How to identify Weathered Rock with Lichen

To correctly identify Weathered Rock with Lichen (Rock (unidentifiable without further analysis) with biological growth (lichen)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   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.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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).

Explore Weathered Rock with Lichen