At the more than 1,000 volcanoes around the planet, when they erupt, they send molten
rock originally from the mantle and eject it onto the surface.
Sometimes, these eruptions can even bring chunks of the mantle with them which then
solidify via what is known as a mantle xenolith.
For example, here is an olivine xenolith from the San Carlos volcanic field in Arizona.
Yet, despite all of this volcanic activity, the mantle remains generally unseen by humans
due to its sheer depth.
This is not the case, however, at a remote Australian island where the mantle is being
actively exposed on the surface above sea level.
The island I am referring to is Macquarie Island, where rock from the mantle and adjacent
oceanic crust is being uplifted and pushed higher above sea level at a rate of 11.5 centimeters
or 4.5 inches every 100 years.
Because of this, the island contains an abundance of highly unusual rock and mineral types,
such as this piece of serpentinite.
Another example is this piece of dunite which contains abundant olivine crystals.
While this mineral is rare to see in large crystals, in the upper mantle it is one of
the most if not the most common mineral in that layer of the planet.
Both of these rock types are part of a rare group of rocks known as an ophiolite.
While there are a scattered assortment of ophiolite deposits around the planet, only
the one at Macquarie island which represents part of the mantle is being actively and increasingly
exposed on a landmass.
For context, Macquarie Island is despite being closer to the South Island of New Zealand,
considered part of Tasmania where it is 1,550 kilometers southeast of the city of Hobart.
This island is more than 5 times as long as it is wide, and this fact can be credited
to how it formed.
Before I describe why this strange geologic oddity of an island exists I want to first
say this.
The exact reason why the island is being uplifted is still debated and despite spending many
many hours reading an assortment of papers I could not find an in depth model.
As a result, what I am about to present is a combination of ideas proposed in several
of those papers intermixed and combined into my own model.
Thus, this information could later be proven incorrect.
At the present, Macquarie Island is located at the intersection of two tectonic plates.
To the east, is the Pacific Plate.
To the west is the Indo Australian Plate.
Instead of colliding, these plates are largely moving horizontally with one another as part
of a large group of transform faults.
Approximately 10 million years ago, the two plates were instead slowly spreading apart
from one another to the east and west as what is known as a mid ocean ridge.
As they spread apart, lava erupted onto the ocean floor, forming new oceanic crust and
depositing large amounts of pillow basalt.
As this process continued for millions of years, the crust which would later become
much of Macquarie Island was formed.
However, eventually the rifting stopped and instead the plates started moving in a horizontal
manner.
Later, most likely at some point in the last 4 million years, the two plates while continuing
to on occasion move horizontally started to also move towards one another.
This eventually caused a section of the denser Pacific Plate to be pushed underneath the
Indo Australian plate.
As the very beginning of a subduction zone continued to form, a section of the Pacific
Plate broke off along with some of the underlying lithospheric rock, and due to the ongoing
collision began to be pushed upwards.
Over time, this tilted large segments of the affected area upwards towards the west.
After millions of years of gradual uplift, the island rose above sea level around 0.6
million years ago.
Since then, the island has continued to rise, with the island's tallest point now more than
381 meters or 1,250 feet above sea level.
Because Macquarie Island is still rising, additional sections of mantle rock are continually
being exposed, at a rate of around 1 millimeter per year.
Today, a wide range of minerals can be found on the island which includes pyrite, actinolite,
olivine, and serpentine group minerals among others.
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