Lake island

• 1. Introduction

  ›

  Manitoulin Island sits in Lake Huron, stretching across 2,766 square kilometres of fresh water. It is the largest lake island on Earth, and yet most people could not name it. A lake island is, in its simplest definition, any landmass sitting within a lake. That definition, however, opens onto a world far stranger and more varied than the phrase suggests. Some lake islands were born from collapsing volcanoes. Some rise and fall on trapped gas. Some were built entirely by human hands. And a few have vanished altogether, swallowed by shrinking water or deliberately buried under it. The questions worth asking are not just what a lake island is, but how they form, what holds them up, and what happens when the water eventually wins.

• 2. How Islands Are Born

  ›

  Sediment is one of the quietest island-makers. Over long stretches of time, material accumulates into shoals beneath the water's surface. Then, when the lake level drops, those shoals break through and become dry land. Other islands begin as ordinary shoreline. Erosion cuts a channel between a headland and the mainland, severing it. Still others were never connected to a shore at all; they are the surviving high points of terrain that flooded when a river was dammed or a waterway rose.

  Glacial lakes add their own mechanism to the list. When ice retreats, it leaves behind ridges of deposited material called moraines. If a moraine runs through the basin of a forming lake, it can emerge as an island. Earthquakes, meteor strikes, and volcanic eruptions complete the roster of origins. Each of these forces can reshape a lakebed or hurl material upward fast enough to leave land standing above the water line.

• 3. Fire and Collapse

  ›

  Volcano Island in Taal Lake on the Philippine island of Luzon carries a peculiar distinction: it holds its own lake, Main Crater Lake, which in turn contains a cluster of cinder cones and craters called Vulcan Point. That makes it one of a handful of known third-order recursive lake-islands on Earth, a landmass inside a lake on an island inside another lake.

  The geological process that creates such formations begins with a large eruption. When enough magma is expelled, the emptied chamber beneath a volcano can no longer bear the weight above it. A roughly circular fracture, called the ring fault, develops around the chamber's edge. The centre of the volcano drops, forming a ring-shaped depression called a caldera. Centuries later, rainwater and groundwater fill the caldera to create a lake. If volcanic activity resumes, the caldera floor can be pushed back upward as a resurgent dome, and a crater lake island emerges.

  Among the notable examples, Wizard Island and Phantom Ship rise from Crater Lake in Oregon in the United States. Samosir Island occupies Lake Toba in Sumatra, Indonesia. Kamuishu Island sits in Lake Mashu on the Japanese island of Hokkaido. Though calderas and craters form through distinct processes and differ in size and depth, the distinction is often set aside in everyday usage, and the term crater lake is applied broadly to lakes formed in either type of depression.

• 4. Rock from Space

  ›

  René-Levasseur Island in Lake Manicouagan, Canada, measures approximately 2,000 square kilometres. It is the world's largest impact crater island and the second-largest lake island of any kind. The lake itself fills the scar left by a meteorite collision, and the island occupies the central peak that rose from the crater floor after the impact.

  Impact craters form when large meteorites or comets strike the Earth. They are relatively uncommon, and most of those that exist have been heavily eroded or buried over geological time. Where a complex impact crater does hold water, a central peak often protrudes above the lake surface. In other cases, geological forces have left only a ring-shaped annular lake, with a broad central island filling what was once the heart of the crater.

  The Sanshan Islands of Lake Tai in China share this impact origin, as do islands within Canada's Clearwater Lakes. The Slate Islands of Lake Superior, also in Canada, were formed the same way. Sollerön Island in Siljan Lake in Sweden, and an unnamed island in Lake Karakul in Tajikistan, round out the list of confirmed impact-origin lake islands. René-Levasseur Island became technically artificial when the Manicouagan Reservoir was flooded in 1970, merging two existing lakes to create the current water body around it.

• 5. Floating and Built

  ›

  Peaty masses on shallow lake floors can rise without any geological drama at all. As plant matter decomposes, it releases gases. When enough gas accumulates, a clump of vegetation breaks free from the bottom and floats to the surface. These ephemeral islands drift until the gas dissipates and the mass sinks again. Because lakes lack the currents and tides of the open sea, floating islands are far more common in lakes than anywhere else.

  Artificial islands represent a different category entirely. Humans have been building them since prehistoric times. The crannogs of Britain and Ireland and the floating Uru islands of Lake Titicaca in South America are among the oldest examples. The Aztec city of Tenochtitlan, now the site of Mexico City, was a notable early artificial island, though islands formed by the deliberate flooding of valleys for hydroelectric projects are generally not counted in that category.

  The Dutch polder Flevopolder in Flevoland, at 948 square kilometres, is the largest human-made island in the world. It sits in the now-freshwater lake IJsselmeer. IJburg is another artificial island in Amsterdam. In South Africa, Flamingo Island in Kamfers Dam was also constructed by human activity. The uses for artificial lake islands range from flood protection and immigration stations to agricultural reclamation, as with the polders of the Netherlands, and the expansion of living and transportation space in densely populated areas.

• 6. Islands That Disappeared

  ›

  Vozrozhdeniya Island in the Aral Sea began at roughly 200 square kilometres. From the 1960s onward, the Aral Sea shrank dramatically. The retreating water exposed more and more of the surrounding lakebed, and the island grew rapidly until mid-2001, when the expanded landmass finally connected to the mainland. By 2014, what had once been an island was simply part of the Aralkum Desert, a dry expanse covering the former lake floor.

  Similar fates have overtaken Kokaral and Barsa-Kelmes, also in the Aral Sea, and the Bogomerom Archipelago in Lake Chad, and Shahi Island in Lake Urmia. Islands can also disappear by sinking. Erosion, subsidence, or a rising water level can pull a landmass below the surface; Sunken Island in Otsego Lake is one documented case. Human engineering can eliminate an island too, as happened with Urk, which was absorbed into the mainland when the former Zuider Zee was enclosed and drained.

• 7. Recursive Worlds

  ›

  Manitoulin Island in Lake Huron holds Lake Manitou on its surface. Lake Manitou covers 104 square kilometres, making it the largest lake on an island within a lake. Lake Manitou itself contains Treasure Island in Mindemoya Lake, which is the largest island in a lake on an island in a lake.

  The recursion extends further. Nettilling Lake on Baffin Island in Canada covers 5,542 square kilometres, making it the largest lake sitting on an island. Within Nettilling Lake sits a nameless island of roughly 10 acres. That island holds a nameless lake of approximately 375 acres, which in turn contains yet another nameless island of around 10 acres. Each layer is the largest known example of its order of nesting. The Pamvotida lake near the city of Ioannina in Greece carries its own smaller version of this phenomenon: a 0.52-square-kilometre island holds a village of 219 permanent residents as recorded in the 2011 census, and the villagers named their home Nisos, simply the Greek word for island.