Vesicular Basalt with Sandstone Inclusion

Igneous (extrusive) with Sedimentary Inclusion

Basalt with sedimentary inclusion

Also known as: Basalt with sedimentary xenolith, Basalt with clastic inclusion

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Description

Vesicular basalt with sandstone inclusion is a composite rock characterized by a dark, fine-grained, often porous (vesicular) basaltic matrix containing distinct, lighter-colored fragments of sandstone. The vesicles in the basalt are typically spherical to ovoid, ranging from sub-millimeter to several centimeters in size, and represent gas bubbles trapped during lava solidification. The sandstone inclusions vary in size, shape, and degree of alteration, often showing signs of thermal metamorphism (e.g., reddening, hardening) at their contact with the basalt.

How to Identify

Color: Basalt matrix is typically dark gray to black; sandstone inclusion is usually light-colored (tan, buff, reddish, white, or gray), contrasting sharply with the matrix.
Luster: Basalt matrix is dull to earthy; sandstone inclusion can be dull to sugary.
Texture: Basalt matrix is aphanitic (fine-grained) with abundant vesicles (porous). Sandstone inclusion is clastic, granular, and typically medium-grained, often showing individual sand grains.
Crystal Form: Basalt minerals are too fine-grained to show distinct crystal forms without magnification. Sandstone grains are typically sub-angular to rounded.
Cleavage: Basalt minerals (e.g., pyroxene, plagioclase) have cleavage, but it's rarely visible in hand samples due to fine grain size. Sandstone grains do not exhibit cleavage, but the rock may fracture along grain boundaries.
Geological Environment: Volcanic settings where basaltic lava flows or pyroclastic eruptions interact with pre-existing sedimentary rock layers (e.g., continental rifts, volcanic plateaus, subduction zones with overlying sedimentary basins).

Key Facts

Hardness: Basalt matrix: 5-6 (Mohs); Sandstone inclusion: 6-7 (quartz grains), but overall rock hardness can be lower due to porosity and cement.
Specific Gravity: Basalt matrix: 2.7-3.0 (less if highly vesicular); Sandstone inclusion: 2.6-2.7 (depending on porosity and cement).
Crystal System: Basalt: Monoclinic (pyroxene, plagioclase); Sandstone: Quartz is hexagonal, feldspars are monoclinic/triclinic.
Color: Dark gray to black (basalt) with light tan, white, or reddish (sandstone).
Luster: Dull to earthy.
Transparency: Opaque (basalt); Translucent to opaque (sandstone grains).
Fracture: Basalt: Conchoidal to irregular; Sandstone: Granular to irregular.
Cleavage: Not typically observed in hand samples for basalt; absent in sandstone grains.
Composition: Basalt: Predominantly plagioclase feldspar, pyroxene, olivine, and iron-titanium oxides. Sandstone: Primarily quartz, with varying amounts of feldspar, rock fragments, and cementing minerals (e.g., silica, calcite, iron oxides).

Quick Check

Color: Dark basalt matrix with light-colored sandstone inclusions.
Luster: Dull to earthy for both components.
Streak: Basalt: grayish-black; Sandstone: typically white or light-colored (depending on composition).

Physical Characteristics

Crystal Habit: Basalt: Microcrystalline to cryptocrystalline, often with phenocrysts of olivine or pyroxene. Sandstone: Detrital grains, typically sub-angular to rounded.
Cleavage Type: Basalt: Not visible in hand sample; minerals have good cleavage. Sandstone: None.
Fracture Type: Basalt: Conchoidal to irregular. Sandstone: Granular.
Tenacity: Brittle.
Luster Type: Dull to earthy.

Formation

This rock forms when molten basaltic lava erupts and incorporates pre-existing sandstone during its ascent or flow. The basalt cools rapidly, often trapping gas bubbles (forming vesicles) and solidifying around the sandstone fragment. The sandstone inclusion (xenolith) is a piece of country rock that was entrained by the magma but not fully assimilated.

Usage

Primarily of scientific interest for understanding magma-country rock interactions, eruption dynamics, and paleogeography. Not typically used for commercial purposes due to its heterogeneous nature.

Age Distribution

Variable, depending on the age of the basaltic eruption and the incorporated sandstone. Basalts can range from Precambrian to Cenozoic.

Where to Find

Columbia River Basalt Group, USA

Extensive flood basalts in the Pacific Northwest, where lava flows have incorporated underlying sedimentary units, including sandstones.

Deccan Traps, India

One of the largest volcanic provinces globally, where basaltic flows have interacted with various country rocks, potentially including sandstones.

Volcanic regions with sedimentary cover

Any active or ancient volcanic area where basaltic eruptions have occurred over or through sandstone formations, such as parts of Iceland, Hawaii, or rift valleys in Africa.

Finding Tips

Look for contrasting lithologies

Search for dark, fine-grained, vesicular igneous rocks that contain distinctly lighter-colored, granular fragments. The contrast in color and texture is key.

Examine outcrops near sedimentary-igneous contacts

These rocks are most likely found where basaltic lava flows or dikes have intruded into or flowed over sandstone layers.

Check for thermal alteration

The sandstone inclusion may show signs of heating, such as reddening, hardening, or a 'baked' appearance at its margins, indicating contact with hot lava.

Similar Rocks

Basalt with Shale Inclusion

Vesicular Basalt with Shale Xenolith

Also known as: Basalt with argillaceous xenolith

Basalt with Granite Inclusion

Vesicular Basalt with Granite Xenolith

Also known as: Basalt with granitic xenolith

Scoria

Also known as: Volcanic cinders

Scientific Classification

Mineral Class: Igneous rock (basalt) with Sedimentary rock (sandstone) inclusion.
Group: Volcanic rock (basalt); Clastic sedimentary rock (sandstone).
Crystal System: Basalt: Predominantly monoclinic (pyroxene, plagioclase). Sandstone: Hexagonal (quartz), monoclinic/triclinic (feldspars).
Chemical Formula: Basalt: Complex silicate mixture (e.g., (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 for pyroxene; NaAlSi3O8-CaAl2Si2O8 for plagioclase). Sandstone: SiO2 (quartz) with other silicates and carbonates.
Composition: Basalt: Mafic, low silica (45-52 wt% SiO2), high in Fe, Mg, Ca. Sandstone: Siliciclastic, dominated by quartz (SiO2).

Explore Vesicular Basalt with Sandstone Inclusion

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