Weathered Igneous Rock with Lichen

Description

This refers to an igneous rock, most commonly basalt or andesite, that has undergone significant weathering and is colonized by lichens. Weathering manifests as discoloration, pitting, rounding of edges, and the breakdown of primary minerals into secondary minerals (e.g., clays, iron oxides). The lichen growth appears as crusts, foliose (leaf-like), or fruticose (shrub-like) forms, varying in color (greens, grays, yellows, oranges, blacks) and texture, adhering tightly to the rock surface. The underlying rock's original features may be obscured by both weathering products and lichen cover.

How to Identify

Color

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.

Luster

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

Texture

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.

Crystal Form

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.

Cleavage

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.

Geological Environment

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.

Quick Check

• Color: Variable, often dark gray to reddish-brown with patches of green, gray, yellow, orange, or black from lichens.
• Luster: Dull to earthy.
• Streak: Not applicable for the rock as a whole; individual weathered minerals might produce a streak (e.g., reddish-brown for iron oxides), but lichens do not produce a streak.

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.

Formation

The primary igneous rock (basalt or andesite) forms from the rapid cooling of mafic (basalt) or intermediate (andesite) lava flows at or near the Earth's surface. Weathering processes (physical, chemical, and biological) then act on the rock, altering its original mineralogy and texture. Lichens, which are symbiotic organisms composed of a fungus and an alga or cyanobacterium, colonize the rock surface, contributing to both physical and chemical weathering through the secretion of organic acids and physical penetration of hyphae.

Usage

Primarily of geological and ecological interest. Used in studies of weathering rates, biogeochemical cycling, and as indicators of air quality and environmental conditions. Not typically used for commercial or industrial purposes due to its altered state and biological cover.

Age Distribution

Ranges from Precambrian to Cenozoic, depending on the age of the host igneous rock. Lichen growth can occur on any exposed rock surface over time.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Igneous Rock (Extrusive)

Group

Volcanic Rocks

Crystal System

Not applicable to the rock as a whole.

Chemical Formula

Variable, complex silicate compositions for the rock; organic compounds for lichens.

Composition

Silicates (plagioclase, pyroxene, olivine, amphibole) altered to clays, iron oxides, and other secondary minerals. Lichens are symbiotic organisms of fungi and algae/cyanobacteria.

Explore Weathered Igneous Rock with Lichen