EPOW - Ecology Picture of the Week

Patterns in Glacial Ice

Crystal patterns of glacial ice
Mendenhall Glacier, southeast Alaska 

Explanation:  Peer closely and deeply into this iceberg ... what is this jumble of lines and patterns?

You are nose to nose with an iceberg locked into the frozen surface of the outflow lake of Mendenhall Glacier, part of the Juneau Ice Field in southeast Alaska.  It is mid-winter and we can trudge the mile or more across the frozen surface of Mendenhall Lake to the very toe of the glacier.  

But this iceberg catches our eye with its exquisite internal patterns.  What is going on here?

These are the edges of crystals.  They form when snow falls onto the glacier, and then freezes to form what is called firn or névé, which is old snow that has thawed and refrozen multiple times.  Eventually, firn is compressed into dense glacial ice.  

When the glacial ice cracks off the face of the glacier -- a process called calving -- the pressure is released.  The ice then cracks and separates internally along the same fracture lines that occurred during its formation.  Thus, we see this intricate, tangled spaghetti of internal fractures.

In fact, during summer, the natural resource interpreters at the Mendenhall Glacier Visitor Center retrieve pieces of iceberg like this one and display them for people to see and touch.  When the ice melts, it breaks apart along those fracture lines like a giant jigsaw puzzle.  

Acknowledgment:
Many thanks to Laurie Craig, Interpreter, Mendenhall Glacier Visitor Center, for the explanation of the patterns.