What sort of rope would be ideal for glacier travel? I have a 10mm 60m rope, but this is quite heavy. Would a 9mm 50m rope be suitable for a team of three, crossing glaciers in the Swiss Alps?
Beal Ropes has a guide that covers this. Read the page but here are two images for quick reference:
A robust half rope such as the Mammut Genesis is probably a very good choice; such a rope is much lighter than a 10mm Single but still has a thick sheath.
Mr. Wizards answer provides nice pictures supplied by Beal about what type of ropes can be used on ice/snow and rock. Quick recap: On ice/snow you can use every rope type (single, half and twin) with a single strand. If there is rock involved the single rope can still be used with one strand, while twin ropes have to be used as two strands immediately parallel to each other at all times. For half ropes there is more to it, but it is not relevant to glaciers, so I do not want to get into it here. The full difference between half and twin ropes is in my opinion not clear from just the second picture and is generally complicated enough for its own question.
This addresses what is ok to use, but not what is optimal. And in recent times static ropes made from dyneema/kevlar have come into use for mountaineering on glaciers as well. I will first shortly introduce the latter and then evaluate the benefits of the options.
Traditionally one uses the same dynamic kernmantle rope made of Nylon (name of various polyamide fibers) for mountaineering in pure snow/ice or on rock/mixed. For bigwalling so called trail/haul lines which are static are often used for rappelling or hauling material up. Recently I have seen such/similar ropes used for ski mountaineering (e.g. the Mammut Rappel Cord 6.0, no affiliation). The line being static is not a problem when holding a fall into a crevasse. While the initial pull is more abrupt (it is already quite abrupt with dynamic ropes anyway), braking is smoother. In tests by the Petzl team they did hold 60% of the falls with the dynamic rope and 80% with the static rope (bergundsteigen article in German). Together with personal tests this seems to indicate that it is just as possible to hold a fall with such a static rope as with a dynamic one.
As dyneema ropes are about half the weight and much less volume than a typical half rope and as an extra do not hold water, this is in my opinion the ideal rope for glacier mountaineering. If you will only use the rope on glaciers and for rappelling, it is perfectly suited. If you need to belay even the shortest of climbs on rock or steep ice, you need a dynamic rope. So if this is the case or you do not feel comfortable for whatever reasons with a static rope, any light and impregnated rope will do, no matter the type. Impregnation of either just the sheet or all involved fibers helps to keep water out of the rope. Water is an obvious issue on glaciers and will make the rope heavier and less dynamic.