Question

In your own words, explain Darwin's hypothesis for the formation of coral atolls.

Short answer

Darwin proposed that coral atolls form as volcanic islands sink, corals grow upwards, leaving a ring-shaped reef around a lagoon.

Step-by-step solution

Introduction to Darwin's Hypothesis

Darwin's hypothesis about coral atoll formation is a geological explanation that relates coral growth to volcanic activity and gradual changes in sea level. It is important to understand the general idea before diving into the details.

Understanding the Role of Volcanic Islands

Coral atolls begin with the formation of a volcanic island. An undersea volcanic eruption creates a volcanic island that rises above the sea level. This geological base is crucial for the initial coral attachment and growth.

Coral Growth Around the Island

Around the volcanic island, corals begin to grow and form a fringing reef. These corals require sunlight and warm water, which they find in the shallow regions near the island. The reef forms a narrow band around the volcanic landmass.

Subsidence and Lagoon Formation

Over time, the volcanic island begins to subside or sink because of tectonic movements or natural erosion. Meanwhile, the corals continue to grow upwards towards the sunlight, maintaining their position near the surface of the water. This process results in the formation of a lagoon between the coral reef and the sinking island.

Formation of the Atoll

Eventually, the island completely subsides below sea level, leaving a ring-shaped coral atoll at the surface. The atoll typically encircles a lagoon, which is formed where the island initially was. This ring structure is what defines a coral atoll.

Key concepts

Darwin's Hypothesis

Darwin's Hypothesis explains how coral atolls, which are ring-shaped reefs, are formed. Charles Darwin proposed this theory during his voyage on the HMS Beagle. He suggested that coral atolls are created through a combination of coral growth and geological processes. Unlike the idea that coral atolls directly form on rising reefs, Darwin proposed a dynamic process involving underwater volcanism and changes in the earth's crust. The hypothesis starts by linking coral grow with volcanic islands, forming around the shorelines of these islands. This theory was groundbreaking at the time because it provided a natural scientific explanation for the formation of atolls, rather than attributing them to sudden appearances or supernatural phenomena.

Volcanic Islands

Volcanic islands are the starting point in the formation of coral atolls. These islands originate from magma eruptions under the ocean, forming a new landmass. As magma is released from volcanic activity, it cools and solidifies, creating an island that juts above the sea. This creates a strong base for coral, which needs a stable surface to anchor upon. Over hundreds or thousands of years, these volcanic islands may rise and alter their shape due to ongoing volcanic activity and erosion. During this time, the islands provide optimal conditions for coral larvae to attach, grow, and establish the basis for what will eventually become a coral reef.

Coral Reefs

Coral reefs are crucial ecosystems that start forming as soon as coral organisms find conditions conducive for growth. Corals are marine invertebrates that secrete calcium carbonate to form a hard skeleton. This allows them to build colonies called reefs. Around volcanic islands, corals form fringing reefs first, which directly encircle the island. Coral reefs require specific conditions to grow: shallow water allows sunlight penetration essential for photosynthesis by symbiotic algae living in coral tissues, and warm waters that do not fall below around 18°C. These criteria are met near the equator and around volcanic islands, promoting expansive reef growth. As they grow, reefs contribute to coastal protection and create habitats for diverse marine life.

Subsidence

Subsidence is a critical stage in Darwin's coral atoll formation process. It refers to the gradual sinking of the volcanic island due to tectonic activity or the weight and erosion of the island itself. As the island slowly subsides, corals continue to survive and grow upwards towards sunlight, maintaining their presence near the water surface. This phenomenon can occur over long periods and may be influenced by shifts in the Earth's crust or volcanic drainage underneath the island. Despite the subsidence, the coral structure remains intact and expands, building upon previous coral generations to keep rising against the sink. Subsidence leads to key changes that facilitate the formation of a central lagoon, fundamental in identifying a coral atoll.

Lagoon Formation

Lagoon formation is the final and defining stage in the creation of a coral atoll. As the volcanic island sinks below sea level, the ring of coral continues to grow upwards and outwards. This creates an isolated body of tranquil water known as a lagoon. The lagoon sits in the middle of the atoll, where the island had sunk, and typically fosters a unique ecological environment, protected by the surrounding coral reef barrier. The water in lagoons is often shallow and calmer compared to open ocean waters, making it a rich habitat for various marine species. The existence of a central lagoon surrounded by a coral reef barrier is a characteristic feature of coral atolls and is essential for differentiating them from other types of reefs or island formations.