Needle Ice

Needle ice forms when the ground is warmer than the air: the temperature gradient creates a pressure gradient, which draws water out of the soil in needle-like formations. As a Coyhaique resident pointed out to me, the formation of needle ice is highly-dependent on soil texture: they observed needle ice on organic, porous, saturated soils.

This aligns with our technical understanding: in order for ice to form at the surface, a steady supply of liquid water is required from below, which involves two factors:
1. Capillarity: In order to prevent downward flow by gravity, water must be retained by the force of capillary action. Soils with the greatest 'capillary pressure' are those with small grain sizes. Clays have the smallest grain size, sand the largest, and silt somewhere in between.
2. Permeability: In order for water to move quickly through the soil to the surface, the soil must be permeable, that is to say, the pores must be well-connected. Clays are the least permeable, sand the most, and silt somewhere in between.

So in order for frost to form, the soil must be both permeable and have a high capillary pressure: clays have high capillarity but are not permeable, sands are permeable but have low capillarity. Since silts have both capillarity and permeability, these are the most frost susceptible soils.

On a slope, the needles will induce soil erosion: the needles form perpendicular to the soil, and when they grow long enough or the sun melts them, they will fall by gravity down the slope, pulling soil with them. This geomorphic process is known as frost creep (see bottom photo).