Convergent Plate Boundaries
convergent plate boundaries are also responsible
for nearly three-fourths of Earth's volcanoes
Which of the following best describes a silica-rich magma?
very viscous
In AD 79
, a huge volcanic eruption of Mt. Vesuvius sent tons of ash and other particles into the air around Pompeii, Italy. The entire town of more than 200,000 people was buried in the ash and forgotten until the town was excavated in 1748.
Volcanic features (Calderas )
A caldera is a cauldron-like volcanic feature formed when land collapses after a volcanic explosion. Crater Lake in Oregon, which you see in this picture, formed when a caldera filled with water.
volcanic features (ashflows)
Because convergent boundary volcanoes are often explosive in nature, they release large amounts of ash. Here you see a massive ash flow from an erupting volcano in the Philippines. These flows can travel at speeds up to 700 km/hour (450 miles/hour) and are extremely hot. Italy's famous Mt. Vesuvius exploded in AD 79, killing hundreds of people when the thick ash flow settled on their homes and buried them alive.
Volcanic Features (Cinder cones )
Cinder cones are steep hills of volcanic fragments that form around the flanks of a stratovolcano. They are made of rock fragments that have a glassy texture and contain "frozen" gas bubbles that were locked into place as magma was ejected into the air and rapidly cooled after a volcanic eruption.
Continental volcanic arcs
Continental volcanic arcs are chains of volcanoes on the margin of the continent above a subduction zone at ocean-continent boundaries. Examples include the Andes Mountains and the Cascades off the west coast of the U.S.
Types of Convergent Plate Boundaries
Convergence of an oceanic plate with a continental plate is similar to ocean-ocean convergence in that subduction will also take place in this env ironment and volcanic activity often results. When an oceanic plate collides with a continental plate, the oceanic plate is always pulled under and subducted because it is denser than the continent al plate. The continental plate then overrides the oceanic plate. Since the continental plate is the non-subducting plate, the volcano often develops on the continental plate as new magma wells up. As the oceanic plate is pulled under the contin ental plate, the continental plate becomes deformed at its margins. It crumples up and forms a folded mountain belt, like the Andes of South America, which are highly deformed mountains with many active volcanoes. The Andes form a long narrow mountain belt down the side of Sout h America's west coast. They are formed at the convergent plate boundary between the Nazca and the South American plates. The oceanic Nazca Plate is being subducted under the continental South American Plate. Here you see that this area has several distinct volcanic regions.
Types of Convergent Plate Boundaries
Do you think volcanic activity is common at areas of continent-continent collision? Why or why not? Volcanic activity is not common at areas of cont inent-continent collision because neither plate is subducted. It is the subduction process that generates new magma to form volcanoes.
Island volcanic arcs
Island volcanic arcs are a chain of islands and mountains that form on the overriding or non-subducting oceanic plate. They include Japan, the Philippines, the Tonga Islands, the Aleutian Islands, and the West Indies Islands, among others. On this map you see the island volcanic arc of the Mariana Islands in the Pacific Ocean. They develop parallel to the direction of subduction.
volcanic features (Tsunamis )
Just like earthquakes, the violence of a volcano can trigger a tsunami, or giant wave of water that washes over the land
1 Types of Convergent Plate Boundaries
Slid e 1 The first type of convergent plate boundary is the ocean-ocean boundary, where two oceanic plates meet and collide against each other. The dens er of the two plates is pulled under the other and is subducted. It descends into the asthenosphere, or upper mantle, where it will lead to the generation of new magma. As you learned earlier, subduction zones are areas of high earthquake activity. The yellow squares in this image show where earthquakes take place at ocean-ocean convergence zones. This animation shows you how plates move at an ocean-ocean convergent plate boundary. Notice the volcano that rides along on the non-subducting plate. When one oceanic plate is subducted under the other, the resulting new ma gma is less dense than the surrounding rock. Therefore it easily rises and erupts on the seaflo or, ultimately building a volcano or a volcanic island in the sea. Sediment gets scraped off of the subducting plate as it moves downward. It collects in what is calle d an accretionary prism, which you see here as the orange collection of sediment straddling the trench. Areas of ocean- ocean convergence are characterized by ocean trenches, seafloor volcanoes, and volcanic islands. Mt. Fuji in Japan and Mt. Pinatubo in the Philippines are examples of volcanoes on volcanic islands formed in this type of environment.
Types of Convergent Plate Boundaries
Slide 2 Why does magma well up and why do volcanoes form at ocean-ocean convergent boundaries? When one plate is subducted under the other, it leads to the generation of new magma. The magma is hot and less dense than the surrounding rocks so it rises and erupts on the seafloor, eventually building a seafloor volcano or volcanic island.
