Weathered Sedimentary Rock (with lichen/algae growth)

Description

Weathered sedimentary rock, particularly sandstone or siltstone, is characterized by its altered surface texture, often appearing pitted, crumbly, or discolored compared to fresh rock. The presence of lichens and algae manifests as patches of various colors (greens, grays, yellows, oranges, blacks) adhering to the rock surface. Lichens can form crusts, foliose (leaf-like) structures, or fruticose (shrub-like) growths. Algae typically form thin, often slimy, green or dark films, especially in damp environments. The underlying rock's original features (bedding, grain size) may be obscured or enhanced by the weathering and biological growth.

How to Identify

Color

Highly variable. The underlying rock color (tans, browns, reds, grays) will be present, but overlaid by patches of green (algae, some lichens), gray, yellow, orange, black (various lichens).

Luster

Dull to earthy, often obscured by biological growth. The rock itself may have a dull luster.

Texture

The rock surface can be rough, granular, friable, or powdery due to weathering. The biological growth adds a textured, often crusty or slightly raised feel, or a smooth, slimy feel if algae are dominant and wet.

Crystal Form

Not applicable for the rock as a whole; individual mineral grains within the sedimentary rock may show crystal forms, but these are typically clastic (broken) and not well-formed. Lichens and algae do not form crystals.

Cleavage

Not applicable for the rock as a whole. Individual mineral grains (e.g., feldspar) may exhibit cleavage, but the rock itself does not have a characteristic cleavage plane. The biological growth has no cleavage.

Geological Environment

Exposed rock outcrops, cliffs, boulders, and rock surfaces in terrestrial environments, particularly in humid to semi-arid climates where moisture is available for biological growth. Common in areas with moderate to high rainfall, or where fog/dew provides moisture.

Key Facts

• Hardness: Variable, depending on the underlying rock and degree of weathering. Fresh quartz sandstone grains are 7 on Mohs, but the weathered rock surface can be very soft (1-3) and friable. Biological growth has no measurable hardness.
• Specific Gravity: Variable, depending on the underlying rock. Typically 2.2-2.7 for sandstone/siltstone. Biological growth has negligible impact on bulk specific gravity.
• Crystal System: Not applicable for the rock as a whole or the biological growth. Individual mineral grains within the sedimentary rock may be crystalline (e.g., quartz is trigonal).
• Color: Mottled, with natural rock colors (tans, browns, grays) overlaid by biological colors (greens, grays, yellows, oranges, blacks).
• Luster: Dull to earthy.
• Transparency: Opaque.
• Fracture: Irregular to conchoidal (for individual quartz grains), but often granular or earthy for the weathered rock mass.
• Cleavage: None for the rock mass; individual mineral grains may exhibit cleavage (e.g., feldspar).
• Composition: Primarily silicate minerals (quartz, feldspar, clay minerals) for the sedimentary rock, with organic compounds from lichens and algae on the surface. Cementing agents (silica, calcite, iron oxides) are also present in the rock.

Quick Check

• Color: Variable, often mottled with greens, grays, yellows, oranges, blacks over the natural rock color.
• Luster: Dull to earthy, often obscured.
• Streak: Not applicable for the rock as a whole; individual mineral grains would have their own streak, and the biological growth does not produce a streak.

Physical Characteristics

• Crystal Habit: Not applicable for the rock or biological growth. Clastic grains within the rock are typically anhedral to subhedral.
• Cleavage Type: None for the rock mass.
• Fracture Type: Granular, earthy, or irregular.
• Tenacity: Brittle to friable, depending on the degree of weathering and cementation.
• Luster Type: Dull to earthy.

Formation

This is not a distinct rock type but rather a description of a sedimentary rock (commonly sandstone or siltstone due to their porosity and permeability) that has undergone physical and chemical weathering processes, and subsequently been colonized by lichens and/or algae. Weathering breaks down the rock, creating micro-environments suitable for microbial and plant life. Lichens (symbiotic organisms of fungi and algae/cyanobacteria) and algae colonize rock surfaces, contributing to both physical (e.g., wedging by hyphae, expansion/contraction) and chemical (e.g., secretion of organic acids) weathering, further altering the rock's surface.

Usage

Not used as a raw material due to its altered and often friable nature. Its primary 'usage' is ecological, contributing to soil formation and nutrient cycling, and as a substrate for pioneer species. Geologically, it provides evidence of surface exposure, weathering processes, and biological activity.

Age Distribution

Any age of sedimentary rock exposed to the surface environment

Where to Find

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Scientific Classification

Mineral Class

Not a mineral; it's a rock type with biological modification.

Group

Sedimentary Rock (clastic) with biological colonization.

Crystal System

Not applicable for the rock as a whole.

Chemical Formula

Variable (e.g., SiO2 for quartz, KAlSi3O8 for orthoclase, plus organic compounds from biota).

Composition

Dominated by silicate minerals (quartz, feldspar, clay minerals), with varying amounts of cementing agents (calcite, iron oxides, silica). The surface includes complex organic molecules from lichens and algae, along with their metabolic byproducts.

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