Science Photo Library The iceberg that is set to break off may be the biggest one that has broken away from the Brunt Ice Shelf since observations began in 1915

Cracks growing across Antarctica's Brunt Ice Shelf are set to cause an iceberg to break off, which will be about twice the size of New York City.

Scientists don't know what will happen to the ice shelf after this happens.

"It's possible that the ice shelf will be destabilised," explains Nasa scientist Joe MacGregor.

When an iceberg breaks away like this, it is called calving.

Nasa says that while calving is a completely normal part of the life cycle of an ice shelf, the recent changes to this particular shelf aren't behaving quite as scientists would expect.

So let's have a look at what's happened.

Compare the top image - taken on 30 January 1986 - with the second image, taken on 23 January 2019, and you can see what's going on.

NASA

NASA

In the second image, it's clear to see the significant rift growing upwards in the middle of the photograph, towards the McDonald Ice Rumples.

Nasa has called this an "immediate concern".

The way that rock rises underneath the ice shelf causes crevasses and rifts on the surface of Antarctica. But this particular rift, which has been stable for about 35 years, is accelerating northward at a speed of 4km per year, towards something called the Halloween crack.

This is a crack growing eastwards from the McDonald Ice Rumples.

It is called this because it first appeared at the end of October 2016 and it has been growing eastwards ever since.

In this next picture, you can see how the cracks are moving closer together.

NASA

The iceberg, which is at least 660 square miles long, is set to break away from the ice shelf as a result of these cracks meeting is not enormous by Antarctica standards.

But it may be the biggest one that has broken away from the Brunt Ice Shelf since observations began in 1915, so it is significant.

BAS This is what the Halloween Crack - which is being closely monitored - looks like

Researchers have been based in this area of Antarctica since 1955.

But in 2015, scientists were forced to move the Halley VI research station because of the cracks that had appeared on the shelf, due to concerns the station could get cut off.

The station has also been forced to close down twice in recent years due to unpredictable changes in the ice.