Sandstone with Lichen

Sedimentary rock (likely quartz sandstone) with biological growth

Also known as: Lichen-covered sandstone, Biologically weathered sandstone

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Description

Sandstone with lichen is a sedimentary rock, predominantly composed of sand-sized mineral grains (0.0625 to 2 mm), cemented together. The most common mineral is quartz (SiO2), making it a quartz arenite. Other common constituents can include feldspar, rock fragments, and accessory minerals. The rock's color varies widely depending on its mineralogy and cement (e.g., iron oxides impart red/brown hues, clay minerals can make it grey/green). Overlying this rock surface are lichens, which appear as crusty, leafy, or shrubby growths. Their colors are diverse, including shades of green, grey, yellow, orange, black, and white, depending on the species and environmental conditions. Lichens adhere tightly to the rock surface, often forming intricate patterns. They contribute to both physical and chemical weathering of the sandstone.

How to Identify

Color: Sandstone color varies (white, grey, yellow, red, brown, green, black) depending on mineral composition and cement. Lichens add patches or extensive coverings of green, grey, yellow, orange, black, or white.
Luster: Sandstone typically has a dull to earthy luster, though quartz grains can exhibit a vitreous luster. Lichens are matte to slightly waxy.
Texture: Sandstone is clastic, with a gritty feel due to sand grains. Grain size is 0.0625 to 2 mm. Lichens have diverse textures: crustose (crust-like, tightly adhering), foliose (leaf-like, somewhat detached), or fruticose (shrubby, branched).
Crystal Form: Sandstone is composed of detrital grains, typically sub-angular to rounded. Lichens do not have a crystal form; they grow in thalli (body structures) characteristic of their species.
Cleavage: Sandstone typically exhibits no cleavage, though it may break along bedding planes. Lichens do not have cleavage.
Geological Environment: Sandstone forms in various sedimentary environments: fluvial (rivers), eolian (deserts), deltaic, and shallow marine settings. Lichens colonize exposed rock surfaces in almost all terrestrial environments, from polar regions to deserts, requiring only light, air, and moisture. They are common in areas with stable, exposed rock outcrops, cliffs, and boulders.

Key Facts

Hardness: Sandstone: 6-7 (Mohs) for quartz grains; overall rock hardness varies with cementation (can be friable to very hard). Lichens: Not applicable, as they are biological organisms, not minerals.
Specific Gravity: Sandstone: 2.2-2.8 g/cm³ (varies with porosity and mineralogy). Lichens: Not applicable, as they are biological organisms.
Crystal System: Sandstone: Not applicable (composed of detrital grains). Quartz grains are trigonal. Lichens: Not applicable (biological).
Color: Sandstone: White, grey, yellow, red, brown, green, black. Lichens: Green, grey, yellow, orange, black, white.
Luster: Sandstone: Dull to earthy (quartz grains vitreous). Lichens: Matte to slightly waxy.
Transparency: Sandstone: Opaque. Lichens: Opaque.
Fracture: Sandstone: Granular, conchoidal (if well-cemented quartz arenite). Lichens: Not applicable.
Cleavage: Sandstone: None (though may break along bedding planes). Lichens: Not applicable.
Composition: Sandstone: Predominantly quartz (SiO2), with varying amounts of feldspar, rock fragments, and cementing agents (silica, calcite, iron oxides, clay minerals). Lichens: Symbiotic association of a fungus (mycobiont) and an alga or cyanobacterium (photobiont), composed of organic compounds (e.g., carbohydrates, proteins, lipids, lichen acids).

Quick Check

Color: Variable sandstone color (e.g., tan, red, grey) with superimposed patches of green, grey, yellow, orange, or black from lichens.
Luster: Dull to earthy for sandstone, matte for lichens.
Streak: Sandstone streak is typically white or the color of its cementing agent (e.g., reddish-brown for hematite cement). Lichens do not produce a streak.

Physical Characteristics

Crystal Habit: Sandstone: Detrital grains, typically rounded to sub-angular. Lichens: Crustose, foliose, or fruticose thalli.
Cleavage Type: Sandstone: None. Lichens: Not applicable.
Fracture Type: Sandstone: Granular, conchoidal (for quartz grains). Lichens: Not applicable.
Tenacity: Sandstone: Brittle. Lichens: Flexible to brittle depending on species and moisture content.
Luster Type: Sandstone: Earthy to dull (vitreous for quartz). Lichens: Matte.

Formation

Sandstone forms from the lithification of sand-sized grains, primarily quartz, through compaction and cementation. Lichens are symbiotic organisms composed of a fungus (mycobiont) and an alga or cyanobacterium (photobiont). They colonize exposed rock surfaces, deriving nutrients from the atmosphere and the rock itself, and are highly resistant to environmental extremes. Their growth is a biological process occurring on pre-existing sandstone.

Usage

The sandstone itself is used as a building material, for paving, and as an aquifer. Lichen growth on sandstone is generally not a 'use' but rather a natural phenomenon. In some contexts, lichens are used in biomonitoring of air quality, and their presence on rock can contribute to aesthetic value in natural landscapes or historical structures. However, extensive lichen growth can also contribute to biodeterioration of stone.

Age Distribution

Sandstone can range from Precambrian to Cenozoic. Lichen growth can occur on any exposed sandstone surface, typically forming over decades to centuries.

Where to Find

Global Distribution

Sandstone is one of the most common sedimentary rocks and is found worldwide in diverse geological settings. Lichens are also globally distributed, colonizing exposed rock surfaces in virtually all terrestrial biomes where sandstone outcrops occur.

Specific Examples

Notable sandstone formations like the Navajo Sandstone (USA), Uluru (Australia), Petra (Jordan), and various formations in the Colorado Plateau (USA) frequently exhibit extensive lichen growth on their exposed surfaces.

Finding Tips

Observe Exposed Rock Surfaces

Look for sandstone outcrops, boulders, and cliffs in natural environments. Lichens prefer stable, undisturbed surfaces.

Check for Color Patches

The presence of distinct patches of green, grey, yellow, orange, or black on the rock surface, which are not mineral stains, often indicates lichen growth.

Examine Texture

Feel the surface. Lichens will add a different texture (crusty, papery, or slightly spongy) compared to the bare gritty sandstone.

Consider Environmental Conditions

Lichens thrive in areas with good air quality and sufficient moisture (rain, fog, dew), but can also be found in arid regions adapted to extreme conditions.

Similar Rocks

Shale with Lichen

Shale with various lichen species

Also known as: Lichen-covered shale

Granite with Lichen

Granite with various lichen species

Also known as: Lichen-covered granite

Limestone with Lichen

Limestone with various lichen species

Also known as: Lichen-covered limestone

Scientific Classification

Mineral Class: Sandstone: Sedimentary rock. Lichens: Fungi (kingdom Fungi), Algae (kingdom Protista) or Cyanobacteria (kingdom Bacteria).
Group: Sandstone: Clastic sedimentary rock. Lichens: Symbiotic organisms.
Crystal System: Sandstone: Not applicable (rock). Quartz grains are trigonal. Lichens: Not applicable.
Chemical Formula: Sandstone: Primarily SiO2 (for quartz), plus other minerals. Lichens: Complex organic molecules.
Composition: Sandstone: Quartz, feldspar, rock fragments, various cements. Lichens: Fungal hyphae, algal/cyanobacterial cells, water, organic compounds.

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