Art. LVII.—On Geyser-action at Rotorua.
[Read before the Auckland Institute, 22nd June, 1891.]
Being appointed engineer in charge of the Government thermal-springs district at Rotorua immediately after the eruption of Tarawera, in 1886, it became part of my duty to observe and report on any changes which might take place in the hot springs, geysers, &c. The eruption seemed to have had great influence over them. Many which had been quiescent, and some which had been considered as dead (having in the course of time become filled with rubbish and overgrown with weeds and brushwood), suddenly burst into renewed activity; and almost daily during the first six weeks after the eruption I could observe some changes in the thermal action—something new here and there throughout the district.
The geysers immediately attracted my attention. Waikite Geyser, at Whakarewarewa, which had been quiescent for about ten years, again burst into full activity, with eruptions about every quarter of an hour. Pohutu, Wairoa, and the other geysers were also playing occasionally, but were very irregular in their action. Sometimes weeks would elapse without one or the other of them showing any sign of activity, whilst at other times they would be active for several days in succession.
Not having had a long acquaintance with the district, I made inquiries of old residents (European and Maori) for any
theory to account for the inequality in the thermal action of these springs and geysers. The generally-received opinion was that the geysers were influenced by the wind: with southerly wind they were quiescent, and with northerly they were active. As I could not well understand how the wind could affect geysers or springs situated in sheltered positions, I began a system of personal observation, and soon found that southerly wind generally meant high barometer, and northerly a low one; and, if I could not understand the wind theory, I could the hydrostatic effect, and the influence of atmospheric pressure, which was simply equivalent to a reduction in the column of water. Every spring and geyser being naturally hydrostatically balanced, the reduction by any means of the weight of the column of water should bring a corresponding increase in the activity of the spring. Acting upon this theory, I determined to experiment privately upon Te Puia, a thermal spring in a secluded spot near an old pa, on the right-hand side and well down in the bed of the Puarenga River, therefore little influenced by winds. It was at that time boiling, but not very actively. By means of a drain which I cut in the sand formation by the level of the river I removed about 2ft. of the water from the pool which formed around the spring. This removal of 2ft. of dead-water had an immediate effect on the spring; it began to boil furiously, and a few minutes afterwards burst into a geyser, throwing water from 30ft. to 40ft. high, discharging at the same time the whole of the dead-water of the pool. I watched this eruption of what I thought a new geyser—for there was vegetable growth of several years' standing around it—with wonder, and with a certain amount of anxiety, as I began to fear that I had started something which I could not control. However, after a few minutes, taking advantage of a decrease in the eruptive force, I ran to the drain I had made, and refilled it as quickly as possible, causing part of the water thrown up by the geyser to be again caught in the pool or basin. Here it soon accumulated, and after a while the geyser-action ceased, and the water of the pool ran down the geyser's tube, together with a considerable quantity of water from the river which had flowed back through the partially-closed drain. In about ten minutes the tube was filled with cold water to the surface.
I watched it for a while, and saw the water getting hotter and hotter. Eventually it began to boil, but without any geyser-action. After a time I again opened the drain, and almost immediately there was another splendid eruption similar to the former. I determined to allow this to play, and see what it would do, as I began to have some confidence in my ability to control it by the same process as before if it was found necessary. It played for about twenty minutes, the
geyser - action getting weaker and weaker, and the cooled water in the pool getting stronger all the time. The water eventually got the best of it, and flowed down the geyser-tube, to repeat the same action as before.
Having made this geyser play and cease playing several times, I reclosed the drain thoroughly and went away. I did not see it play again that day; and the following day about noon, when I went near, I found by the marks I had left that it had not been in eruption since I left it the night before. The water of the pool would boil up violently at times, but there was no eruption. I then again tried what the opening of the drain would do. The result was the same as on the previous day—a splendid eruption of the geyser. I again watched the action for three successive times, and eventually went away leaving the drain open; and from the volume of steam which went up periodically from that spot I could see that intermittent geyser-action was taking place.
