Igneous Structures

Vesicular

Formerly gas bubbles in a melt.

Scoriaceous

Highly vesicular and cindery.

Pumiceous

Frothy vesicular structure characteristic of pumice.

Vesicular Pipes

Cylindrical bodies highly charged with vesicles.

Pipe Vesicles

Tube-like elongated vesicles.

Diktytaxitic

Texture of certain basalts containing abundant angular interstitial gas cavities.

Vugs

Vesicles or cavities lined with crystals. Sometimes called a drusy.

Miarolitic Cavities

Small gas cavities present in certain granite.

Amygdales

Vesicles filled entirely with secondary minerals such as quartz, calcite, or zeolites.

Directionless

(Massive) structurally homogenous, lacking parallelism, preferred orientation, or flow structure of the constituent minerals.

Flow

Parallel structure developed as a result of magmatic flow.

Flow flotation

Parallel alignment of planar minerals due to flow.

Layering

Planar parallelism of compositional zones.

Lineation

Parallel alignment of linear minerals due to flow

Schlieren

Flow structure marked by steaky layers of mafic composition.

Xenolith

Inclusion of country rock or other material genetically unrelated to the surrounding magma.

Enclave

Inclusion of rock of unknown relationship to the igneous rock or country rock.

Pillow

Structure developed in the subaqueous eruption of basalt, consisting of heaps of pillow-like blobs of lava.

Block lava (aa)

Rough blocky structure formed on the surface of certain lava flows by fragmentation of the congealed crust during certain flow.

Ropy lava (Pahoehoe)

Smooth, glazed, and ropy surface structure resembles cow dung in form.

Joints

Fractures along which no visible displacements have occurred, that form during or after the cooling of a magmatic body.

Sheeting

Type of jointing produced by pressure release or exfoliation that forms large tabular blocks or sheets roughly parallel to the Earth’s surface, especially in granitoids.

Columnar Jointing

Long, polygonal prisms with three to eight sides, usually oriented with their long axes parallel to the direction of greatest heat flow; reflect volume decrease in igneous rocks during cooling.

Perlitic

Concentric fracture pattern resulting from contraction on cooling of certain volcanic glasses.

Protoclastic

Deformation and granulation of early-formed minerals due to movement during the final emplacement of magma in the late stages of consolidation.

Hyaloclastite

Fragmental rock derived from rapid chilling and brecciation of volcanic glass. Glassy fragments may be extensively altered.

Pyroclastic

Fragmented products of volcanoes (ash, pumice, agglomerate).