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Lava rock, or volcanic rock, is a general term for any igneous rock that forms from lava erupted from a volcano. It encompasses a wide range of compositions and textures, from dense, fine-grained basalts to vesicular (porous) scoria and pumice, and glassy obsidian. The appearance is highly variable, but common characteristics include a fine-grained or aphanitic texture, often with vesicles (gas bubbles) or phenocrysts (larger crystals) embedded in a finer matrix. Colors typically range from dark gray to black, but can also be reddish-brown, green, or even light-colored depending on the mineralogy and oxidation state.
How to Identify
- Color
- Highly variable, commonly dark gray to black (basaltic), reddish-brown, or lighter shades (andesitic, rhyolitic).
- Luster
- Dull to earthy in vesicular varieties; vitreous (glassy) in obsidian; sub-vitreous to dull in fine-grained types.
- Texture
- Aphanitic (fine-grained), porphyritic (larger crystals in fine matrix), vesicular (porous with gas bubbles), scoriaceous (highly vesicular), or glassy.
- Crystal Form
- Crystals are typically microscopic or absent due to rapid cooling. Phenocrysts (larger crystals) may be present, exhibiting euhedral to subhedral forms.
- Cleavage
- No distinct cleavage planes in the rock mass; individual mineral grains within the rock may exhibit cleavage.
- Geological Environment
- Volcanic flows, volcanic cones, shield volcanoes, stratovolcanoes, mid-ocean ridges, and subduction zones.
Key Facts
- Hardness: Variable, typically 5-7 on Mohs scale for constituent minerals, but the rock mass can be brittle or friable depending on texture (e.g., pumice is very soft).
- Specific Gravity: Highly variable: 0.5-1.0 for pumice (due to porosity), 1.5-2.5 for scoria, 2.7-3.3 for dense basalt.
- Crystal System: Not applicable to the rock as a whole; individual mineral crystals within the rock will have their own crystal systems (e.g., plagioclase is triclinic, pyroxene is monoclinic).
- Color: Dark gray, black, reddish-brown, green, or light-colored.
- Luster: Dull, earthy, sub-vitreous, or vitreous.
- Transparency: Opaque to translucent (thin sections).
- Fracture: Conchoidal (obsidian), irregular, or hackly.
- Cleavage: Absent in the rock mass; present in individual mineral grains.
- Composition: Silicate minerals, primarily feldspars (plagioclase), pyroxenes, olivine, amphiboles, and quartz, with varying amounts of volcanic glass. Composition ranges from mafic (basalt) to intermediate (andesite) to felsic (rhyolite).
Quick Check
- Color: Variable, often dark gray to black, reddish-brown, or lighter.
- Luster: Dull, earthy, or vitreous (glassy).
- Streak: White to light gray (for most common types like basalt), but often difficult to obtain due to hardness and fine grain size.
Physical Characteristics
- Crystal Habit: Aphanitic (microscopic crystals), porphyritic (macroscopic phenocrysts in a fine-grained matrix), or glassy (no crystals).
- Cleavage Type: Not applicable to the rock; individual minerals may exhibit cleavage.
- Fracture Type: Conchoidal (obsidian), irregular, or hackly.
- Tenacity: Brittle.
- Luster Type: Dull, earthy, sub-vitreous, or vitreous.
Formation
Formed from the rapid cooling and solidification of molten rock (lava) extruded onto the Earth's surface or ocean floor. The rapid cooling prevents the formation of large crystals, resulting in fine-grained or glassy textures. The specific mineralogical composition and texture depend on the original magma composition and the cooling conditions.
Usage
Construction aggregate, landscaping, decorative stone, barbecue briquettes, filtration media, lightweight concrete, insulation, abrasive, and as a growing medium in hydroponics.
Age Distribution
From Precambrian to present day, depending on volcanic activity.
Where to Find
Hawaii, USA
Extensive basaltic lava flows and volcanic cones from active shield volcanoes (e.g., Kilauea, Mauna Loa).
Iceland
Abundant basaltic lava fields and volcanic structures due to its location on the Mid-Atlantic Ridge.
Pacific Ring of Fire
Numerous volcanic arcs and active volcanoes producing a wide range of volcanic rocks (e.g., Japan, Indonesia, Andes Mountains).
Italy
Regions around active volcanoes like Mount Etna and Vesuvius, producing basaltic and tephritic lavas.
Cascades Range, USA
Andesitic and dacitic volcanic rocks from stratovolcanoes like Mount St. Helens, Mount Hood, and Mount Rainier.
Finding Tips
Look for Volcanic Regions
Lava rock is found in areas of past or present volcanic activity. Consult geological maps to identify volcanic provinces.
Examine Outcrops
Look for exposed rock faces, road cuts, or riverbeds in volcanic terrains. Freshly broken surfaces reveal true color and texture.
Observe Texture
The presence of vesicles (small holes from gas bubbles) is a strong indicator of volcanic origin. Glassy textures (obsidian) or very fine-grained textures are also characteristic.
Consider Density
Pumice is exceptionally lightweight and floats on water, while scoria is also light but typically sinks. Basalt is denser.
Safety Precautions
When collecting in volcanic areas, be aware of unstable ground, sharp edges of rocks, and potential for active volcanic hazards. Wear appropriate safety gear, including sturdy boots and gloves.
Similar Rocks
Gabbro
Gabbro
Also known as: Black Granite (misnomer)
Diorite
Diorite
Also known as: Salt and Pepper Rock
Granite
Granite
Also known as: Granitic Rock
Scientific Classification
- Mineral Class
- Igneous Rock (Extrusive)
- Group
- Volcanic Rocks
- Crystal System
- Not applicable to the rock; constituent minerals have various crystal systems.
- Chemical Formula
- Variable, depending on specific rock type (e.g., Basalt: predominantly (Mg,Fe)SiO3, CaAl2Si2O8; Rhyolite: predominantly SiO2, KAlSi3O8, NaAlSi3O8).
- Composition
- Primarily silicate minerals, including plagioclase feldspar, pyroxene, olivine, amphibole, quartz, and volcanic glass. The silica content varies significantly, defining the rock type (e.g., basalt <52% SiO2, andesite 52-63% SiO2, rhyolite >69% SiO2).
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