Basaltic magmas have erupted throughout most of Earth's history, in oceans and continents and every tectonic environment. The main areas are:
- Mid-oceanic ridges where they make most of the oceanic crust.
- On oceanic islands, where they build up large shields.
- At orogenic (mountain-forming) continental margins and island arcs, related to subduction zones (where one tectonic plate slides under another).
- On stable continental areas.
All fragmental volcanic rocks can be described as volcaniclastic. Blocks are angular fragments of solid rock whereas bombs are fragments flung from volcanic vents as liquid blobs of magma that were streamlined as they solidified during their flight.
In addition to size, fragments also vary in composition. They include:
- vitric (glass)
- lithic (rock)
- crystal (crystal fragments)
Ash-sized fragments are usually vitric or crystal. Tuffs are volcaniclastic rocks made of ash-sized fragments. Those from ash falls are often well layered and look like fine-grained sedimentary rocks.
The terms for rock composition and size can be combined in the rock name. Common examples are lithic lapilli tuff, crystal tuff and dacitic vitric tuff. Agglomerate is a term for an accumulation of unsorted deposits of bombs near the volcanic vent.
The term pyroclastic includes fragmental volcanic rocks produced during explosive volcanic eruptions. Pyroclastic rocks can be classified by their mode of formation into three main groups: ash fall deposits, ash flow deposits and surge deposits
Ash fall deposits
These form when volcanic material is explosively ejected from the vent up into an eruption column. There are five main types based on degree of fragmentation (i.e. size of ash particles):
Hawaiian-type eruptions have low eruption columns (typically fire fountains) so the deposits have large particles dispersed over a small area. In contrast, Ultraplinian eruptions have towering eruption columns (commonly over 25 km high) and deposits consist largely of small particles dispersed over an extensive area. Ash fall deposits usually drape over the landscape with a uniform thickness. They are generally coarser-grained near the source vent and finer-grained away from the vent, and can deposit both on land or in water. Accretionary lapilli are small round masses (2 mm - 10 mm across) made of concentrically-layered ash or fine lithic particles, condensed by moisture in the air. They usually fall within a few kilometres of the vent.
Ash flow deposits
Pyroclastic flows move under gravity and hug the ground as hot, high concentrations of gas and solid material. Large pumice-rich examples are called ignimbrites. Ash flows are formed in two main ways:
- Lava dome/flow collapse: This occurs on steep-sided, andesitic volcanic cones, when an unstable actively-growing lava dome or flow collapses under gravity or through explosion. The flows rush down the flank of the volcano.
- Eruption column collapse: When an erupting column of ash becomes too dense to support itself, gravitational collapse back to the ground occurs, generating a pyroclastic flow. All ash flow deposits follow the topography and fill valleys and depressions. The deposits are generally poorly sorted, with some lacking any layering.
Pyroclastic surge deposits form by sideways blasts of expanded, turbulent, low concentration mixtures of gas and solids. Formation occurs:
- during phreatomagmatic and phreatic eruptions : Here a base surge forms a collar-like, low cloud expanding radially in all directions from the centre of explosion. They result from the explosive interaction of hot magma or gas with water.
- associated with flows: Thin, stratified pumice and ash deposits are often found associated with pyroclastic flows. They can be found at either the base of the ash flow unit (ground surges) or at the top (ash cloud surges).
- associated with falls : These are similar to ground surges but no pyroclastic flow is generated. Surge deposits run over the landscape and accumulate more thickly in depressions.