Page 39 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

volcaniclastic rocks. Correct interpretation depends on: lithofacies geometry and contacts; (3) study of thin-
(1) a search for features such as flow folds, relict sections in order to distinguish formerly vitriclastic
phenocrysts, spherulites or lithophysae (or altered from devitrified or recrystallized, coherent textures.
equivalents), and columnar jointing; (2) mapping the

Fig. 16 Folded flow foliations in a rhyolite lava that indicate the local sense of relative movement. Detached
synforms closely resemble detached antiforms. In synforms, the detachment surface is at the top of the fold. In
antiforms, the detachment surfaces is at the base. ds: trace of detachment surface. Modified from Christiansen
and Lipman (1966).

Joints (9) be columnar jointed, especially the densely welded and
vapour-phase crystallized zones (26.1-2). In some cases,
Contraction that accompanies cooling of hot volcanic columnar joints in ignimbrite are rectangular rather than
deposits and high-level intrusions produces a variety of hexagonal in cross-section.
more or less regular joints: columnar joints, radial
columnar joints, concentric joints, tortoise shell joints, The pattern of jointing in columnar-jointed sheets can
"tiny normal joints" and quench fractures. Joints and be organized into two or three distinct zones (Waters,
fractures related to cooling are very conspicuous 1960; Spry, 1962) (Fig. 17; 9.5). Regular, well-
features of lavas, especially those emplaced under developed, wide columns in the basal zone or lower
water, and they strongly influence the shapes of clasts in colonnade are oriented perpendicular to the flow base.
associated autoclastic deposits. The overlying entablature consists of thinner, less
regular columns in complex arrangements. An upper
Columnar joints divide the affected rock into elongate zone of regular columns (upper colonnade) oriented
prismatic units or columns (9.1-4, 13.7, 19.2, 22.2, 26.1- normal to the top cooling surface may overlie the
2). Joints that define the sides of the columns are entablature. No significant compositional variations
continuous and dominant over joints that cut across the occur between the various zones (Spry, 1962). The
prisms (Spry, 1962). Columns typically have hexagonal boundaries of the entablature are typically very distinct,
shapes in cross-section, although 3-, 4-, 5- and 7-sided and could easily be mistaken for flow unit contacts.
columns are relatively common. Column diameters Although the colonnade-entablature pattern is most
range from a few centimeters to several metres. The commonly associated with basaltic lavas, it also occurs
surfaces of some columns are striated by alternating in some peralkaline silicic sheets (comendite ─
smooth and rough zones, each a few centimeters wide Schmincke, 1974).
and oriented normal to the column axis (chisel marks ─
Spry, 1962; Ryan and Sammis, 1978). Grain size and Cooling of magma results in increases in the viscosity
composition of the centers and margins of separate and in contraction. When thermally induced stresses
columns are uniform, or else show only very minor exceed the tensile strength, intersecting contraction
changes (Spry, 1962; Macdonald, 1968). fractures form at right angles to surfaces of equal tensile
stress (Spry, 1962). The fractures migrate inward from
Columnar jointing occurs in coherent igneous bodies the cooling surface, most probably by incremental
(lava flows, lava lakes, domes, sills, dykes), in both fracturing events which are recorded by complementary
subaerial and subaqueous settings, and in a wide range subhorizontal joints or by striated zones on the fracture
of magma compositions. Parts of ignimbrite sheets can surface (Ryan and Sammis, 1978). In magma bodies

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