I repeated and watched these experiments on many occasions during the months of August and September, 1886. Once, the river being rather high, I turned the cold water from it on the geyser when in full eruption. This almost instantly stopped its action, but at the same time it caused a great noise, probably owing to the sudden condensation of steam within the geyser-tube. After a while, however, the noise ceased, the pool filled up, and all was quiet; and as long as I allowed the cold water to flow across over the mouth of the geyser-tube there was no eruption, nor even any perceptible action of the springs.
Having thus acquired some little practical knowledge of the working of this particular geyser, I began to compare it with that of others, to see if any of them could be made to play at will. I then watched and studied the action of Pohutu, which is situated on the principal geyser - fissure of Whakarewarewa. This fissure supplies no less than seven active geysers and blowholes, besides quite a number of old geyser-tubes which have been inactive for many years, though they still emit steam and make a rumbling noise as of boiling water at some considerable depth below the surface. They do not seem to affect or be affected by the working or the non-working of the active geysers.
The annexed plan (Pl. XL.) gives the position of the seven active geysers.
A. The “Torpedo” is situated in the bed of the Puarenga River, and is so named from the peculiar noise it makes in its explosions, which occur every few minutes, through the river-sand under 3ft or 4ft. of cold water. This commenced in September, 1888, at the same time as the renewed activity of Pohutu.
B. The “Kereru” is an old, well-established geyser, with intermittent irregular action, sometimes throwing up water 40ft. high. Its water deposits a black sinter formation, quite different in character from that of the others. I am therefore inclined to think that, though on the same line of fissure, it is not directly connected with them.
C. The “Indicator” is a small hole, 6in. by 1ft., on the common fissure. It generally plays intermittently for an hour or so before Pohutu, throwing a jet of steam and water, at an angle of 45°, to a height of 5ft. to 10ft.
D. “Pohutu” is a perpendicular oval tube or funnel, about 2ft. long by 18in. wide (formed in the fissure), worn quite smooth. It is an intermittent geyser—the greatest in Whakarewarewa, if not in the whole thermal district—throwing a splendid column of water to a height of from 50ft. to 80ft., and lasting from one to two hours at each eruption. Previous to September, 1888, it was very irregular in its action, sometimes inactive for several months.
E. “Te Horo.”—This is a splendid pool of blue water, about 15ft. diameter and 20ft. deep, with a kind of drain leading out of it, thus:
This pool is hydrostatically connected with Pohutu. It is usually full of water to the line a (section, Pl. XL.); but when Pohutu is about to play the water becomes hot and boiling, and, rising to the level of b, it overflows through the outlet. The action then becomes more intense and furious, large volumes of water being thrown up from 10ft. to 20ft. in height, and falling back into the pool in beautiful showers, the large drops sparkling like so many diamonds. It is a splendid sight, well worth seeing. Sometimes this ebullition goes on for an hour or so, when all at once the water in the tube of Pohutu rises to overflowing and the eruption of Pohutu takes place.
F. “Waikoroihi” is a small geyser-tube situated about 40ft. south of Te Horo, and a few feet higher up. It plays almost constantly, throwing water from 5ft. to 20ft. high.
G. “Wairoa” is a splendid geyser, and the last southwards in the fissure. When in eruption it somewhat resembles Pohutu; it is very irregular, being sometimes inactive for six months at a time.
Having noticed the great irregularity of action of the different geysers, I thought that it must, to a great extent, be
attributed to outside or surface influence. I noticed that when Pohutu was in eruption Waikoroihi would stop, and vice versá. This showed that they were hydrostatically connected, and, as long as Waikoroihi played, the water ejected, finding its way into the Blue Pool of Pohutu (E, Te Horo) at a considerably-reduced temperature (about 160° Fahr.), would so cool the water in the Blue Pool that it would not boil up whilst this lasted. As it did not cause the water to rise in that pool, I concluded that it might find its way back into Waikoroihi, and thus be thrown up again and again. I tested this by discolouring the water in the Blue Pool with loam, and found that, though the small bits of grass, moss, &c., were not re-ejected, the water from Waikoroihi was to some extent discoloured.
