Pillow lavas consist of masses of interconnected ellipsoidal lava masses, and represent effusive subaqueous or submarine eruptions, most commonly of basaltic magma. They are also characteristic of the initial phases of subglacial basaltic eruption, principally occuring in ponded subglacial water (Walker and Blake, 1966; Jones, 1968; etc.). Pillow units in tuyas and hyaloclastite ridges are similar to pillows formed in other subaqueous or submarine settings.
Columnar joints are common in lava flows of all compositions, and consisting of parallel columns with polygonal cross-sections, centimeters to more than a meter across, formed by cooling and contraction. Column size generally decreases as cooling rate increases, thus, in rapidly cooled flows (e.g. against ice), column size may be extremely small (less than 10 cm across). At glaciovolcanic edifices, columnar-jointed flows may form subaerially or subglacially. Orientation of columns is always perpendicular to the cooling surface, and thus, if a lava flow is chilled against the edge of an ice sheet (a vertical surface), the columns will be oriented horizontally.
|Small columns in andesite (less than 5 cm across) at Ember Ridge, Mount Cayley volcanic field, British Columbia.(Click on the image to view it full-size.)|
Hyaloclastite is a fragmental, medium to fine-grained rock, consisting of glass clasts (which may or may not be vesicular) which are commonly less than 1 mm to several cm in size. Sideromelane (basaltic glass) within hyaloclastite commonly interacts with surrounding water to form palagonite, an amorphous yellow glass. The hard cementation produced by palagonitization enhances the preservation of hyaloclastite deposits. Deposits may be massive to bedded, and hyaloclastite may form the matrix surrounding pillow lavas or pillow breccias. Bedded hyaloclastite units may contain large-scale features such as cross-beds and channel-like structures (Bergh and Sigvaldason, 1991; Hickson et al., 1995).
Hyaloclastite forms by quench fragmentation or through phreatomagmatic eruption (explosive magma-water interaction). Hyaloclastites formed by phreatomagmatic activity may, in some cases, be distinguished from those formed by quench fragmentation by the presence of accretionary lapilli or bomb sags.
|Photomicrograph of basaltic hyaloclastite. (Click on the image to view it full-size.)|
|Hyaloclastite from Hoodoo Mountain, northwest British Columbia.(Click on the image to view it full-size.)|