Volcanic Features (Stratovolcanoes)
Stratovolcanoes have the following features: steep slope near the summit due to thick viscous lava flows that do not travel far down the slope gentler slopes near the base due to accumulations of eroded material explosive eruptions layered structure made of successive outpourings of explosive materials sometimes have a crater at the summit formed by explosive ejection of material can be inactive for hundreds to thousands of years before suddenly exploding with little warning
Continental Collisions
The Himalayas have been forming over the past 100 million years. The continental collision that formed the Himalayas deformed the shallow marine sedimentary rocks that existed prior to the mountains. As India and Asia converged, a double thick layer of continental crust emerged, essentially "welding" the continents together.
Volcanic Features (Caldera Lake Formation)
This diagram shows you how a caldera formed at Crater Lake National Park in Oregon. Part of the Cascades continental volcanic arc, this landscape feature developed when a massive volcanic eruption led to the subsistence of the land around it, probably in the year 5,700 B.C. After it cooled, rain and snow accumulated in the caldera to form a lake.
Volcanic Features (Volcanic Domes )
Volcanic domes, also called lava domes, are mound-shaped protrusions resulting from the slow flow of viscous lava from a volcano. Because the magma is very viscous, it resists flow and thus mounds up, resulting in the characteristic mound shape of volcanic domes.
Volcanism at Convergent Plate Boundaries
Volcanoes are tremendous examples of geologic activity and the consequences of plate tectonics. Like earthquakes, volcanoes are concentrated along tectonic plate boundaries. Nowhere is this truer than in the Pacific Ring of Fire, where subduction is taking place at numerous trenches that border the continental shores. The area has 450 volcanoes, more than 75% of all the volcanoes on Earth. This makes plate convergence responsible for nearly all volcanic activity on Earth. As you learned earlier, volcanic activity is common at convergent plate boundaries because of the generation of new magma when one plate is subducted and consumed under another one. When subduction takes place, partial melting of the crust generates a silica-rich magma. Silica-rich magma is strong and very viscous, like honey. It does not easily reach the surface, but builds up for a long time. Eventually, it builds up so much pressure that it explodes outward with great violence. Therefore, volcanoes at convergent plate boundaries tend to be explosive and violent. While they are infrequent, which often gives people a false sense of security, they are very destructive when they do erupt. (In contrast, magma rich in basalt is thin and runny and does not usually cause violent eruptions. The low viscosity is associated with relatively small and quiet volcanic eruptions. This type of magma is associated with volcanoes at divergent plate boundaries, which you will learn about later.)
The Effects of Volcanoes
Volcanoes at convergent plate boundaries, like those at other tectonic settings, have a number of effects on the hydrosphere and the atmosphere. Some of these effects are listed below: pose hazards to human life as well as plant and animal life; can destroy towns, homes, forests, and farmland trigger offshore tsunamis that sweep over the land place gases in the atmosphere that lead to the formation of acid rain; for example, sulfur dioxide, a common volcanic gas, is converted to sulfuric acid in the atmosphere place gases in the atmosphere that lead to destruction of Earth's ozone layer release sulfur compounds that increase Earth's albedo, which is the reflection of solar radiation back into space lower the temperature of Earth's lower atmosphere because of increased albedo return several important nutrients to the soil in volcanic ash; volcanic soils are often very fertile release water vapor to the atmosphere (this is the most common volcanic gas) release carbon dioxide gas to the atmosphere, increasing Earth's atmospheric carbon load
Continental Collisions
When there is continental-contintental conversion and the masses become compressed and fold forming a mountain ranges or belts.
Volcanic Features
While there are several different types of volcanoes depending on the tectonic environment, the type most common at convergent plate boundaries is shown here. This is called a stratovolcano (also called a composite volcano). Stratovolcanoes and other volcanic features of convergent plate boundaries are discussed below.
Types of Convergent Plate Boundaries
Why are volcanoes common at ocean- continent convergence zones? When the oceanic plate is subducted under the continental plate, it leads to the generation of new magma. This magma wells up and forms volcan oes on the non-subducting plate, or the continental plate.
Which volcanic feature is formed when land collapses after a volcanic eruption?
caldera
Which volcanic feature is made of volcanic frag ments that flank the sides of a stratovolcano?
cinder cone
An island volcanic arc
may include islands that develop in the sea from the build-up of volcanic rocks.
Types of Convergent Plate Boundaries
ocean-ocean convergence
What type of volcano is common at convergent plate boundaries?
shield volcano
What type of magma is most often found asso ciated with convergent plate boundary volcanoes?
silica-rich
Which of the following is a benefit of volcanic eruptions?
soil fertilization
At ocean-continent boundaries
the resulting body of volcanoes is called a continental volcanic arc.
ocean-ocean convergent boundaries,
the resulting body of volcanoes is called an island volcanic arc
Types of Convergent Plate Boundaries
© KC Distance Learning Slide 5 The final type of convergent plate boundary is the continent-continent boundary. Because continental crust is too light and buoyant to be subd ucted, neither plate is subducted at this type of convergent plate boundary. Both continental masses press against the other, and both become compressed and ultimately fused into a single block with a folded mountain belt forming between them. This is the type of activity that is fo rming the Himalayas, and is still going on today. The Himalayas are still growing. Metamorphism is common in these zones because of the intense pressure between the colliding plates.
