These ice spikes, called penitentes, form in high altitudes, where sunlight turns ice directly into water vapor, rather than melting it to water.

Sun beams vaporize small dimples in the snow’s surface. Then, the uneven surface directs sun into the dips and away from the peaks, exacerbating the trend.

The process begins with sunlight shining on the surface of the snow. Due to the very dry conditions in these desert regions, the ice sublimes rather than melts — it goes from solid to gas without melting and passing through a liquid water phase. Surface depressions in the snow trap reflected light, leading to more sublimation and deeper troughs. Within these troughs, increased temperature and humidity means that melting can occur. This positive feedback accelerates the growth of the characteristic structure of the penitentes.

They attain heights ranging from a few centimetres, resembling low grass, up to five metres, giving an impression of an ice forest in the middle of the desert.

Article:
https://www.eso.org/public/images/potw1221a/