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Permafrost: Cryostructures

When examining permafrost within a centimeter scale, the matrix of soil and ice can be described as textures or cryostructures. The combination of various cryostructures and how they layer on top of each other makes up the cryostratigraphy. Then the study of the cryostructures with the frozen ground's formation and structure is cryolithology. And finally, cryolithology is a branch of geocryology which is the study of geological materials that are below 32°F.

Cryostructures are predominately determined by the types of segregated ice within the soil, which forms as the ground freezes. Through out the winter, the ground freezes from the surface downwards, causing a freezing front to move from the surface downwards as seen in the one winter portion of the diagram. In fine-grained soils, available water within the soil is attracted to the freezing front by capillary action and other forces. This water freezes and forms ice lens, which is seen in blue in the diagram. The lateral extent of an ice lens is dependent on the amount of water available and exploitation along the bedding planes. This process causes volume expansion within soil columns and the ground surface to rise upwards, and is known as frost heaving.

In permafrost areas, the segregated ice forms each winter in the top active layer. Then the following summer, the entire active layer along with segregated ice will generally thaw during the summer or occasionally a small layer at the bottom of the active layer will remain frozen and become captured within the permafrost. The capturing can be caused by periods of colder climates, such as ice ages, or as a part of syngenetic permafrost growth as seen in the diagram below. Over the decades, silt is slowly deposited and raised the surface elevation. If the thickness of the active layer remains approximately the same, the top of the permafrost will rise in elevation and captures the bottom of the active layer. As variations in climate and the local ecosystem happen throughout the decades, some evidence can be captured within permafrost with variations in cryostructures and organic inclusions.


Diagram of segregated formation.
Diagram of segregated formation.

Types of Cryostructures

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 Lenticular-Layered Cryostructure
Cryostructure where the ice lenses are black. In lenticular-layered cryostructures, the ice lenses form long layers through the silt. The ice lenses range in thicknesses from 0.5 to 1.5 millimeters and lengths varying up to 1 centimeter. In the photo, the ice lenses are black in color.
 Micro-Lenticular Cryostructure
Cryostructure where the ice lenses are dark brown to black and sunken. In micro-lenticular cryostructures, the ice lenses are thin and short with thicknesses usually less than 0.1 millimeters and lengths ranging from less than 1 millimeter to a few millimeters. The lenses form in both straight and wavy lines where the spacing between the lines ranges from less than 1 millimeter to a few millimeters. While the ice lenses are small, the weight of the water often greatly exceeds the weight of the silt alone. In the photo, the ice lenses are dark brown to black in color and sunken into the slightly melted ice core.
 Massive Crysostructure
No visible ice. Massive cryostructures refer to a texture that is massive in soil with very fine segregated ice. There generally is no visible ice to the naked eye. This silt matrix contains microscopic, randomly-oriented segregated ice. The ice varies in shape from lenses to spheres and in thickness between 0.02 and 0.05 millimeters. The ice generally does not exceed the silt's unfrozen pore space, unless the soil was unconsolidated when it was frozen. Massive cryostructures can be linked to thermal erosion, where the unconsolidated silt falls into eroded openings in permafrost. This unconsolidated silt can be seen in the tunnel, where the weight of the water is approximately 65% of the weight of the soil alone.
 Layered Cryostructure
The dark brown section in the top right is the ice wedge. Layered cryostructures are continuous ice layers that form perpendicular to ice wedges. The ice layers will curl upwards as it approaches the ice wedge edge. The layer thickness ranges between 0.2 and 1 centimeter, with the spacing in between the layers vary between 2 and 5 centimeters. Usually, micro-lenticular cryostructures will exist in between the ice layers. In the photo, the dark brown section in the top right is the ice wedge and the black-colored layered ice is seen approaching from the left side of the photo; the photo beneath shows reticulate.
 Reticulate-Chaotic Cryostructure
Photo depecting reticulate. The ice lenses form in a randomly-oriented, multi-directional pattern. This ice pattern is seen around thermokarst-cave ice. The pattern forms from enclosed areas of thawed permafrost, where the soil is in a closed-system and freezes inward from many directions instead of only in the downward direction. In the photos, the ice is dark brown to black in color.
Reticulate.

References for This Page

 References

Bray, Matthew T., Hugh M. French, and Yuri Shur (2006) Further cryostratigraphic observations in the CRREL Permafrost Tunnel, Fox, Alaska. Permafrost and Periglacial Processes, 17(3): 233–243.

Fortier, Daniel, Mikhail Kanevskiy, and Yuri Shur (2008) Genesis of Reticulate-Chaotic Cryostructure in Permafrost. In Vol. 1 of Proceedings of the Ninth International Conference on Permafrost: University of Alaska Fairbanks, June 29–July 3, 2008, ed. D.L. Kane and K.M. Hinkel, 451–456. Fairbanks, AK: Institute of Northern Engineering.

Shur, Yuri, Hugh M. French, Matthew T. Bray, and D.A. Anderson (2004) Syngenetic permafrost growth: cryostratigraphic observations from the CRREL tunnel near Fairbanks, Alaska. Permafrost and Periglacial Processes, 15(4): 339–347.