We (my wife Chris and I) enjoyed our fourth visit to Big Island Hawaii in May 2013 so much that we decided to return to the same places in October 2014. We were hoping to see similar events and activities, which we had found particularly interesting and accessible over the years. Every time we visit, something changes or isn’t possible, but this time was a little more changeable than most. The intervention of three ladies altered a few of our plans – Iselle, the hurricane that visited the southeast of Big Island two months before we arrived; Madame Pele, the Hawaiian Goddess of the Volcano; and Ana, the hurricane that hit the area during our stay. The three interventions illustrate the simple fact that we and our little plans have to be adaptable and show that some of the great locations will be discussed in these articles and will be missed if you only make one visit.
This is the first of three articles on Big Island in Hawaii. In them, I will talk about the major highlights of our visit in connection with the volcanic activity of this wonderful island. This first part will mostly illustrate the different volcanic concepts that need to be understood to appreciate what can be seen, and will also provide a general background to the location and the significant summer 2014 flow towards Pahoa.
About lava
Traditionally, lava is described as pahoehoe or a’a. These are taken to mean ropey and blocky (or broken). However, in the Puna area, it is possible to see several distinct variations on these two forms (Figs. 1, 2 and 4 show maps of Hawaii).
In Hawaii, the lava is tholeiitic basalt, that is, it has a high iron and magnesium content, and contains relatively less silica and aluminium than other lavas. Sometimes referred to as mafic lava, it tends to be hot and can allow the polymerisation of some minerals within the matrix; that is, crystals of various constituent minerals can form as it begins to cool, or existing crystals, which form at depth, remain in crystal form. For example, olivine crystals are common in the a’a lavas in the west of Puna. Pyroxene is another mineral that can begin to crystallise at a fairly high temperature (around 950°C). It is found in the form of augite crystals and is a silicate of calcium, iron, aluminium, magnesium and titanium. Bubble cavities within the rock, generally formed by steam, can become part-filled with secondary minerals, such as chlorite, calcite and various zeolites. They usually show as small whitish masses in cavities or as white encrustations along the edges of cracks. They also often contain sulphurous minerals.
The surface morphology of lava is most affected by two factors – its temperature and the nature of the ground over which it travels, particularly the angle of slope. The fluidity of the lava from Kilauea is high, because it is hot and has a basic silica-rich content – so it runs a long way. However, the slopes are not so steep that it runs over the surface at extreme speeds – it can easily be out-walked on most slopes. And, because it is so runny, it spreads a long way and forms volcanoes with only slight slopes – that is, shield volcanoes. So, the shape of the volcano remains largely unchanged as a flattish gentle rise. A flow rate is quick at 15m an hour, as occurs some days in the current flow towards Pahoa. Of course, where the slope is steep, the lava will run much more quickly if there is a sufficiently large amount to keep it hot and not begin to crust over.
A brief look at the lava forms and types found in the Puna area might include the following, in an approximate order of smoothness. These and the sections describing them are neither definitive, nor technical – one merges into another, and sometimes back again. (Others that could be included, such as Lapilli, Pele’s hair, Pele’s tears, Limu o Pele sheets and reticulite foam are more commonly found in the Ka’u area.)
A smooth sheet. The first lava to be exuded by a volcano such as Kilauea is generally, but not always, hot (1,050°C) and quite runny. Because it is at its hottest, it is more fluid than any of the other forms. Where it flows onto a flat, near-horizontal surface, the lava may spread quite evenly and form a relatively smooth surface over quite a wide area. This is most likely to occur if the rate of lava emission is high, such that, any patches and lobes that begin to form are quickly re-melted and coalesced into a flat sheet of lava. The surface cools rapidly and turns black by 900°C and forms a crust as the temperature falls to 750°C. Between these temperatures, the surface is ‘visco-elastic’ and this is the temperature range in which most variations of surface formation take place. Below these temperatures, the lava becomes rigidly solid and will only re-melt if buried by a considerable amount of hotter, fresher lava. Often, such a fluid lava flow will grow and deepen by inflation rather than by being flooded-over: fresh molten lava beneath the surface can push the surface upwards for a time, and may or may not then burst out. It is strange to watch a newly-crusted lava lobe begin to rise and expand like a balloon, before eventually splitting at some point and giving rise to a fresh lobe or flow.
Disrupted surface lava. A relatively fresh smooth surface of a lava flow or flood can be subjected to a resurgence of molten lava below. If this doesn’t last long, the hardening crust begins to crack and disturb the surface, perhaps with minor break-outs of lava. It can also break up the original surface into rafts of crust, which are shifted about by the underlying hot lava. The smooth lava can also be disturbed if the molten lava below drains away and the crust sinks; cracking and splitting, and perhaps forming mounds of radiating slabs.
