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Weathered Rock with Lichen refers to any rock type (igneous, sedimentary, or metamorphic) that has undergone physical and/or chemical weathering processes and subsequently, or concurrently, hosts lichen growth on its surface. The rock itself is unidentifiable without further geological analysis, as the term describes a surface phenomenon rather than a specific rock type. Lichens are symbiotic organisms composed of a fungus (mycobiont) and an alga or cyanobacterium (photobiont). They adhere to rock surfaces, contributing to both physical (e.g., wedging by hyphae, freeze-thaw cycles exacerbated by water retention) and chemical (e.g., secretion of organic acids) weathering. The appearance of the rock is highly variable, dictated by the underlying lithology, the extent and type of weathering, and the species and coverage of the lichen.
How to Identify
- Color
- Highly variable, depending on the underlying rock and the lichen species. Lichens can be green, grey, yellow, orange, black, or white. The rock itself may show colors indicative of weathering (e.g., reddish-brown for iron oxidation).
- Luster
- The rock's luster may be obscured by lichen growth. Exposed rock surfaces will exhibit the luster characteristic of the rock type (e.g., dull, earthy, vitreous). Lichen surfaces are typically matte or dull.
- Texture
- The rock's texture will be modified by weathering (e.g., roughened, pitted, exfoliated) and by the presence of lichens, which can be crustose (crust-like), foliose (leaf-like), or fruticose (shrub-like).
- Crystal Form
- Not applicable to the composite. The underlying rock may or may not exhibit crystal forms depending on its type (e.g., igneous rocks may show interlocking crystals, metamorphic rocks may show foliation). Lichens do not have crystal forms.
- Cleavage
- Not applicable to the composite. The underlying rock may exhibit cleavage planes if it is a mineral or a rock with preferred mineral orientation (e.g., slate). Lichens do not have cleavage.
- Geological Environment
- Ubiquitous in terrestrial environments where rocks are exposed to the atmosphere and light, from polar regions to deserts, and from high mountains to coastal areas. Requires stable rock surfaces for colonization.
Key Facts
- Hardness: Variable, dependent on the underlying rock (e.g., quartz: 7, calcite: 3). Lichens are soft and easily scraped.
- Specific Gravity: Variable, dependent on the underlying rock (e.g., granite: 2.6-2.7 g/cm³, sandstone: 2.0-2.6 g/cm³). Lichen biomass is negligible in terms of overall density.
- Crystal System: Not applicable (composite). The underlying rock may or may not have a defined crystal system.
- Color: Highly variable, a combination of rock color and lichen pigmentation.
- Luster: Dull to earthy for lichen; variable for the underlying rock.
- Transparency: Opaque (rock and lichen).
- Fracture: Variable, dependent on the underlying rock (e.g., conchoidal, irregular, splintery). Lichens do not fracture in a geological sense.
- Cleavage: Not applicable (composite). The underlying rock may or may not exhibit cleavage.
- Composition: Rock component: Silicates, carbonates, oxides, etc., depending on lithology. Lichen component: Fungal hyphae (chitin), algal/cyanobacterial cells (cellulose, peptidoglycan), various organic compounds, water, and absorbed minerals.
Quick Check
- Color: Variable (rock color + lichen color: green, grey, yellow, orange, black, white)
- Luster: Dull to earthy (lichen) or characteristic of the underlying rock (exposed areas)
- Streak: Not applicable (lichen) or characteristic of the underlying rock (if tested on exposed rock)
Physical Characteristics
- Crystal Habit: Not applicable (composite).
- Cleavage Type: Not applicable (composite).
- Fracture Type: Variable, dependent on the underlying rock.
- Tenacity: Variable, dependent on the underlying rock (e.g., brittle, tough). Lichens are generally fragile when dry, pliable when wet.
- Luster Type: Dull to earthy (lichen); variable for the underlying rock (e.g., vitreous, metallic, resinous).
Formation
The rock component forms through standard geological processes (igneous, sedimentary, metamorphic). Lichen colonization occurs on exposed rock surfaces, initiating and contributing to biological weathering.
Usage
Ecological indicators, bio-weathering agents, aesthetic value in natural landscapes. Not typically used as a raw material due to its composite and often superficial nature.
Age Distribution
Ubiquitous across geological ages, dependent on rock exposure and lichen colonization time
Where to Find
Mountainous Regions
Common on exposed bedrock, boulders, and scree slopes due to harsh weathering conditions and suitable light exposure for lichens.
Coastal Areas
Abundant on sea cliffs, rocky shores, and coastal outcrops, where salt spray and wind contribute to weathering and provide specific niches for halophilic lichens.
Arid and Semi-arid Environments
Found on desert pavements, rock outcrops, and canyon walls, where specialized lichens (e.g., cryptogamic crusts) thrive despite limited moisture.
Forests and Woodlands
Present on boulders, fallen logs (though this is wood, not rock), and exposed bedrock within forested areas, often in shaded, humid conditions.
Urban and Suburban Areas
Common on old stone walls, monuments, gravestones, and concrete structures, demonstrating their adaptability to anthropogenic substrates.
Finding Tips
Observe Surface Features
Look for discolored, pitted, or flaking rock surfaces, often with visible biological growths that appear crusty, leafy, or bushy. The color and texture of the lichen itself will vary greatly.
Consider the Environment
Weathered rocks with lichen are found in almost any outdoor environment where rocks are exposed to the elements and sunlight. They are particularly prevalent in areas with long-term rock exposure and moderate humidity.
Examine Closely
Use a hand lens to distinguish between different types of lichen growth (crustose, foliose, fruticose) and to observe the underlying rock's texture and mineralogy where visible.
Note the Rock Type (if possible)
While the primary focus is the weathering and lichen, try to identify the underlying rock type (e.g., granite, sandstone, schist) as this influences weathering patterns and lichen colonization.
Similar Rocks
Moss-covered Rock
Bryophyta on various lithologies
Also known as: Bryophyte-rock association
Algae-covered Rock
Algae on various lithologies
Also known as: Phycobiont-rock association
Biofilm-covered Rock
Microbial communities on various lithologies
Also known as: Microbial mat on rock
Scientific Classification
- Mineral Class
- Not applicable (composite).
- Group
- Not applicable (composite).
- Crystal System
- Not applicable (composite).
- Chemical Formula
- Not applicable (composite).
- Composition
- Heterogeneous, comprising inorganic rock minerals and organic lichen biomass.
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