why does crushed ice melt so much faster than an equal mass of ice cubes?

air pressure from all sides

Easy, crushed ice has more surface area to volume. While the volume (mass) is what holds the heat, it's the surface area that transfers the heat into the ice from the outside.

So the ratio A/V, surface area to volume, determines how fast that ice will melt. Let's assume the shape of the ice, cubes and crushed bits, is spherical, for easy calculation.

A = 4 pi R^2 is the surface area for a sphere; V = 4/3 pi R^3 for the volume.

So A/V = 4 pi R^2//4/3 pi R^3 = 3/R and there you are. As the radius gets smaller and smaller, the ratio A/R gets larger and larger. That's means, per volume, more and more heat can be transferred into the crushed ice; thus, it melts faster than the larger bits.

Less surface area (: