How to identify Weathered Igneous Rock with Lichen

To correctly identify Weathered Igneous Rock with Lichen (Igneous Rock (likely basalt or andesite) with lichen 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. The underlying weathered rock can range from dark gray, black, or greenish-black (basalt) to medium gray or reddish-brown (andesite), often with reddish, yellowish, or brownish stains from iron oxidation. Lichens add patches of green, gray, yellow, orange, or black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy due to weathering and lichen cover. Original vitreous or sub-vitreous luster of fresh rock minerals is typically absent.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The rock itself may show signs of original aphanitic (fine-grained) texture, but weathering often creates a rough, pitted, or friable surface. Lichens add a crusty, powdery, or leafy texture to the surface.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual crystals are generally not discernible without magnification due to the fine-grained nature of basalt/andesite and subsequent weathering. Phenocrysts (larger crystals) might be visible in some andesites, but their forms would be rounded by weathering.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the rock as a whole. Individual minerals within the rock may exhibit cleavage, but the weathered state and fine grain size make it difficult to observe. Lichens do not exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in areas with exposed volcanic rocks, such as lava flows, volcanic plateaus, mountain ranges, and coastal regions. Requires exposure to the atmosphere, moisture, and sunlight for both weathering and lichen growth to occur.

Key Facts

Hardness

Variable, generally softer than fresh rock (Mohs 5-6) due to weathering, but the overall rock mass can still be tough. Lichens have no measurable hardness.

Specific Gravity

Variable, typically lower than fresh rock (2.5-2.9) due to porosity from weathering and the presence of lighter secondary minerals. Lichens have very low density.

Crystal System

Not applicable to the rock as a whole. Constituent minerals (e.g., plagioclase, pyroxene) have specific crystal systems (e.g., triclinic, monoclinic).

Color

Dark gray, black, greenish-black, or reddish-brown for the weathered rock; diverse colors for lichens.

Luster

Dull to earthy.

Transparency

Opaque.

Fracture

Irregular to conchoidal in fresh rock, but often earthy or crumbly in weathered specimens.

Cleavage

Not applicable to the rock as a whole; individual minerals may exhibit cleavage.

Composition

Primarily silicate minerals (plagioclase feldspar, pyroxene, olivine in basalt; plagioclase, pyroxene, amphibole in andesite) altered to secondary minerals like clays (kaolinite, smectite), iron oxides (hematite, goethite), and carbonates. Lichens are composed of fungal hyphae and algal/cyanobacterial cells.

Physical characteristics

• Crystal Habit: Aphanitic (fine-grained) to porphyritic for the original rock, but obscured by weathering. Lichens exhibit crustose, foliose, or fruticose growth forms.
• Cleavage Type: Not applicable to the rock as a whole.
• Fracture Type: Irregular to earthy.
• Tenacity: Brittle to friable, depending on the degree of weathering.
• Luster Type: Dull to earthy.

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