Chapter 9: Volcanoes And Other Igneous Activity

The statement "All magmas have the same composition" is false. Magmas can have different compositions depending on the types of rocks they originate from and the processes they undergo. Magma composition can vary in terms of the types and proportions of minerals, gases, and elements present. Factors such as the depth of magma formation, melting of different rock types, and the addition or removal of certain components can all contribute to variations in magma composition. Therefore, it is incorrect to assume that all magmas have the same composition.

A large volcano with a broad slightly domed shape is a shield volcano. Shield volcanoes are characterized by their low, broad profiles and gentle slopes. They are formed by the eruption of basaltic lava, which has low viscosity and can flow easily over long distances. The lava spreads out in thin layers, creating a wide and shallow volcano. Shield volcanoes are typically not very explosive and have frequent eruptions that release large amounts of lava.

A low viscosity magma is more likely to flow easily when erupted. Viscosity refers to the resistance of a fluid to flow. In the case of magma, low viscosity means that it is less sticky and more fluid-like. This allows it to flow more easily through volcanic conduits and erupt onto the surface with less resistance. On the other hand, high viscosity magma is thicker and more sticky, leading to slower and more explosive eruptions. Therefore, the correct answer is that low viscosity magma flows easily when erupted.

Composite cones, also known as stratovolcanoes, are the most violent type of volcanic activity. These volcanoes are characterized by their steep slopes and explosive eruptions. They are formed by alternating layers of lava flows and pyroclastic materials, such as ash, pumice, and volcanic bombs. The explosive eruptions of composite cones occur due to the high viscosity of their magma, which traps gases and creates pressure until it is forcefully released. This results in powerful explosions, ejecting large quantities of ash, gas, and volcanic material into the atmosphere, making composite cones the most violent type of volcanic activity.

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Volcanoes associated with the Ring of Fire are violent. The Ring of Fire is a major area in the basin of the Pacific Ocean where a large number of earthquakes and volcanic eruptions occur. This region is characterized by tectonic plate boundaries and subduction zones, which create intense geological activity. Volcanoes in this area often experience explosive eruptions, releasing large amounts of ash, lava, and gases. These eruptions can be highly destructive and pose significant risks to nearby communities and the environment. Therefore, the correct answer is "violent."

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The correct answer is plate boundaries. Most active volcanoes are associated with plate boundaries because these are the areas where tectonic plates interact and collide. Convergent boundaries occur when two plates collide, causing one plate to subduct beneath the other, leading to the formation of volcanoes. Divergent boundaries occur when two plates move away from each other, creating a gap where magma rises to the surface and forms volcanoes. Therefore, plate boundaries encompass both convergent and divergent boundaries, making them the most common location for active volcanoes.

A dike is a tabular, discordant igneous body produced when magma is injected into fractures that cut across rock layers. This means that when magma forces its way into cracks in the rock layers, it forms a vertical or near-vertical sheet-like intrusion called a dike. Dikes are usually composed of volcanic rock and can be seen cutting across the layers of existing rock, indicating their intrusive nature. They are different from sills, which are tabular intrusions that form parallel to the existing rock layers. Caldera refers to a large volcanic crater, and fissure is a long, narrow crack in the Earth's crust.

The three zones of volcanism are convergent and divergent plate boundaries and plates proper that are not associated with any plate boundaries. Convergent plate boundaries occur when two tectonic plates collide, causing one plate to be forced beneath the other, creating a subduction zone and leading to volcanic activity. Divergent plate boundaries occur when two plates move away from each other, creating a gap where magma rises to the surface, forming new crust and volcanic activity. Plates proper that are not associated with any plate boundaries can also experience volcanic activity due to hotspots or mantle plumes, where hot magma rises from deep within the Earth's mantle, creating volcanoes.