Question

Why do oceanic islands sink after they form?

Short answer

Oceanic islands sink due to cooling, subsidence, and erosion processes.

Step-by-step solution

Initial Formation

Oceanic islands are generally formed by volcanic activity at tectonic plate boundaries or hotspots. As the molten rock from beneath the Earth's surface erupts, it builds up to form a new landmass above the sea level, creating the island.

Cooling and Contracting

After the island forms, the volcanic rock that constitutes the island begins to cool down. As it cools, it contracts slightly, leading to a reduction in volume. This physical contraction contributes to a slight sinking of the island.

Subsidence of the Oceanic Crust

The oceanic crust under the island is initially uplifted by the additional weight of volcanic material. Over time, the oceanic crust undergoes a process called isostatic adjustment, where it slowly sinks under its own weight and the additional weight of the island.

Erosion and Weathering

The island is continuously subjected to erosion and weathering due to wind, rain, and ocean waves. This process gradually wears away the island's surface, lowering its overall height and contributing to its sinking appearance.

Conclusion of the Sinking Process

Over time, these processes—cooling and contracting, subsidence, and erosion—combine to cause the oceanic island to seemingly sink into the ocean. The island may eventually disappear below sea level.

Key concepts

Volcanic Activity

Volcanic activity plays a critical role in the creation of oceanic islands. This process begins deep under the Earth's surface, where intense heat and pressure cause rocks to become molten, forming magma. When a volcano erupts, this magma is forced through cracks or openings in the Earth's surface. Once the magma reaches the surface, it cools rapidly and solidifies, forming volcanic rock. With repeated eruptions, this rock accumulates and builds up over time. Eventually, the mass can become large enough to emerge above the ocean's surface, forming an island.
Volcanic activity is often associated with specific areas that are particularly prone to eruptions. These include tectonic plate boundaries and hotspots. At these locations, movement and interaction of the Earth's plates or mantle plumes increase the likelihood of volcanic activity.

Tectonic Plate Boundaries

Tectonic plate boundaries are crucial areas where oceanic islands often form. The Earth's crust is divided into several large and rigid pieces known as tectonic plates. These float on the semi-fluid layer of the mantle below and are constantly in motion relative to one another. There are three main types of plate boundaries:

• Convergent Boundaries: Where two plates collide, one may be forced below the other in a process called subduction. This can result in volcanic activity as the subducted plate melts into magma.
• Divergent Boundaries: Here, plates move apart, and magma from the mantle wells up to fill the gap, creating new crust. If this occurs under the ocean, it can lead to the formation of an island.
• Transform Boundaries: These occur where two plates slide past each other. While they mostly cause earthquakes, they can sometimes be linked to volcanic activity at nearby locations.

Understanding tectonic plate boundaries helps explain why some regions are more prone to island formation than others.

Isostatic Adjustment

Isostatic adjustment is a process that affects the elevation of oceanic islands over time. It refers to the way the Earth's crust responds to changes in weight. When an oceanic island forms through volcanic activity, it adds significant weight to the oceanic crust below. Initially, this causes the crust to uplift and accommodate the increased load. However, as time passes, the crust begins to adjust to this weight through a process called subsidence.
During subsidence, the crust gradually sinks as it "settles" under the new weight, balancing itself like a floating object in water. This is similar to how a boat might sink slightly lower in the water when additional weight is added. As the crust adjusts downward, the island may appear to sink slowly over time. This ongoing adjustment ensures that the Earth's crust remains balanced.

Erosion and Weathering

Erosion and weathering are natural processes that continuously shape landforms, including oceanic islands. Once an island is formed, it is exposed to the elements. Rain, wind, and ocean waves exert relentless force on the island's surface. Over time, these forces gradually wear down the rocks and soil in a process known as erosion.
Weathering is closely related and involves the physical and chemical breakdown of rocks. This can occur due to temperature changes, water flow, or biological activity. Both erosion and weathering act together to lower the island's elevation by removing material from its surface.
Over the years, these processes can significantly alter the island's landscape, making it appear as if it's slowly sinking. Although erosion and weathering are natural and continuous, their impact becomes a critical factor in the eventual disappearance of some oceanic islands beneath the waves.