Volcanic islands can seem to appear out of nowhere, emerging from the ocean like breaching monsters of the deep. Below, Mika McKinnon explains how these odd geological formations are born, how they evolve, and how they eventually vanish back beneath the waves.
Under the sea, volcanoes erupt, building land until they break the surface as a new island. These islands evolve and change as they age, following a predictable pattern of landform evolution until one day returning to the sea.
The Emerging Volcano: Niijima and Nishino-shima, Ogasawara Islands
Volcanic islands are born below the waves, growing from the seafloor in countless eruptions. The volcano breaches the surface, and a new land greets the morning sun.
Prior to 1973, Nishino-shima was a tiny island barely poking above the waves. In 1973, new eruptions from fissures and submarine cones of the volcano spewed ash, tephra, and smoke into the sea. By December, the new land had glommed onto the existing island, expanding the island's size. Since 1974, we haven't seen any eruptions and the land has been rapidly colonized by plants, but ongoing reports of discolouration in the surrounding sea suggest that more have been taking place underwater.
In November 2013, a next-door neighbour breached the surface, Niijima. By December, the visible landmass had tripled in size; by late December, the land had merged into the older Nishino-shima. By the end of March 2014, the island had tripled in elevation compared to its December height above the waves. The eruptions show no signs of stopping, with lava flows steadily adding to the size of the island.
The Bare Volcano: Pinta, Galápagos Islands
The eruptions continue, building a new land emerging from the sea. As they reach the light, their flanks become home to corals, colonizing in a protective fringe.
Pinta is a dry volcanic island, with dark lava flows staining its flanks. Like most volcanic islands, Pinta is composed of runny basaltic lavas. The low silica content of oceanic crust ensures a low silica lava. This low-viscosity fluid releases gas easily, resulting in gentle, even effusive eruptions.
Pinta grew as a rounded shield volcano 800,000 years ago. Faulting steeped the island's slopes, and later fissure eruptions spilled dark lava flows down the flanks. These flows are still visible as brown stains on the northern and eastern flanks, while a small ring of forest and grasslands engulfs the tiny collapse pit.
Pinta is one of the smaller islands within the Galápagos island chain, almost due north of Santiago. The other islands also show their volcanic origins, with the largest island, Isabela, sporting three distinct craters.
The Fringed Volcano: Tahiti, French Polynesia
Eventually the volcano stops erupting, settling into dormancy. No new rock is formed, and life takes hold as rock weathers into soil. Erosion gradually wears the rock into sand, while the volcano shrinks and sinks as the hot, buoyant rock cools from its younger days.
Tahiti is an island formed by volcanic eruptions and shaped by rain.
The pair of shield volcanoes grew from countless eruptions. Tahiti-Nui to the northwest is the older volcano, 870,000 to 1.4 million years old. The dome was originally symmetric, but the north and south flanks collapsed 860,000 years ago. Afterwards, a growing volcano overtopped the nothern flanks and spilled lavas down the collapsed basins. Now, the dome looks symmetrical again, but the mountains to the north are higher than those to the south. As the Pacific Plate moved relative to the upwelling hot spot source in the mantle, Tahiti-Nu was left behind, and Tahiti-Iti to the southwest was born.
Heavy tropical rains eroded deep valleys in the young volcanoes, carving canyons up to a kilometer deep. The heavy erosion means a higher risk of landslides, but also deeper soils and rapid growth of the lush plants colouring the island its picturesque green. The rain, erosion, and vegetation work in a positive feedback loop, breaking the volcanic rock apart into rich soils and black sands.
The Protected Volcano: Maupiti Island, French Polynesia
The coral colonies continue to thrive, piling upwards to stay close to the precious surface light even as the volcano submerges. A gap grows between the volcano and coral, with the reef forming a protective barrier around a new lagoon.
Maupiti is the tip of a volcano poking barely 213 meters above the sea, ringed by a barrier of islands and coral reefs. The barrier cuts off a quiet lagoon from the open ocean, dissipating wave energy. While Tahiti has rocky black sands from the eroding volcanic basalt, the white sands of Maupiti are broken sea shells and coral fragments.
It is the oldest of the volcanos formed by the hot spot in the Society island chain, building its shield 3.9 to 4.5 million years ago. Since then, the Pacific Plate has moved on at 11 mm per year, dragging across the plume to form Bora Bora (3.1 to 3.5 million years ago), Moorea (1.5 to 2.0 million years ago), Tahiti (0.6 to 1.2 million years ago), and Mehetia (less than a million hears ago).
The Sunken Volcano: Atafu Atoll, Tokelau Islands
One day, the volcano slips below the waves, and only the corals remain. The volcanic island is gone, and a new atoll is