Taking the opportunity of a visit of the Hon. Mr. Mitchelson, the late Minister for Public Works, to our district, I explained my views on these matters to him, with an imaginary sketch of the geyser-tubes. Mr. Mitchelson took considerable interest in it, and authorised my making some experiments. At the beginning of September, 1888, I built a temporary wall, diverting the Waikoroihi water away from the Blue Pool. This soon had the effect of raising the temperature of the Blue Pool to from 200° to 210° Fahr. The water rose a few feet and began to boil furiously; then the pipe which I call the Indicator became active, and as soon as this took place the water of the Blue Pool would cease boiling and go down again to the low-water line. I watched this same action for several hours, but unless the water of the Blue Pool rose to the level of the overflow-drain there was no eruption of Pohutu. Seeing that this small Indicator tube acted as a kind of safety-valve, I tried to close it up with bags, stones, &c., but failed, the steam and water finding its way through small fissures in the rock. It then occurred to me to build a kind of dam around the Indicator, so as to collect the water ejected by it, and also lead some of the water from Waikoroihi into this dam, thus causing this cooled water to flow back down the Indicator tube. This had the desired effect. The Indicator stopped playing altogether as long as I could keep a small stream of cooled water running down it. On the following day Waikoroihi stopped playing. The water of the Blue Pool rose to the level of the overflow-drain, became more and more active, and on the 9th of September, two days after the works were finished, Pohutu gave a grand eruption, lasting nearly two hours, throwing large volumes of water from 60ft. to 80ft. high. This eruption was repeated in the evening, and from that date until December, 1889, it played regularly about twice in twenty-four hours.
During the time I was absent from the colony as Commissioner at the Paris Exhibition there was no one on the spot to look after these special works. The consequence was that Pohutu again stopped playing regularly. On my return, in February, 1890, I was informed that Pohutu had not played or been in active eruption for the last nine weeks. I at once went over to see it, and, finding that the works I had made had been tampered with, I had them put into temporary repair, with the result that Pohutu played up again a few hours after the work was finished; and its action has continued ever since, though not so regularly as before, but this is no doubt only due to the defective repairs of former walls, &c.
As a further illustration of what may be done in regulating the action of geysers, or even in creating or starting new ones, I may state that in the Sanatorium grounds there are two hot springs, with concrete basins around them, which were never known to have geyser-action, though the formation of the surrounding rocks shows that they had been geysers at some remote period. These springs supply the hot swimming-bath, but during the year 1889 they had gone so low, and were so much influenced by the atmospheric pressure, that sometimes they would remain for several days 2in. or 3in. below the level of the outflow-pipe, thus discharging no water. This became a matter of great importance, as the bath, which had cost £1,000, threatened to become useless, owing to the impossibility of keeping it at a proper and regular temperature.
It occurred to me that by contracting the springs proper into pipes it would prevent the hot water from becoming cold by admixture with the water in the basin, for I had noticed that when the springs were active the temperature of the water of the basins would rise from 140° minimum to 180° maximum. I thought that this increased activity of the springs when the water was hot was owing to the difference in the specific gravity between hot and cold water which the spring-tube or fissure might contain in its column, and that this difference might be sufficient to cause the water to rise a foot or two above present level, according to the depth at which this influence (on the temperature of the water) would take place. I had some temporary works carried out to prove the correctness of this theory, and to my delight found that it was quite true, and that, instead of a small rise of 2ft., which would have been quite sufficient for our purpose, there was force enough in the springs under these altered circumstances to form geysers. Having further acquired the knowledge that the whole of the springs contained in the Oruawhata and Chameleon basins were hydrostatically connected, I arranged a system of pipes over the three principal springs, connecting each of them by secondary pipes to three valves, by means of
which either of the springs can be made to play as a geyser at will. (See Pl. XLI.)
