How to identify Weathered Rock with Algae/Lichen

To correctly identify Weathered Rock with Algae/Lichen (Indeterminate rock type with biological growth), 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 species of algae/lichen. Algae often appear as green, dark green, or black films. Lichens can be gray, white, yellow, orange, red, brown, or black, forming crusts, leafy structures (foliose), or branching forms (fruticose). The weathered rock itself may show discoloration (e.g., reddish-brown from iron oxidation, bleached white from leaching).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, due to the weathered surface and biological growth. The biological component itself does not exhibit a mineral luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The rock surface will typically be rough, pitted, or granular due to weathering. The biological growth adds a distinct texture: smooth and film-like for algal mats, crusty or powdery for crustose lichens, leafy for foliose lichens, and bushy for fruticose lichens.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable to the overall weathered rock with biological growth. The underlying rock may have crystalline components, but these are often obscured or degraded. Algae and lichens do not form crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the biological growth. The underlying rock may exhibit cleavage, but it is often obscured or compromised by weathering.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Ubiquitous in terrestrial environments, from polar regions to deserts, and from high mountains to coastal areas. Requires exposed rock surfaces, sufficient moisture (even intermittent), and light for photosynthesis. Common in outcrops, boulders, cliffs, gravestones, and man-made structures.

Key Facts

Hardness

Not applicable to the biological growth. The underlying rock's hardness will vary (e.g., 1-7 on Mohs scale), but the weathered surface may be softer.

Specific Gravity

Not applicable to the biological growth. The underlying rock's specific gravity will vary (e.g., 2.5-3.5 for common rocks).

Crystal System

Not applicable to the biological growth. The underlying rock may have a specific crystal system if it's crystalline.

Color

Highly variable, depending on the specific algae/lichen species and the underlying weathered rock.

Luster

Dull to earthy.

Transparency

Opaque (for both the weathered rock and the biological growth).

Fracture

Not applicable to the biological growth. The underlying rock's fracture will vary (e.g., conchoidal, irregular, splintery), but weathering may make it more friable.

Cleavage

Not applicable to the biological growth. The underlying rock's cleavage will vary, but weathering may obscure it.

Composition

The rock's composition is indeterminate (silicates, carbonates, oxides, etc., depending on rock type). The biological component consists of organic compounds (carbohydrates, proteins, lipids) from algae (photosynthetic organisms) and fungi (heterotrophic organisms) in the case of lichens, often with accumulated dust and mineral particles.

Physical characteristics

• Crystal Habit: Not applicable to the biological growth. The underlying rock may have a specific crystal habit if it's crystalline.
• Cleavage Type: Not applicable to the biological growth. The underlying rock's cleavage type will vary.
• Fracture Type: Not applicable to the biological growth. The underlying rock's fracture type will vary.
• Tenacity: Not applicable to the biological growth. The underlying rock's tenacity will vary (e.g., brittle, friable), but weathering often makes it more friable.
• Luster Type: Dull to earthy.

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