Macquarie Island: The Only Place Where You Can See Earth’s Mantle Actively Exposed Above Sea Level

Far out in the Southern Ocean, halfway between Tasmania and Antarctica, lies an extraordinary island where you can walk on rocks that were once buried 6 kilometers beneath the ocean floor. Macquarie Island, a small strip of land just 34 kilometers (21 miles) long and 5 kilometers (3 miles) wide, offers geologists and curious visitors something found nowhere else on Earth: exposed rocks from our planet’s mantle thrust above sea level.

This geological oddity exists because Macquarie Island lies at the boundary between the Indo-Australian and Pacific tectonic plates, in a tectonically active region known as the Macquarie Ridge. Over millions of years, these plates were moving apart, allowing molten rock from the mantle to rise and form new oceanic crust – at a rate of approximately one millimeter (0.04 inches) per year.

Then, the plates reversed direction and began pushing against each other. The resulting pressure squeezed these deep rocks upward, lifting a five-kilometer-wide (3 miles) section of oceanic crust and mantle above the water, similar to toothpaste being pushed out of a tube.

Over its slow 12-million-year ascent, the Macquarie Ridge has been shaped by tectonic uplift rather than surface volcanic activity, glaciation, or significant sedimentary deposition, preserving its unique geological composition. This process eventually resulted in the emergence of Macquarie Island starting around 600,000 to 700,000 years ago, today providing a unique outdoor laboratory where scientists can study rocks from Earth’s mantle that normally remain hidden deep beneath the oceans.

Earth’s mantle is a thick, mostly solid layer between the crust and the core, making up about 84% of the planet’s volume. It extends from the base of the crust (the Mohorovičić discontinuity, or “Moho”) down to the outer core, about 2,900 kilometers (1,800 miles) deep. The mantle consists of silicate rocks rich in magnesium and iron, and its temperature ranges from about 500 °C (932 °F) near the crust to over 4,000 °C (7,200 °F) near the core-mantle boundary. It’s the immense pressure that keeps most of the mantle in a solid state, despite these high temperatures.

The unique exposure of the Earth’s mantle and oceanic crust on Macquarie Island not only reveals its geological history but also serves as a critical resource for studying the mantle’s role in plate tectonics. It is the only known location where mantle rocks are currently being actively exposed above sea level due to ongoing tectonic processes. While other places and geological formations, such as ophiolites, also have exposed mantle rocks, these exposures are typically the result of ancient tectonic events and are not currently active. Therefore, Macquarie Island’s distinction lies in the active nature of its mantle exposure.

One of the island’s most striking geological features is the presence of pillow basalts, which form when lava erupts underwater and cools rapidly. These rocks, along with other volcanic formations, offer insight into processes that typically unfold miles beneath the ocean’s surface. The absence of deformation in these rocks allows scientists to observe their original relationships, offering clues about oceanic crust formation and the dynamics of plate tectonics.

The island’s geological story is still being written. Small earthquakes frequently shake the land, and major quakes registering above 6.2 on the Richter scale occur about once per year. This ongoing geological activity, combined with the pristine nature of the exposed rocks, makes Macquarie Island an invaluable site for understanding how the Earth’s crust forms and moves.

Beyond its geological significance, Macquarie Island hosts an abundance of wildlife. The island serves as home to over 3.5 million breeding seabirds of various species. Among them are the royal penguins, a species found nowhere else, whose population on the island exceeds 850,000 breeding pairs. The island also supports around 80,000 elephant seals, along with several other seal species that use its shores as breeding grounds.

The island’s history includes both conservation triumphs and cautionary tales. After its discovery in 1810, sealers and penguin hunters nearly drove several species to extinction. Later, introduced cats, rabbits, rats, and mice caused severe damage to the island’s ecosystem. However, through dedicated conservation efforts, these invasive species were successfully eliminated by 2014 – the largest island pest-eradication program ever completed at that time. Since then, the island’s native vegetation has flourished, and several bird species have made remarkable recoveries, including the white-headed petrel, which had nearly vanished by 2001.

The island’s landscape reflects its harsh subantarctic location in the “Furious Fifties” latitudes, known for their strong winds and stormy seas. Despite these conditions, the island supports diverse plant life, though none grow very tall – most plants stay under one meter in height, with only one grass species reaching two meters in sheltered areas.

Stay part of the Mission! Get our latest articles to your inbox!

Your Email:

I consent to receiving email updates from Earthly Mission

Please prove you are human by selecting the flag.

Δ

Macquarie Island’s significance earned it UNESCO World Heritage status in 1997, recognizing it as a site of major geoconservation importance. Today, it’s carefully managed as a nature reserve, with access restricted to protect its unique geological features and recovering ecosystem.

For scientists, Macquarie Island offers unparalleled opportunities to study rocks that would otherwise require extremely deep drilling to access. These rocks tell the story of how oceanic crust forms at mid-ocean ridges, making the island a key location for understanding the processes that shape our planet’s surface. The exposed mantle rocks are particularly valuable because they’re remarkably pure, showing no contamination from continental crust – a rare find that helps geologists understand the composition of Earth’s interior.

As both a natural laboratory for understanding Earth’s structure and a vital habitat for Antarctic and subantarctic species, this small island in the vast Southern Ocean deserves continued protection. Its restricted access and careful management will help ensure that future generations can learn from its unique geology and its remarkable wildlife can continue to flourish.

Sources: 1, 2, 3, 4