Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Scoria with xenoliths is a dark-colored, highly vesicular, mafic volcanic rock (scoria) that contains inclusions of foreign rock material (xenoliths). The scoria matrix is typically reddish-brown to black, characterized by abundant, irregularly shaped vesicles (gas bubbles) that give it a low density. The xenoliths can vary widely in size, shape, and composition. They may appear as angular to sub-rounded fragments, often lighter in color than the scoria matrix, and can be composed of felsic igneous rocks (e.g., granite, rhyolite), metamorphic rocks (e.g., gneiss, schist, quartzite), or even other volcanic rocks. The xenoliths often show signs of thermal alteration or reaction with the host magma, such as discoloration, partial melting, or the formation of reaction rims.
How to Identify
- Color
- Scoria matrix is typically dark, ranging from dark reddish-brown to black. Xenoliths can be lighter (e.g., white, grey, pink for felsic fragments) or vary depending on their original composition.
- Luster
- Dull to earthy for the scoria matrix due to its vesicular nature and fine-grained texture. Xenoliths may exhibit vitreous, greasy, or dull luster depending on their mineralogy.
- Texture
- Highly vesicular (porous) and rough to the touch for the scoria matrix. Xenoliths will have a distinct texture from the host scoria, often crystalline (phaneritic or aphanitic) or foliated if metamorphic.
- Crystal Form
- Scoria itself is typically amorphous or microcrystalline, with phenocrysts (larger crystals) of olivine, pyroxene, or plagioclase occasionally present. Xenoliths will display crystal forms characteristic of their original rock type (e.g., interlocking grains in granite, aligned minerals in schist).
- Cleavage
- No distinct cleavage in the scoria matrix due to its glassy or microcrystalline nature. Xenoliths may exhibit cleavage planes if they contain minerals with good cleavage (e.g., feldspar, mica).
- Geological Environment
- Volcanic cones, lava flows, cinder fields, and explosive eruption deposits in active or recently active volcanic arcs, rift zones, and hot spots.
Key Facts
- Hardness: Variable, typically 5-6 on Mohs scale for the constituent minerals, but the overall rock is brittle due to porosity.
- Specific Gravity: Low, typically 1.0-2.0 g/cm³ due to high porosity (can float on water if extremely vesicular).
- Crystal System: Amorphous to microcrystalline for the scoria matrix; xenoliths will have crystal systems characteristic of their constituent minerals (e.g., monoclinic for pyroxene, triclinic for plagioclase).
- Color: Dark reddish-brown to black (scoria); xenoliths vary.
- Luster: Dull to earthy.
- Transparency: Opaque.
- Fracture: Conchoidal to irregular, very brittle.
- Cleavage: Absent in scoria matrix; present in some xenolith minerals.
- Composition: Scoria is primarily basaltic (mafic), composed of plagioclase feldspar, pyroxene, olivine, and volcanic glass. Xenoliths can be felsic (quartz, feldspar, mica), metamorphic (various silicates), or ultramafic (olivine, pyroxene).
Quick Check
- Color: Dark reddish-brown to black matrix with lighter, contrasting xenoliths.
- Luster: Dull to earthy.
- Streak: Brownish-black to grey.
Physical Characteristics
- Crystal Habit: Scoria: Aphanitic to glassy matrix with occasional phenocrysts. Xenoliths: Granular, interlocking, or foliated depending on origin.
- Cleavage Type: Not applicable for scoria matrix; varies for xenolith minerals.
- Fracture Type: Conchoidal to irregular, very brittle.
- Tenacity: Brittle.
- Luster Type: Dull to earthy.
Formation
Scoria forms during explosive volcanic eruptions when gas-rich basaltic magma is ejected. The rapid depressurization allows dissolved gases to exsolve, forming bubbles (vesicles) that are preserved as the lava cools quickly. Xenoliths are fragments of pre-existing country rock (either crustal or mantle material) that are entrained in the rising magma but are not fully assimilated before eruption and solidification. The presence of xenoliths indicates that the magma ascended through and incorporated material from the surrounding rock.
Usage
Scoria itself is used as lightweight aggregate in construction, for landscaping, and as a growing medium in horticulture. Scoria with xenoliths is primarily of scientific interest, providing insights into the composition of the Earth's crust and mantle, magma ascent processes, and eruption dynamics. It is also valued by collectors for its aesthetic and scientific uniqueness.
Age Distribution
Ranges from Cenozoic to recent, typically associated with active or recently active volcanic regions.
Where to Find
Hawaii, USA
Kilauea and Mauna Loa volcanoes, where basaltic eruptions are common and xenoliths of mantle peridotite or crustal gabbro can be found.
Cascades Volcanic Arc, USA
Volcanoes like Mount Shasta, Mount Hood, and Mount St. Helens, where eruptions can entrain fragments of underlying crustal rocks.
Canary Islands, Spain
Volcanic islands such as Tenerife and La Palma, known for basaltic volcanism and occurrences of various xenolith types.
Iceland
A highly active volcanic region where basaltic eruptions are frequent, potentially carrying xenoliths from the underlying mantle or crust.
Eifel Volcanic Field, Germany
Known for its maar-diatreme volcanoes and abundant mantle xenoliths in scoria and other volcanic rocks.
Finding Tips
Volcanic Regions
Focus your search in areas with recent or active basaltic volcanism, particularly near cinder cones, lava flows, and explosive eruption sites.
Exposed Outcrops
Look for road cuts, quarry exposures, or eroded slopes where volcanic rocks are well-exposed. Xenoliths are often more visible on weathered surfaces.
Examine Vesicular Rocks
Carefully inspect dark, highly porous volcanic rocks (scoria) for embedded fragments that differ in color, texture, or mineralogy from the host rock.
Safety Precautions
Always prioritize safety in volcanic areas. Be aware of unstable ground, sharp volcanic glass, and potential for active volcanic hazards. Wear appropriate protective gear, including sturdy boots and gloves.
Similar Rocks
Pumice
Pumice
Also known as: Volcanic Foam
Basalt
Basalt
Also known as: Volcanic Rock
Obsidian
Obsidian
Also known as: Volcanic Glass
Scientific Classification
- Mineral Class
- Igneous Rock (Extrusive)
- Group
- Volcanic Rocks
- Crystal System
- Amorphous to microcrystalline (scoria); variable for xenoliths.
- Chemical Formula
- Complex silicate mixture (basaltic for scoria; variable for xenoliths).
- Composition
- Mafic (scoria) with inclusions of various crustal or mantle rock types.
Explore Scoria with Xenoliths
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.