Weird Geological Formation:

[beecee]

Around 400 miles [650kms] north of the Fijian group of Islands, is a small group called Rotuma, a Polynesian group of Islands, and a dependency of Fiji. 

While I am fairly familiar with most of the Fijian group, I was less knowledgable of Rotuma. So I did some research and came upon this strange Island named Hafliua sometimes called Split Island. The following photos will reveal the reason....

As is clearly visible,caught between the two walls is a large boulder that has been there throughout recorded history.

Some more info on this weird formation..... https://en.wikipedia.org/wiki/Hạfliua

The Island is obviously part of a sunken atoll and long extinct volcano, so any ideas as to what happened here and what was responsible for this formation and split?

[Kartazion]

5 minutes ago, beecee said:

Looks like the highest point of a volcano crater.

[beecee]

1 minute ago, Kartazion said:

Looks like the highest point of a volcano crater.

Yes, part of a half sunken Atoll and extinct volcano, but what would have caused the split? Answering my own question, probably due to a somewhat asymetric eruption when the volcano was active? A section blown out sort of?

[Kartazion]

The sea currents would have gnaw the part sunken.

 

The sysmic movements would have split the rock.

[studiot]

6 hours ago, beecee said:

The Island is obviously part of a sunken atoll and long extinct volcano, so any ideas as to what happened here and what was responsible for this formation and split?

Spectacular pictures and great question. +1

(Volcanic) Horseshoe islands are uncommon but not unknown.

Here is a link to another.

https://earthobservatory.nasa.gov/images/146164/the-island-shaped-like-a-horseshoe

 

As you say the eruption was asymmetric.
The obvious dipping of the volcanic layers down from the high side to right under water suggest this.
The layering can be traced through the cleft as though a slice of cake was removed.
The cleft is wider at the top than the bottom.
There is evidence of several less dramatic vertical breaks segmenting the island in the photos.

So how did it happen ?

Well there are three conceivable mechanism I can think of.

Firstly the eruption may have issued through sea bed faults.
This would have accounted for the original asymmetry of the eruption.

But it would not account for the clefts or large scale vertical joints.

Then there may have been a series of small eruptions, perhaps many, building up the layers around a central plug.
This is common in such marine islands.

One day the central plug was blown clear in a massive explosion and the surrounding ring fell backwards and outwards.
This would have increased the ring diameter and perhaps caused the original segmentation.
This would also account for the taper in the gap.

I say perhaps because jointing is also a common feature of volcanic rock which shrinks as it cools.
A determination of the rock type would help identify any such activity. Jointing is most common in granitic and basaltic roc, as opposed to the tuff lavas associated with pyroclastic events.

Kartazion has mentioned the third mechanism, subsequent erosion.
 

5 hours ago, Kartazion said:

The sea currents would have gnaw the part sunken.

The sysmic movements would have split the rock.

 A gentle correction here to help your English. The word is seismic not sysmic.

There is little scope for erosion by the usual forces but I suspect that the lodged rock was the result of rain erosion of the upper dipping layer to the high side of the cleft, causing a large boulder to separate and lodge lower down in the narrower part of the cleft.
The rounded shape suggests it is within the reach of the spray from the sea.
The general rounded shape of formation itself further suggests soft rock and tropical rain erosion.

 

So the story of this island is entirely different from the famous 'rock of ages' cleft in limestone scenary on the english Mendip hills.
Here the result was from the interplay of limestone scenary with first ice and then running water.

https://en.wikipedia.org/wiki/Burrington_Combe

Edited July 6, 2021 by studiot

[Jamey]

Perhaps there are two coincidental happenings.

1 volcanic build up and eruption, loss of the central cap 

2 meteor shower sends rock into side of eruption chamber while still hot

questions to think about: 

what material is the rock made of and is there any other similar " islands" without cracks obviously, in the same general area that would indicate "volcanic" or "celestial events"?  

[mistermack]

Looking at the pictures, I would guess that the split was caused by land slides under water, like in the case of Mount St Helens. With or without an explosive eruption. If you imagine Mt. St. Helens flooded up to it's neck, it looks quite similar :