Mapping the crystal structure of ice

Ice consists of individual ice crystals, each with different physical properties. The typical size of the crystals in glacier ice is 1 - 5 mm, but much larger crystals can be found, e.g. in very old ice near the base of the Antarctic ice sheet.

When ice flows, the crystal structure of ice is changed, and conversely, the orientation and size of the ice crystals influence how ice deforms when exposed to stresses. Crystal structure measurements can therefore reveal the "flow history" of an ice sample and also improve our ability to understand and model how ice deforms. Read more about the "flow history" of an ice sample in the Flow of ice section.

The crystal structure of the ice is investigated by making so-called thin sections. The technique relies on the fact that ice crystals are able to alter the polarization of light. A 5 mm thick slab of ice is cut using the band saw. Because of vibrations in the saw blade the ice will break if it is cut thinner than 5 mm. The ice slab is placed on a glass plate and ground down to a thickness of 0.5 mm. The thin section is then placed between two crossed polarization filters. Normally, no light is transmitted when the two filters are perpendicular, but because the ice crystals alter the polarization, each crystal will take on a colour that depends on the orientation of the crystal. In this way, the size and orientation of the crystals can be measured.

Crystal structure

A picture of a thin section of glacier ice placed between two crossed polarizers. The different orientation of the individual crystals shows up as colour differences, although the ice itself is clear. The sample is about 6 cm ⋅ 6 cm.

Microtome knife

A slab of ice cut from an ice core is ground down to a thin section, only 0.5 mm thick. The upper part of the machine with the microtome knife moves back and forth, each time removing a thin layer of ice.










Read more about the correlation of ice crystal size and the visual layers in the ice.