

Question 93

What are island arcs?

Answer 93

When plates collide on the ocean floor, an island arc can form. Subduction also occurs at these boundaries. Lava plumes formed by melting of the leading edge of the subducted plate rise through the ocean floor, slowly building volcanic mountains. Over long periods of time, the mountains can break the ocean’s surface to become chains of volcanic islands. Just as with volcanic arcs on land, trenches can form in the subduction zones next to island arcs.

Question 94

Island Arc Definition

Answer 94

Line of volcanoes in the ocean near a convergent plate boundary

Question 95

What are some of the violent volcanoes and eruptions in the islands of Southeast Asia?

Answer 95

Many of the islands of Southeast Asia contain volcanoes that formed due to subduction at convergent boundaries on the ocean floor. Some of these volcanoes have produced historically large eruptions. They include composite volcanoes Krakatau and Tambora, in Indonesia. Krakatau ripped its island apart in 1883 with an eruption so powerful that it could be heard 4,500 km (2,800 mi.) away. In 1815, Tambora erupted with an explosion 100 times more powerful than that of Mount St. Helens.

Question 96

Hotspot Definition

Answer 96

Hot area under a plate that creates plumes of magma that form volcanoes

Question 97

What are Earth’s oceans split by?

Answer 97

Earth’s oceans are split by divergent plate boundaries that form a chain of mid-ocean ridges. In several places at these boundaries, upwelling magma has formed volcanoes.

Question 98

How is the divergent plate boundary in Iceland?

Answer 98

Iceland is an example of volcanism at a divergent plate boundary. The country straddles the dividing line between the North American Plate to the west and the Eurasian Plate to the east. Here, the the mid-Atlantic ridge comes to the surface, and the plate boundary becomes visible on land. Beneath the surface lies a reservoir of magma that combines with the natural upwelling of magma at a divergent boundary to create many volcanoes found on Iceland.

Question 99

What is Africa’s Triple Junction Rift? What are its affects?

Answer 99

Iceland is an example of volcanism at a divergent plate boundary. The country straddles the dividing line between the North American Plate to the west and the Eurasian Plate to the east. Here, the the mid-Atlantic ridge comes to the surface, and the plate boundary becomes visible on land. Beneath the surface lies a reservoir of magma that combines with the natural upwelling of magma at a divergent boundary to create many volcanoes found on Iceland.

Question 100

Where in the United States do most Earthquakes occur? How many earthquakes occur worldwide? How many are detectable by instruments? How many can we feel? How many cause damage?

Answer 100

In some parts of the United States, earthquakes are a common occurrence. Alaska has the most earthquakes by far, but earthquakes also occur frequently in California, Hawaii, Nevada, Montana, Oklahoma, and Wyoming. Several million earthquakes occur every year worldwide. Most are too small to be felt by humans, but about 500,000 per year are detectable with instruments. Humans can feel the vibrations caused by about 100,000 of these earthquakes, and about 100 earthquakes per year cause damage.

Question 101

How do earthquakes occur? Where do 90% of earthquakes occur? What does the Ring of Fire consist of?

Answer 101

Earthquakes occur when Earth’s plates move against or past each other at their boundaries. This movement releases energy in waves that cause the ground to shake. About 90 percent of the world’s earthquakes occur in the “Ring of Fire,” an area surrounding the Pacific Ocean where the Pacific Plate collides with several other plates. The Ring of Fire, which stretches for 40,435 km (25,000 mi.), includes more than 450 (mostly undersea) active volcanoes.

Question 102

Besides the Ring of Fire, which area of the world has the most earthquakes?

Answer 102

Besides the Ring of Fire, the area of the world that has the most earthquakes is known as the Alpide Belt. It stretches from Indonesia up into Asia; through the Himalayan Mountains, India, Iran, and Turkey; through the Mediterranean Sea; and finally, into the Atlantic Ocean.

Question 103

How do earthquakes form? What are the parts of an earthquake?

Answer 103

The rock on both sides of a fault moves because Earth’s plates are always moving. The rough edges of the rocks get caught on each other at the fault. As the plates continue to move, energy builds up where the edges are stuck. An earthquake begins when the rocks finally move past each other along the fault, at a point called the focus (also called the hypocenter). The contact releases waves of energy that move outward from the focus in all directions and cause the shaking that people feel. The epicenter of the earthquake, which is located directly above the focus at Earth’s surface, is often where the most damage takes place. Sometimes, an earthquake causes rock to be thrust above the Earth’s surface at the fault line, forming a fault scarp that looks like a giant step.

Question 104

Fault Definition

Answer 104

A fracture or zone of fractures in Earth’s crust

Question 105

Focus Definition

Answer 105

The location in Earth’s crust at which an earthquake begins

Question 106

Epicenter Definition

Answer 106

The point on Earth’s surface directly above where an earthquake begins

Question 107

At what depths do most earthquakes occur at? What are foreshocks and aftershocks?

Answer 107

Most earthquakes happen at a fairly shallow depth in Earth’s crust—about 80 km (50 mi.) below the surface—but they can occur as deep as 750 km (400 mi.). Some earthquakes have one or more foreshocks, which are smaller earthquakes that happen before the mainshock, or main earthquake. All main earthquakes have aftershocks, which are smaller earthquakes that happen after the mainshock. Foreshocks and aftershocks happen in the same place as the mainshock.