To keep the springs quiet, low, and cool during the time the works were being carried out, cold water from the town main was injected into one of the three spring-tubes, pumping it with an ejector out of another, whilst the work of cementing the geyser-tube was going on in the third; and by shifting the injector- and ejector-pipes from one spring-tube to another I had the three geyser-tubes firmly secured. These works were finished early in May, 1890, and the springs were thus kept quiescent for three weeks to allow the concrete to set properly, and eventually four days longer, so as to start them into action for the first time on the Queen's Birthday at 2 p.m. A considerable number of people gathered to see this novel experiment. The new fountains were christened the Malfroy's group of geysers, their distinctive names being the “Victoria,” the “Nelly,” and the “May.”
The three geyser-tubes, A, B, and C, on Plate XLI., consist of 6in. earthenware pipes sunk in the ground from 6ft. to 9ft. deep, right over the outlet of the springs, secured with cement concrete, and allowed to project about 2ft. above the water-level of the basin. a, b, and c, the three valve-pipes connected with A, B, and C, consist of 4in. earthenware pipes from 6ft. to 10ft. long, laid zig-zag, and thus forming double traps; they are provided with wooden valves to regulate the quantity of return water to be admitted into any of the geyser-tubes.
D is a supposed steam-chamber, showing probable connection between the geyser-tubes and the hot fissure or fissures which supply the heat or superheated steam and water to the springs.
When the three geyser-tubes are allowed to work without interference on the surface, C begins to get heated, the water rises in its tube and discharges through the small pipe c, the action and heat increasing steadily for a quarter of an hour or so; eventually the water boils furiously, and, as the water and steam come up the tube with considerable velocity, it takes the path of least resistance, and is shot up the tube and ejected into the air to a height of from 15ft. to 30ft.
This geyser-action lasts from five to ten minutes, dying away gradually until the cooled water gathered in the basin penetrates into the tube C from the valve-pipe c, which eventually stops the geyser-action, the water sinking rapidly down the tube. It is then quiescent for about ten minutes, while the heat from below seems to accumulate its force. The column of water in the geyser-tube again rises, begins to discharge, heats up, and repeats the geyser-action as before explained.
This intermittent action of C goes on for an hour or two, during which time the temperature of the water in tube B is
gradually augmented. The water now rises in this tube, discharges through pipe b, boils up, &c., as in the case of C, and eventually bursts into a beautiful geyser, throwing water 20ft. to 40ft. high for a period of one or two hours, when it again stops, as previously described in the case of C. Sometimes B and C will play up simultaneously for a short time. B, however, eventually takes the lead; C will then stop, the water in its tube receding 5ft. to 6ft., the cooled water which runs down the tube in considerable quantity being re-ejected by B. I have noticed that the eruption of B is of longer or shorter duration in proportion to the temperature of the water taken in at C.
While B and C are thus active a small stream of cooled water constantly finds its way down the tube A, keeping it silent; but it can be made to play by the following arrangement of valves: If A is required to play constantly, close the valve a and open b and c, so as to allow a considerable quantity of the cooled water ejected by B and C to find its way back down these tubes. If intermittently, allow rather more water to flow in the tubes B and C, and also a little in tube A: this will cause A to stop occasionally for a few seconds or for a minute; but, owing to the quantity of water being, say, half what finds its way down B or C, the heated water will still follow the tube A, as it is the one offering least resistance, and every now and then the accumulated or pent-up steam will eject the water. If, now, the tube A is required to be transformed into a mere boiling-spring, close the valve a, and open the valves b and c to allow as much of the cooled water to find its way down B and C as they will take. This has the effect of swamping the force; the superheated steam seems to be condensed by coming into contact with this large quantity of cooled water, and the action almost disappears.
If, on the contrary, B or C is required to play up separately and constantly, it is only necessary to reverse the working of the valves in a similar manner to what has been described to make A play up.
Further, the springs supplying the baths at the Palace Hotel having become quiescent some two or three years ago, I advised Mr. McRae, the proprietor, to do certain works to try and bring them into geyser-action: this proved entirely successful, and the spring became a small active geyser, and has remained so ever since, supplying his baths with plenty of hot water.