Columnar lava. Where a large amount of lava has flowed and formed a layer several metres thick, it cools relatively slowly. As it cools, it shrinks. The slower it cools, the more regular are the shapes and cracks created by the shrinkage. These can form hexagonal columns that penetrate the depth of the lava. In the area of Kilauea, such columns are not formed as beauthifully and perfectly as, for instance, at the Giant’s Causeway in Northern Ireland. Instead, because they form at a lesser depth and have cooled quicker, the columns are only roughly formed.
Lobate lava. Most commonly in Hawaii, the outflow is not over a horizontal surface, nor a steep one, but down a gentle slope. In this case, it forms lobes of lava, which begin to crust over, burst out under their internal pressure and begin a new lobe. These lobes overtake, solidify, slide alongside and ride over the top of each other. I think of this kind of lava as ‘lobate’. It is almost always the kind that features on television recordings of flows, such as the one that took the houses at Kalapana in 2010.
Finger lava. The small lobes sometimes take the form of long protrusions, creating ‘finger lava’, which can seem to squirm and writhe slowly as the nose keeps pushing forwards, sometimes rising above the general surface as though to look around. The imagination can play with some of these forms – taking on the appearance of a swan’s neck, a Hawaiian ‘aloha’ wave, or a person, for instance. They are sometimes referred to as ‘toes’ of lava.
Folded lava. As the lobate lava flows, perhaps in some quantity, and the surface begins to cool, and harden, so the surface is pushed from beneath by the still very hot lava. Depending on the thickness of the hardening crust and the pressure of moving lava below, one thing that frequently happens is that the surface creases up and presses together, much like a table-cloth might rise in folds when pushed across a smooth table surface. This is ‘folded’ lava and it seems to form when the smooth or lobate lava surface has cooled for a time, and then slowly recommences its movement. At Kalapana, there are numerous examples of this beautiful lava formation. The surface is often a pearly silver-grey and can be very attractive in the sunlight.
Ropey lava. This is the one that is most commonly called pahoehoe lava. If the surface is continually cooling slightly and hardening just a little, the underlying movement can cause it to press together: the front edge cools the most; the following lava surface is being pushed and catches it up very quickly, and all gang up behind the leading edge. This creates the ropey appearance that is so well known, and which forms some of the most wonderful and creative patterns in the natural world. They can become extremely complex, as the pressure of moving lava varies with tiny changes in slope, the quantity being emitted and heat – including the weather on the day (cold rain, for instance). Patches of this ropey lava can be re-worked, overlain, twisted over and piled high in complex masses. Often, a flow will alternate between lobate and ropey lavas – beginning to heap up in a long chain one moment, and then a burst out of the side will begin the lobate sequence all over again.
The word ropey (or ropy) actually has two origins in this context: the appearance is like rope and, technically, any substance that is of a viscous nature and tends to form ropes or filaments can be referred to as ropey before it has solidified into shape – in the same way that it might be called sticky or plastic. Beneath the surface, there may well be a great depth of tangled sheets of ropey pathways and lobate masses, piled one on top of another as one phase of an eruption overtakes another. These may be minutes apart, or a year, within the same overall eruption – or up to 31 years in the case of Puʻu ʻŌʻō flowing south and east to the sea. The varied layers can be seen in cracks all over the area – often very clearly marked by colour changes and with fresh seepages of lava squeezing between lower layers, showing that they are separate and not solidly fused together. Such intermediate squeezes are often signs that the lava surface has been pushed upwards – or inflated – by such later intrusions.
Cable lava. Depending on the exact temperature, rate of cooling, angle of slope and constituent minerals, the ropes of lava can become rolled and re-rolled over themselves by the moving mass beneath, creating lava surfaces like coils of great cables. This probably occurs if the lava has a considerable bulk, which is sufficient to keep the surface hot enough to remain pliable as it rolls and twists over a long enough period to produce such complicated formations. Some of these are very spectacular and awesome in their complexity.
Dribbles and spatter. Very vesicular lava can build up large bubbles below the surface, especially in vents, and these can burst upwards, sending a shower of lava across the local – or not so local – area. If they haven’t travelled far, they dribble and drip down anything they land on, forming spatter surfaces. Some definitions of lava may not include this as lava – as the material was thrown out of the volcano in explosive (sort of) eruptions, rather than being extruded, or oozed out, as lava should be. It seems to be a matter of where to draw the line: really explosive eruptions tend to produce material that isn’t always referred to as lava – ash, lapilli (pea-sized), tephra, scoria and cinder. However, these spatters and dribbles have the consistency of lava, look like lava and act like lava, so I include them here. Identical dribble formations can be formed when lava trickles over, for instance, a cliff – as at the abandoned and recent ‘ocean entry’ cliffs, west of Kalapana.
READ MORE...To view the rest of this article, you need A subscription. FROM JUST £2.95.
If you are already a subscriber, login here.
♦ Buy Fossils, Crystals, Tools
♦ Subscribe to Deposits
♦ Join Fossil Hunts
♦ UK Fossil Locations