Schist: the story on HearLore

Schist

On the 17th of August 1959, a magnitude 7.2 earthquake shattered the silence near Hebgen Lake in Montana, triggering a catastrophic landslide that claimed the lives of 26 campers. The mountain that gave way was not made of granite or limestone, but of schist, a medium-grained metamorphic rock whose very name comes from the Greek word schízein, meaning to split. This geological feature, which defines the rock by its ability to cleave into thin flakes, turned a place of recreation into a graveyard in moments. The schistosity, or the layered texture of the rock, created planes of weakness that allowed the mountain to slide with terrifying ease, demonstrating how the internal structure of a stone can dictate the fate of human lives. Before the mid-19th century, the terms slate, shale, and schist were not sharply differentiated by those involved with mining, but today geologists define schist as a rock that shows well-developed schistosity, permitting it to be split into flakes or slabs less than 1 centimeter thick. The mineral grains in a schist are typically from 0.5 to 2 millimeters in size, making them easily visible with a 10x hand lens, and over half of these grains show a preferred orientation that defines the rock's character.

Origins Of Mountain Building

Schist typically forms during regional metamorphism accompanying the process of mountain building, known as orogeny, and usually reflects a medium grade of metamorphism. This transformation occurs when the rock is subjected to elevated temperature and nonhydrostatic stress, meaning it is more strongly compressed in one direction than in others. As mountain building takes place, platy minerals are rotated or recrystallized into parallel layers perpendicular to the direction of greatest compression. While platy or elongated minerals are most obviously reoriented, even quartz or calcite may take up preferred orientations, creating a texture that distinguishes schist from other metamorphic rocks. The composition of the rock must permit the formation of abundant platy minerals, such as mica, chlorite, talc, or graphite. For instance, the clay minerals in mudstone are metamorphosed to mica, producing a mica schist, which is particularly common. Early stages of metamorphism convert mudstone to a very fine-grained metamorphic rock called slate, which with further metamorphism becomes fine-grained phyllite. Further recrystallization produces medium-grained mica schist, and if the metamorphism proceeds further, the mica schist experiences dehydration reactions that convert platy minerals to granular minerals such as feldspars, decreasing schistosity and turning the rock into gneiss.

Mineral Identities And Names

Common questions

What caused the Hebgen Lake landslide on the 17th of August 1959?

The Hebgen Lake landslide on the 17th of August 1959 was caused by a magnitude 7.2 earthquake that triggered a catastrophic failure of schist bedrock. The schistosity planes within the rock created zones of weakness that allowed the mountain to slide with terrifying ease, resulting in the deaths of 26 campers.

How is schist defined by geologists today compared to the mid-19th century?

Geologists today define schist as a medium-grained metamorphic rock that shows well-developed schistosity and splits into flakes or slabs less than 1 centimeter thick. Before the mid-19th century, the terms slate, shale, and schist were not sharply differentiated by those involved with mining.

What minerals are typically found in schist and how large are they?

Schist typically contains mineral grains ranging from 0.5 to 2 millimeters in size that are easily visible with a 10x hand lens. These grains often include platy minerals such as mica, chlorite, talc, or graphite, and over half of them show a preferred orientation that defines the rock's character.

How does schist form during the process of mountain building?

Schist forms during regional metamorphism accompanying orogeny when rock is subjected to elevated temperature and nonhydrostatic stress. This process rotates or recrystallizes platy minerals into parallel layers perpendicular to the direction of greatest compression.

What is the difference between schist, gneiss, and granofels?

Schists make up one of the three divisions of metamorphic rock by texture, with gneiss having poorly developed schistosity and thicker layering, and granofels having no discernible schistosity. Schists are formally described as such only when the original type of the rock prior to metamorphism is unknown and its mineral content is not yet determined.

Why does schist pose a challenge for civil engineering projects?

Schist bedrock poses a challenge for civil engineering because its pronounced planes of weakness create discontinuities that influence the strength and deformation of rock masses. These schistosity planes develop perpendicular to the direction of greatest compression, allowing the rock to slide with minimal resistance during tunnel, foundation, or slope construction.