Question 108

Can scientists predict earthquakes?

Answer 108

There is a widely held misconception that scientists can predict earthquakes. Unfortunately, scientists are not yet able to make accurate earthquake predictions.

Question 109

What is the location and severity of an earthquake affected by?

Answer 109

The location and severity of an earthquake is affected by the composition of the plates involved and the type of boundary between them. Some plates are composed of either thick continental crust or thinner oceanic crust, while other plates are composed of both continental and oceanic crust.

Question 110

At what type of plate boundary do the weakest earthquakes occur?

Answer 110

The weakest earthquakes occur at divergent boundaries

Question 111

At what type of plate boundary do earthquakes of middle strength occur?

Answer 111

Earthquakes of middle strength tend to occur at transform boundaries, with somewhat weaker earthquakes occurring when the transform boundary is underwater (as most are). The many faults in the San Andreas Fault Zone in California are examples of transform boundaries—in this case, between the Pacific Plate and the North American Plate.

Question 112

At what type of plate boundary do the most aggressive earthquakes form?

Answer 112

Convergent boundaries tend to spawn the strongest earthquakes as the two plates collide, with slightly weaker earthquakes occurring as one plate continues to move under the other.

Question 113

The magnitude of an earthquake is measured on which scale?

Answer 113

The Richter Scale which runs from 1 to 10

Question 114

What is the magnitude of earthquakes at divergent boundaries?

Answer 114

6 or less

Question 115

What is the magnitude of earthquakes that occur at convergent boundaries?

Answer 115

Up to 9.5

Question 116

What is the magnitude of earthquakes at transform boundaries between land and ocean?

Answer 116

Up to 8

Question 117

How do tectonic plate form and affect landforms?

Answer 117

The movements of Earth’s plates, including the resultant earthquakes, are responsible for the formation and alteration of many different types of landforms. When plates are subducted at convergent boundaries, they often form deep ocean trenches. Subduction can also form mountain ranges. For example, when the oceanic Nazca Plate was subducted under the continental South American Plate, the South American Plate was lifted up, forming the Andes Mountains. Convergent boundaries don’t always result in subduction, though. The Indian and Eurasian Plates are both composed of continental crust, so when they collided at a convergent boundary, neither was subducted. Instead, they continued to push against one another, slowly forming the Tibetan Plateau and the towering Himalayan Mountains. Divergent boundaries form mid-ocean ridges such as the Mid-Atlantic Ridge, an undersea mountain range that formed at the boundary between the North American Plate and the Eurasian Plate. It extends through the Atlantic Ocean from the Arctic Ocean to the tip of Africa. At the top of the ridge is a rift, or valley, where molten rock bubbles up from the mantle and forms new crust, pushing the plates away from each other. Over 100 to 200 million years, this divergent boundary has made the Atlantic Ocean into the immense expanse of water it is today. Earthquakes at all three types of plate boundaries can reshape mountains and create valleys and lakes. They can change how groundwater flows and cause mudslides and landslides that change Earth’s surface.

Question 118

What is magnitude and intensity and how are they measured?

Answer 118

Magnitude, which is measured by seismographs, is a measure of the energy released at the focus of the earthquake. It can also be thought of as a measure of the relative “size” of an earthquake. Magnitude is the same, no matter where it is taken. People have used different instruments to measure the strength of earthquakes for about 1,800 years. In 1935, a scientist in the United States, Charles Richter, developed the Richter scale, which was a system of determining the magnitude of earthquakes from the information recorded by seismographs. This scale was used for many years. Today, scientists use different magnitude scales for different purposes, but the most accurate scale is the Moment Magnitude (Mw) Scale, which uses values of 1 to 10. Intensity is a measure of the damage the earthquake does to the surface of Earth’s crust, including people and buildings. It is based on subjective observations of the strength of ground shaking and its effects. Unlike magnitude, intensity differs, depending on where the observations are made. Intensity is measured with the Modified Mercalli (MM) Intensity Scale.

Question 119

What is a seismograph?

Answer 119

An earthquake releases vibrations that travel through the ground in waves of seismic energy. These waves are called earthquake waves or body waves. Ther e are two types of earthquake waves that travel inside Earth’s crust: P (primary) waves and S (secondary) waves. Seismographs, which are used to gauge the strength of earthquakes, measure the P waves and S waves

Question 120

What are P waves?

Answer 120

P waves, or compressional waves, compress and expand (shake) the ground in the same direction and in the opposite direction that they are moving. These are the weakest earthquake waves, but they also travel the fastest. This means that seismographs detect them before other waves

Question 121

What are S waves?

Answer 121

S waves, or shear waves, deform the ground perpendicular to the direction that they are moving. They are the strongest earthquake waves, and they do the most damage. However, they move more slowly than P waves.

Question 122

What is the earthquake early warning systems?

Answer 122

When scientists detect P waves on a seismograph, they can send out an earthquake warning alert, which gives people seconds to minutes to get into a safer place before the stronger S waves (and, later, the surface waves) arrive. People can take this time to stop cars and trains, get out of elevators, and (if outside) to get away from buildings.

Question 123

What is the moment magnitude scale?

Answer 123

The moment magnitude (Mw) scale measures the magnitude (size) of earthquakes, as determined from the information provided by seismographs. The scale ranges from 1 to 10, with 1 being the weakest earthquake and 10 the strongest.