From the experience thus gained, I support the theory that the geyser-tubes are connected with subterranean caverns or chambers, and that heat or superheated steam penetrating through fissures supplies the natural or motive force; and I conclude that the difference between the specific gravity of
hot and cold water within the geyser-tube will thus produce every phenomenon of geyser-action to be observed at Rotorua; and I am led to believe that, by studying the action of geysers and springs in this district, they could in most cases and to a certain extent be regulated and controlled.
Geyser-action may be briefly explained according to the foregoing, as follows: Supposing that an even-sized tube full of water becomes so hot that steam generated at the bottom, under heavy pressure, rises through it without being condensed, there comes a time when several globules of this steam will be in the tube at the same time, and as they rise to the surface they will expand in proportion to the release of the pressure exerted upon them, and when coming near the surface they, as it were, explode, throwing the small quantity of water contained in the tube into the air, forming irregular intermittent explosions. Eruptions of longer duration can be explained thus: The actual weight of water in the tube, acting as a valve on the force, may by means of these globule explosions find itself suddenly released by, say, half the pressure of the column of water. The equilibrium being thus destroyed, the pent-up steam rushes up the geyser-tube with a force proportionate to the depths at which the reservoir containing this force may be situated, and, acting on the principle of a Giffard ejector, the pent-up steam rushes up the tube, taking up with it a certain quantity of the water which may find its way into the tube, and ejecting it in the air, in the form of high, low, or intermittent geysers, in proportion to the different size, position, force, and volume of the spring, and other circumstances of the case.
I have also observed that the chemical composition of the water is sensibly altered by the different actions of the geysers: thus, if the geyser is made to play very actively, the water becomes softer to the touch, it being more silicious and oily than when the geyser-action is subdued and allowed to boil up quietly. This will account for the comparative rapidity observed in the formation of terraces or mounds around the most active geysers, and the very small amount of silica deposited by springs of less pressure and activity.
Doubts having been expressed, by some gentlemen who have studied and written on the thermal action of this district, as to the theory propounded in the foregoing paper, I have constructed an apparatus (see Pl. XLII.), with which I can illustrate the hot spring, the constant, short- or long-interval intermittent geyser, the steaming fumarole, and the mudvolcano—in fact, all the different series of phenomena to be
observed in this district—by simply altering and regulating the admission of cold or cooled water into the flask or retort, and altering the length and relative positions of the glass tubes.
I have adopted 27in. as the length of the glass tube, as this represents about 1lb. pressure to the square inch; but the longer the tube the better the influence and effect of the difference between the specific gravity of cold and hot water, the water boiling under pressure, and the ascending steam-globules will be demonstrated; and the greater the pressure the stronger will be the geyser action, as explained in my original paper.
I would also mention that the flask need not necessarily represent a cavern or cavity, for it will act equally well when filled with pieces of pumice or stones, thus representing broken rubbly ground, where water can lodge and accumulate from any source or direction.
I will now endeavour to describe how some phenomena may be demonstrated.
First, fill the tray (B) with water so as to cover the top of the glass tubes, say, ⅛in. above them; light the spirit-lamp (F), and set it so that it will cause the water in the flask to rise to a temperature of, say, 3° or 4° above boiling-point. Then, to illustrate—
1. A Hot Spring (say, 140° to 180°),—Set the two glass tubes C, D, to the same length both at the top and bottom, leaving both fully open. After a short time, when the water of the flask reaches a temperature of, say, 60° to 100° above the temperature of the water in the tray B, introduce a piece of sponge on the end of a thin wire into one of the tubes (say C) nearly to the bottom; then draw it smartly up: this will cause the hot water in the flask to fill the tube C, while the cool water in the tray will run down D. This will, owing to the difference between the specific gravity of hot and cold water, destroy the hydrostatic equilibrium; and, by putting a little sawdust in the tray, it will be seen that the hot water will continue to ascend in C, and the cold to descend in D, fast or slow in proportion to the quantity of heat generated by the spirit-lamp. The ascending hot water, on being exposed in the large cooling-surface of the tray B, will naturally lose its temperature, re-enter the tube D, descend into the flask F, get reheated, and ascend C; and this action will continue as long as the heat and the water last. If it is desired to change the action from one tube to the other, it is only necessary to repeat the action with the sponge in the opposite tube, and, when once the trend of the hot water is established, the reverse action will be caused and continue as before described, thus illustrating that the heat generated by the spirit-lamp
can be absorbed by the discharge of a rather large quantity of water at a comparatively low temperature.
