Is this 70km crack in an ice shelf of Antarctica remarkable, or a regular occurrence?

Question

I've just seen the LiveScience article 70-Mile-Long Crack Opens Up in Anatarctica. I'm not sure if the title is a bit sensational or not, the crack is in an ice shelf, not the continent of Antarctica.

An ominous crack in an Antarctic ice shelf as wide as a football field is long takes on an otherworldly beauty in a new aerial image.

Snapped by scientists on NASA's IceBridge mission, the shot shows a rift in Larsen C, an ice shelf that is floating off the Antarctic Peninsula. When the crack eventually spreads across the entire ice shelf, it will create an iceberg the size of the state of Delaware, according to IceBridge. That's around 2,491 square miles (6,451 square kilometers).

As of Nov. 10, when the IceBridge scientists observed this crack, it was 70 miles (112 km) long and more than 300 feet (91 meters) wide. The dark depths of the crack plunge down about a third of a mile (0.5 km), all the way through the ice to the ocean below.

[...] Larsen C is Antarctica's fourth-largest ice shelf, and it holds back the land-based glaciers just behind it: Once the ice shelf goes, those slow-flowing glaciers have one less barrier in their journey toward the sea.

My primary question: is this a remarkable event, or something that over time happens regularly? Isn't this just a natural part of "those slow-flowing glaciers... journey toward the sea." ?

I'm also wondering 1) Is an iceberg the size of Delaware actually remarkable, or something that just happens from time-to-time? And 2) how they (actually) know the crack goes all the way to the ocean - can they actually see the water in images, or is this a hypothesis based on understanding of cracks of this length and width? I can ask as a separate questions if it's too much to ask here.

above: Image from here. "A huge crack can be seen in the Antarctic Peninsula's Larsen C ice shelf in this aerial image snapped on Nov. 10, 2016, as part of NASA's IceBridge mission. Credit: NASA/John Sonntag"

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update:

The rift continues to be tracked. If I understand correctly, it is now over 350 meters wide and growing:

above: From A growing rift on Larsen C at the Project MIDAS website.

More can be read about NASA's operation Ice Bridge program at its official website. These are a series of areal flights over antarctic regionss to collect images and data. The image above of the crack in the Larsen-C ice shelf was a featured photo and more can be read about it here. In the image gallery the caption states:

Rift in Antarctica's Larsen C Ice Shelf

On Nov. 10, 2016, scientists on NASA's IceBridge mission photographed an oblique view of a massive rift in the Antarctic Peninsula's Larsen C ice shelf. Icebridge, an airborne survey of polar ice, completed an eighth consecutive Antarctic deployment on Nov. 18.

The photo was also featured in @NASA_ICE.

A short video explaining the mission and showing details of the aircraft's science equipment can be seen below (and is linked here):

https://www.youtube.com/watch?v=v3yMHHzLTCc

Answer 1

Is an iceberg the size of Delaware [6,451 square kilometers] actually remarkable, or something that just happens from time-to-time?

In March 2000 iceberg B-15 formed which was 11,000 sq. km. and 295 km long.

In October 1998 iceberg A-38 formed which was 6,900 sq. km.

A Delaware size iceberg would be the 3rd largest in the past 20 years.

The 1990 article The calving and drift of iceberg B-9 in the Ross Sea, Antarctica Antarctic Science Vol. 2 , pages 243-257 says:

The largest iceberg in recent times (95 km x 95 km) calved from the Larsen Ice Shelf in early 1986

So a 9000 sq. km. iceberg broke off in 1986.

Contribution of giant icebergs to the Southern Ocean freshwater flux says:

an iceberg of 10,000 km2 in area broke from the Amery ice shelf in 1964

According to Antarctica shed a 208-mile-long berg in 1956:

A record iceberg seen in Antarctic

Little America V, Antarctica, Nov. 17- The U.S.S. Glacier, the Navy's most powerful icebreaker, has sighted an iceberg more than twice the size of Connecticut.

The berg was sighted by the Glacier early this week about 150 miles west of Scott Island. The ship reported it was 60 miles wide and 208 miles long- or more than 12,000 square miles, as against Connecticut's 5,009.

So 12,000 sq. mi. or 31,000 sq. km. is the largest known iceberg.

Both of the two previous references also mention a 1927 Antarctic iceberg that was 170 km long, but area is not given. American Practical Navigator, says the 1927 iceberg was 100 mile long and 100 miles wide, so 25,000 sq. km.

In summary, an iceberg the size of that contemplated in the OP can be expected on the once every 5-20 years time scale.

update 7/12/2017:

according to the MIDAS project:

The calving occurred sometime between Monday 10th July and Wednesday 12th July 2017, when a 5,800 square km section of Larsen C finally broke away. The iceberg, which is likely to be named A68

Answer 2

Developing since 2014, the crack was projected in 2015 to lead to "the largest calving event since the 1980s," according to this report in The Cryosphere journal by glaciologists studying the region.

From 2010, "Overview of areal changes of the ice shelves on the Antarctic Peninsula over the past 50 years" says that the largest break-up event in that period was the crack-up of Larsen B in 2002, which totalled 3,250 square kilometers -- approximately half the size of the area projected to break off from Larson C as a result of the current crack.

"Growing Crack in Antarctica's Larsen C Ice Shelf Spotted by NASA's MISR" explains that the Multiangle Imaging SpectroRadiometer (MISR) instrument aboard NASA's Terra satellite examine's Earth's surface with nine cameras pointed at different angles, which provides information about the texture of the surface. In composite images, the crack is clearly distinct from the surrounding ice shelf -- it is explained that the texture of the ice shelf is smooth and the texture of the sea ice on the ocean surface within the crack is rough.