Pyroclastic Deposits

© Copyright, 2001 by R.A.Kanen, All Rights Reserved

Pyroclastic rocks are the products of volcanic explosions; that is, they are fragmental pieces of rock, whether they be minerals, crystals or glass, ejected from the vent. Characteristically there are more pyroclastics associated with acid magmas than basic. Acid magmas are more viscous, hence they are reluctant to release gas and this results in high explosiveness. Pyroclastics form by the expansion of gas contained in the parent magma. This may occur when the rising magma comes into contact with ground water. Large amounts of ash from the country rock are associated with eruption when this happens. Pyroclastics may also form when a lava flows into the sea, or even a lake. On such occasions, the products are usually crossbedded breccias that have a foreset dip of approximately 25 degrees. Submarine eruptions also produce pyroclastics, however, the hydrostatic pressure at extreme depths prevents explosions, limiting the production of pyroclastics to shallow depths. Rocks formed from this type of eruption include pumice and hyaloclastites, which is the brecciated product. In general, pyroclastics have been divided into two types: (1) pyroclastic fall deposits and (2) pyroclastic flow deposits.

    Pyroclastic fall deposits are those which have traveled through the air as some kind of projectile during a volcanic eruption. All ejecta which have traveled through the air are collectively referred to as tephra. Tephra is the main product of many volcanoes, therefore, it is important to be able to recognize the different types of tephra deposits. Tephra is classified according to size and in some instances, shape. A tabulated classification is shown below.

> 32 mm blocks, bombs
>4 mm > 0.32 mm lapilli, pumice, scoria, etc.
<4 mm > 0,25 mm ash
<0.025 mm fine ash, dust

    Bombs are large fragments of rock that form in a characteristic tear drop shape with a twisted tail. They usually contain a central core which is somewhat vesicular. It may be composed of basalt, peridotite, country rock or, rarely, aggregations of crystals, such as olivine. This is covered by an adhering layer of lava which may be glassy in appearance. Bombs may obtain their shape by twisting through the air, but it seems most is derived from the hurling of lithic fragments through molten lava in the throat of the volcano. Dana has suggested that some bombs could have been formed by the rolling movement of the stream front. He names these lava balls. Bread crust bombs form by the contraction of the exterior skin and expansion of the interior; thus forming a cracked surface, exposing the interior. Most bombs vary up to the size of a football (Ollier, 1969) although some very large ones are known. Some have been said to weigh up to 65 tons.

    A fragment of rock that is the same size as a bomb, but angular in shape is termed a block. Blocks are made from preexisting rock which probably formed from a previous eruption. Most commonly, they are formed when an explosion tears apart the vent of the volcano, sending angular rock fragments flying through the air. Blocks are definitely not made from molten magma.

    Obviously, the large mass of many volcanic bombs and blocks will restrict the distance they are ejected from the vent. Commonly they form heaps of variously sized material adjacent or close to the vent. Much of this material has rolled down the sides of the volcano. These deposits are chaotically arranged with no order to them and are referred to as volcanic agglomerates. They contain lapilli as well as bombs and blocks, and commonly have a dust sized matrix which is post depositional in origin. Their major feature is very poor sorting. This is due to their closeness to the vent. With increasing distance from the vent sorting will be better and average grain size will be smaller, since heavy fragments will not travel as far through the air as smaller fragments will. At some distance from the cone there is sometimes an area which is well sorted but not too fine and suffers a lack of fine ash, meaning it has a large amount of voidspace. The voidspace commonly holds groundwater and this can affect erosion of the area. Agglomerates are sometimes found filling the necks of craters or conduits. These have been observed at the Firth of Fourth (Beckie, 1879). Sometimes agglomerates are found to be well rounded. These are sometimes referred to as volcanic conglomerates and may indicate the presence of water around the volcano. These have also been found associated with non volcanic rocks, such as sedimentary and metamorphic rocks. This may indicate that they were formed from one of the first eruptions in the area. When volcanic activity begins in a new area it begins with an explosion of the crust of the Earth, which may be granite, schist, sandstone, limestone, etc. In areas where volcanic activity stopped after the first eruption all that may be present is a mixture of volcanic and non-volcanic rocks. If volcanism continued, successive layers of volcanic material, representing each eruptive phase, would be piled upon the initial non-volcanic fragments. This has been observed at Haystack Mountain, Montana (Iddings, 1909) where the basal andesitic breccia contains massive amounts of angular gneiss.

