Volcanoes are thrilling and challenging to climb. From non-technical hikes like South Sister to technical climbs on Mount Baker, the Cascade Mountain Range offers a range of difficulty levels for climbers.
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Convergent Plate Boundaries
Converging plates can be either oceanic or continental, or a combination of both. When two continental plates collide, they form mountain ranges. However, when an oceanic plate collides with a continental one, subduction occurs, leading to the creation of volcanoes. Therefore, areas where at least one plate is oceanic at the boundary are characterized by volcanic activity.
Melting
Melting at convergent plate boundaries can be caused by various factors. As the subducting plate sinks into the mantle, it heats up. Additionally, water present in the sediments on top of the subducting plate mixes in and rises into the mantle, lowering its melting point. This melting in the mantle leads to the formation of volcanoes within an island or continental arc.
Why does melting occur at convergent plate boundaries? The subducting plate heats up as it sinks into the mantle. Water mixed with the sediments on top of the subducting plate lowers the melting point of the mantle material. Consequently, this increases the occurrence of melting. Volcanoes resulting from convergent plate boundaries are primarily found along the edges of the Pacific, Cocos, and Nazca plates within the Pacific Ocean basin. Subduction zones are marked by trenches, with only the Aleutian Trench and the Java Trench appearing on the map in Figure 1.
It is important to recall the principles of plate tectonics. Convergent plate boundaries are commonly associated with large earthquakes. Given that the Pacific Ocean is rimmed by convergent and transform boundaries, around 80% of all earthquakes occur within the Pacific Ocean basin, also known as the Ring of Fire. The abundance of convergent plate boundaries around the Pacific explains why approximately 75% of the world’s volcanoes are found in this region.
Pacific Rim
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The Pacific Ring of Fire is the area where the majority of volcanic activity on Earth takes place. Describing the Pacific Ring of Fire along western North America is akin to describing the plate boundaries themselves.
- Subduction at the Middle American Trench gives rise to volcanoes in Central America.
- The San Andreas Fault represents a transform boundary.
- Subduction of the Juan de Fuca plate beneath the North American plate creates the Cascade volcanoes.
- Subduction of the Pacific plate beneath the North American plate in the north results in the Aleutian Islands volcanoes.
Volcanoes formed through convergent plate boundaries can be found throughout the Pacific Ocean basin, primarily along the edges of the Pacific, Cocos, and Nazca plates. These subduction zones are marked by trenches.
The Cascade Range comprises a chain of volcanoes located at the convergent boundary where an oceanic plate is subducting beneath a continental plate. More specifically, these volcanoes are the result of the subduction of the Juan de Fuca, Gorda, and Explorer Plates beneath North America. While the Cascades have been active for 27 million years, the current peaks are no more than 2 million years old. Since the region is located to the far north and is prone to storms, many of the volcanoes are covered by glaciers.
You can find the Cascades on this interactive map, which includes photos and descriptions of each volcano.
Divergent Plate Boundaries
At divergent plate boundaries, hot mantle rock rises into the space created by the plates moving apart. As the hot mantle rock convects upward, it rises higher in the mantle. This decrease in pressure lowers the melting temperature of the rock, resulting in its melting. Lava then erupts through long cracks in the ground, known as fissures.
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Why does melting occur at divergent plate boundaries? Hot mantle rock rises where the plates are moving apart, which reduces pressure on the mantle, thereby lowering its melting temperature. Lava erupts through long cracks in the ground, or fissures.
Volcanoes erupt at mid-ocean ridges, such as the Mid-Atlantic Ridge, where seafloor spreading creates new seafloor in rift valleys. Additionally, when a hotspot is present along the ridge, as seen in Iceland, volcanoes can grow high enough to form islands (Figure 4).
Mid-Ocean Ridges
Volcanoes erupt at mid-ocean ridges, like the Mid-Atlantic Ridge, where seafloor spreading generates new seafloor in rift valleys. When a hotspot is located along the ridge, as observed in Iceland, volcanoes can grow tall enough to create islands.
Continental Rifting
Eruptions also occur at divergent plate boundaries when continents undergo rifting. The volcanoes in Figure 5 are situated within the East African Rift, between the African and Arabian plates. Baja California, which is breaking apart from mainland Mexico, serves as another example of continental rifting.
Summary
To summarize:
- Melting is common at convergent plate boundaries.
- Convergent plate boundaries are prevalent along the Pacific Ocean basin, resulting in the formation of volcanic arcs.
- Melting at divergent plate boundaries is caused by pressure release.
- At mid-ocean ridges, seafloor spreading pulls the seafloor apart, creating new seafloor.
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