2. An Intermittent Geyser.—Supposing the action of the hot spring above described to be going on, take the piece of sponge before described (in this case it should be of such a size as to allow about a quarter of the tube to be open); introduce it into one of the tubes (say C), pushing it well down near the bottom: this will stop part of the cold-water supply. The heat of the spirit-lamp being still the same, the temperature of the water in the flask will rise to the boiling-point due to altitude and pressure; steam will then form and accumulate in the neck of the flask, and force the water up the tube D, which water will be seen full of small steam-globules as it ascends the tube. These steam-globules, displacing a certain quantity of the water of the tube, naturally reduce the specific gravity of the column of water in the tube D, thereby reducing the pressure exerted on the water in the flask. The water, finding itself released from the pressure, will begin to boil furiously; large globules of steam mixed with a certain quantity of water will enter and ascend the tube D, and be ejected in the air to a height corresponding with the pressure exerted by the column of cold water contained in the opposite tube C: this furious boiling-action of the water in the flask will continue until such time as the water in the flask is reduced to some considerable distance below the bottom end of the glass tubes, D being then quite empty. The whole of the pent-up force due to the pressure being exhausted, the water in the flask will continue to boil quietly, whilst a small quantity of the cooled water contained in the tray (B) will find its way back into the flask through the tubes C and D, and as soon as the level of the water of the flask reaches the bottom of the glass tubes it will begin to ascend the tube D; and the pressure thus exerted on the water of the flask will totally stop the boiling-action until such time as the water again acquires the necessary temperature to overcome altitude and pressure, when the same action as before described will repeat itself as long as the circumstances remain the same.
3. A Constant Geyser.—From the foregoing it will readily be understood that, by allowing only a very small quantity of water to find its way down the flask, say through C, and raising the tube D so that it projects above the water in the tray, the action of a constant geyser will be illustrated.
4. A Fumarole.—If a still smaller quantity of water be allowed in the flask,—in fact, just as much as is consumed in steam,—a steam-jet or fumarole will be produced.
5. A Mud-volcano.—If a boiling mud-volcano is to be demonstrated, act the same as for the fumarole, and fix a funnel (see figure, p. 590) full of some soft kind of mud on the top of
the tube D (Pl. XLII.), leaving the tube C full of cool water. By having the sponge nearly closing the tube down near the bottom, so as to allow the mud in the funnel to be the point of least resistance, the steam generated will, when it acquires a certain force, escape through that point of least resistance, and the condensed steam about the mud will keep it constantly soft; thus, after every steam-bubble, the mud will refill the hole, and a constantly boiling mud-volcano be illustrated.
With this apparatus most of the phenomena which excite the wonder of tourists to our district can readily be explained. Thus, we are told that at Wairakei a packhorse accidentally fell into a mud-volcano: this caused the volcano to change itself into an intermittent geyser, which lasted for six months, after which the mud-volcano resumed its former quiescent normal action — the packhorse in this case causing the same effect as the introduction of the sponge into the tube.
Explanation of Plates XL.-XLII.
Plan and section of geysers at Whakarewarewa.
Plan and section of Oruawhatua and Chameleon Springs.
Apparatus designed to show geyser-action.
A. Wooden stand, 12in. by ¾in. by 4ft. high, fixed on a 12in. by ¾in. board, 1ft. 6in. long at bottom, with brackets to keep same in position, and brackets at top supporting tray (B).
B. Tray or tin vessel 14in. by 9in. by 2in., with hole in centre to admit of large cork through which the glass tubes are passed.
C, D. Glass tubes, ¼in. diameter and 2ft. 3in. long.
E. Glass retort or flask, to hold about a quart.
G. Iron tripod supporting flask.
H. Maximum thermometer introduced through cork of flask.