    Lapilli are commonly associated with bombs and blocks in agglomerates, as well as tuff. Basaltic lapilli which is dark and vesicular are also called scoria or cinders. Acid volcanoes produce highly vesicular lapilli size fragments called pumice. Pumice has such a low density it can often float on water. If the gas bubbles continue to expand it can shatter lapilli into pieces of vesicle walls. These are seen as curved splinters and are known as shards (microscopic). They are common in fine ash and tuff. Lapilli are frequently present in graded beds consisting of bombs and lapilli at the base and ash on the top. These occur due to the falling out of the bombs, followed by lapilli and finally ash, from the air. If strong winds were present at the time of deposition, aeolian crossbedding may occur. Ollier suggests that since the cross bedding is randomly located around some volcanoes it may be due to the blast of the volcanic eruption. At Tower Hill, in Victoria, it has been determined that cross bedding is due to south westerly winds (Marshall, 1967).

    Much of the solid material of the cone and the dust which has formed is heated so intensely that it forms ash. Together with dust, ash is the last material to settle after an eruption, sometimes being carried hundreds of kilometers before it's deposited. A good example of this is the recent Mount Saint Helens eruption in which a blanket of ash, up to several inches thick, was deposited as far away as Portland. The characteristic feature of ash; in fact bombs, blocks, and lapilli too, is to follow the lay of the topography, covering hill tops as well as valleys. This is called mantle bedding. Consolidated ash or tuff exhibits this feature. Tuff may be further classified as lithic tuff, if it contains many rock fragments; vitric tuff if most of the fragments are glass; and crystal tuff if well formed crystals are dominant. A general term, tuff breccia or ash fall tuff, is used to describe all of the varieties. Tuff breccia is sometimes bedded in layers representing eruption phases. In some places, such ad the Cascade Mountains and the Andes, the thickness of the breccia is 4000 ft and more (Iddings, 1883 & Reiss, 1892). Each bed varies in thickness up to three or four feet. They are usually horizontal or with a dip of about 5 degrees. Some tuffs, especially crystal tuffs, are useful in stratigraphic correlation. Determination of the refractive indices of the phenocrysts and glass in deposits with a known stratigraphic position and comparing them with refractive indices of tuffs from other areas enables correlation. Chemical and mineralogical comparisons may also be done to a limited extent where post depositional processes have not affected the deposits. Where ash overlies dateable materials, such as carbon, accurate ages can be assigned to the deposits. Other information that can be obtained includes: (1) rates of infilling of depositional basins, (2) rate of alluvial fan building; for example, where ash is buried by alluvium, (3) erosion studies; for instance, where there has been no erosion of ash deposited on hillsides, (4) study of sea level changes; for example; at Gisborne, N.S.W. tephra deposits are mantle bedded over old dunes and beach ridges, indicating the paleo-shoreline and role of progradation of the coast, (5) terrace correlation and chronology, (6) archaeology, (7) tectonics. Many other uses of pyroclastic deposits are possible.

    The explosion of gas within molten lava can result in the disintegration of the lava into minute pieces of glass and angular fragments collectively called dust. Dust may also form from the massive pulverization of solid material during the explosion. The minuteness of dust particles and the velocity which they are ejected can result in massive dust clouds, finally being deposited hundreds of miles away from the source. The most striking example of this is the eruption of Krakatoa in which dust was thrown seventeen miles into the atmosphere and circled the Earth for months before it finally settled, completely disseminated.

    The second type of pyroclastic deposits are called pyroclastic flow deposits. The majority of these form when hot fragmenting material made buoyant by hot gas begins to flow as a fluid. This process is called fluidization. It occurs when the hot gasses accompanying the ejecta, together with trapped air and gas being released by the ejecta as it vesiculates, forms an air cushion around each particle preventing it from coming into contact with adjacent particles. Thus, the whole mass behaves like a fluid with low viscosity, enabling it to travel great distances down slopes and, in many instances, up relatively steep slopes. Such clouds of gas can be initiated in several ways. The cloud may just spill over the lip of the volcano and flow downslope as a density current; or an eruption cloud may collapse due to a lack of momentum and heavy load of pyroclastics, forming a pyroclastic flow; or material may pile up on the upper slopes of the volcano and collapse, forming a flow.

