Question

Explain the origins of, and differences between, seamounts and island arcs.

Short answer

Seamounts form underwater from volcanic activity at hotspots or ridges, while island arcs form above water at subduction zones, leading to visible chains of islands.

Step-by-step solution

Understanding Seamounts

Seamounts are underwater mountains formed by volcanic activity. These structures originate when volcanic eruptions occur on the ocean floor but do not break the surface of the water. They can be found in all ocean basins and are typically created at hotspots or along mid-ocean ridges where tectonic plates are diverging.

Understanding Island Arcs

Island arcs are chains of volcanic islands typically positioned in a curved row. They form as a result of the subduction of one oceanic plate beneath another at convergent plate boundaries. The descending plate melts, forming magma that rises to create volcanoes, which can break the ocean surface to form islands.

Differences in Formation

The main difference in formation is that seamounts arise primarily from hotspots and diverging plate boundaries, while island arcs originate at convergent plate boundaries through subduction processes. This difference in tectonic activity leads to varied geographical features under the sea and at the surface.

Differences in Appearance and Location

Seamounts are singular or grouped underwater volcanoes that do not reach the ocean surface, usually not disrupting the waterline. In contrast, island arcs are visible chains of islands above water, typically found along the boundaries of oceanic plates in areas like the Pacific Ring of Fire.

Key concepts

Underwater Volcanic Activity

Underwater volcanic activity is an exciting phenomenon that occurs when magma from within the Earth's mantle breaks through the ocean floor. These underwater eruptions can create seamounts, which are mountains formed by accumulating volcanic material. These structures remain submerged and become home to unique ecosystems. Underwater volcanic activity is primarily driven by tectonic movements and heating under the Earth's crust. When the magma finds a weakness or a vent on the ocean floor, it erupts and cools down rapidly, forming solid rock. Over time, repeated eruptions can build mighty underwater structures like seamounts or even contribute to the formation of island arcs, if the volcano eventually breaks the surface of the water.

• Magma from the Earth's mantle creates underwater formations.
• Volcanic activity generally occurs at tectonic plate boundaries.
• Builds structures like seamounts and, if reaching the surface, island arcs.

Tectonic Plate Boundaries

Tectonic plate boundaries are the edges where two tectonic plates meet. There are three main types of boundaries: divergent, convergent, and transform. These are crucial locations for geological activity, such as earthquakes, volcanic activities, and mountain building.
Divergent boundaries are where two plates move apart from each other. Magma rises to fill the gap, creating new crust as seen at mid-ocean ridges. Meanwhile, convergent boundaries occur where plates are moving towards each other, often resulting in one plate being forced beneath another, a process known as subduction. This can lead to the formation of mountain ranges or volcanic arcs. Transform boundaries involve plates sliding past each other horizontally, which can cause intense seismic activity.

• Divergent boundaries create new crust and occur at mid-ocean ridges.
• Convergent boundaries can lead to subduction and form island arcs.
• Transform boundaries involve lateral plate movement causing earthquakes.

Hotspots

Hotspots are unique volcanic regions that are not necessarily located at tectonic plate boundaries. They occur due to mantle plumes, which are areas of hot, upwelling mantle that create volcanic activity on the surface. A famous example is the Hawaiian Islands, formed as the Pacific Plate moved over a hotspot. Unlike plate boundary volcanoes, hotspot volcano chains reveal the plate's movement over time, as the stationary hotspot burns through the moving crust above it.
Hotspots can create clusters of volcanic islands and seamounts. As the tectonic plate continues to move, new volcanoes form over the hotspot, while older ones become dormant and erode away. This leads to a trail of islands, which provides valuable information about past plate movements.

• Formed by stationary mantle plumes creating volcanic activity.
• Occur within the interior of tectonic plates, not just at boundaries.
• Create chains of volcanic islands and seamounts over time.

Mid-ocean Ridges

Mid-ocean ridges are underwater mountain ranges formed by the upwelling of magma at divergent tectonic boundaries. As plates pull apart, magma rises to fill the space, forming new oceanic crust. This continuous process can create extensive underwater mountain ranges that span the globe.
Mid-ocean ridges are also known as the sites of seafloor spreading, where new ocean floor is constantly being generated. This geological activity is a key component of the Earth's lithosphere recycling system, as the old crust is moved away to eventually be subducted elsewhere. These ridges can host a variety of geothermal activity, supporting diverse life forms around hydrothermal vents.

• Created at divergent boundaries where new crust forms.
• Major sites of seafloor spreading and crust formation.
• Span the globe, forming extensive underwater mountain ranges.

Subduction

Subduction is a process in which one tectonic plate moves under another and sinks into the mantle due to gravitational forces. This usually occurs at convergent boundaries, where two plates collide. The denser oceanic plate is forced beneath the lighter continental or oceanic plate, leading to geological phenomena like earthquakes, volcanic activity, and the creation of trenches and mountain arcs.
Subduction zones are often marked by deep ocean trenches and a line of volcanoes known as a volcanic arc. These arcs can form island chains when oceanic plates subduct beneath each other, contributing to the creation of island arcs. Over time, the intense pressure and heat cause the descending plate to melt, generating magma that rises to form new volcanic activity on the surface.

• Occurs at convergent plate boundaries with one plate diving beneath another.
• Leads to the formation of volcanic arcs and deep ocean trenches.
• Generates intense geological activity such as earthquakes and volcanoes.