    There are several types of fluidization flows. General terms used to describe them include ash flows, pumice flows and nue'e ardente or glowing cloud. The term nue'e ardente was derived from the Mt. Pele'e eruption in 1902 which has since become known as the classic example of a pyroclastic flow. Pyroclastic flows have been classified into dense, intermediate, and vesicular flows; and ash flow to vesicular types. A more detailed classification is shown in the table below:

Pyroclastic Flow Classification
Dense Nue'e Ardente
  1. Pelee' type (flow from side of dome)
  2. Merapi type (flow from collapsing dome)
  3. Lakurajiima type (flow from open crater)
0.001 - 0.3 Km3
Intermediate Intermediate 0.05 - 1 Km3
Vesicular Ash Flow
  1. St. Vincent (Vert. eruption from crater)
  2. Krakotoa (as St. Vincent but > magnitude)
  3. Valley of Ten Thousand Smokes (magma discharge through fissures)
0.1 - 90 Km3

    The term used to describe the type of deposit formed this type of pyroclastic flow is ignimbrite. Terms such as ash flow deposit, welded tuff, tuff flow deposit and nue'e ardente deposit have also been used. Ignimbrite is a general term and it is specifically used to describe the rock unit. Silicic ignimbrites are most abundant, although ryholite to basaltic ignimbrites also occur. The length of the deposit and area of deposition depends upon the topography. Ignimbrites are usually deposited in low lying areas, such as valleys, since the density of the flow does not permit it to stop on high ground. This is in contrast to pyroclastic fall deposits, which are mantle bedded.

    Ignimbrite deposits generally consist of three layers. The basal layer, or sillar layer, generally has an absence of large fragments which are probably expelled by frictional forces. It is of a finer grain size with small glass and pumice fragments and is generally a white or gray color. The middle layer is often welded and is very poorly sorted. It often shows a reverse grading of pumice blocks and normal grading of large lithic fragments. It varies in color from a brown to deep black. The middle layer merges into the top layer which consists of unwelded ash tuff.

    Another type of deposit which is not strictly a flow deposit but which is commonly associated with and mistaken for one is a surge deposit. Base surge deposits, also called Maar deposits, are due to the horizontal blast from an explosion transporting material. They are usually found on flat ground, are low density deposits and form similar structures to those found in a river bed (thin bedded with localized dome shaped crossbeds). Ground surge deposits are similar to base surge deposits in origin. They mainly form from central crater type volcanoes where gravity forces as well as horizontal blast forces assist their flow. They are less than 10m thick and can be composed of juvenile magma, lithic fragments, and crystals in any combination or proportion.

    Lahars, or mudflows of volcanic material, are very common in high rainfall areas. They form from fresh ejecta, which is still hot, mixed with water or cold ejecta which is mixed with rain water or surface water. They are very mobile, poorly bedded, with occasional layers of crossbeds and have poor sorting. They characteristically form hillocks, ranging in size from a few meters to tens of meters, with a core of boulders. They sometimes have fine basal regions and can be indistinguishable from ignimbrite deposits. Generally, flows with greater than 10% of water content are called mud flows and flows with less than 10% are called debris flows. Debris flows consist of highly concentrated, high density material.

    Avalanche deposits can also resemble ignimbrites. They usually form from very viscous, acidic lavas which are almost solid when ejected. They crumble, break up, and collapse downslope (hot rock avalanche if they are vesicular). They resemble ignimbrites and are best distinguished by their overall geometry.

    Two types of pyroclastic deposits are recognized, pyroclastic flow deposits and pyroclastic fall deposits. Pyroclastic fall deposits usually have good sorting and cover vast areas. Pyroclastic flow deposits are commonly poorly sorted and sometimes welded. It is important for the geologist to be able to recognize these deposits because of their economic significance, not only in terms of mineralization but also in environmental interpretation.


References

Fielder, G & Wilson, L "Volcanoes of the Earth, Moon and Mars: Pg 49-56, 1975

Iddings "Igneous Rocks" 1909

Holmes "Principles of Physical Geology" Ed. 3

Ollier, C "Volcanoes" Vol. 6 1969

Strahler, A, N "Principles of Physical Geology" 1977