YELLOWSTONE NATURE NOTES

CHANGING GEYSERS AND HOT SPRINGS

The association of geysers, hot springs, mud pots and fumaroles is believed to represent a declining phase of volcanic activity. Steam and other gases rising through fractures from the magmatic source at depth come in contact with cold descending ground water. Convection current circulation of the water permits hot springs to attain various temperatures depending on the ease and freedom of circulation. Where circulation is impeded by a twisting tube underground, the water cannot heat uniformly and as steam pockets form they expand and lift the overlying column of water into the air to create an eruptive hot spring technically known as a geyser. A sedimentary deposit of siliceous sinter or geyserite is precipitated within the plumbing systems of the springs and also around their orifices. The source of the silica is the rhyolite lava flows that underlie the basins. Where there is a deficiency of ground water the wall rock is commonly intensely altered causing rhyolite to break down into clay and with the oxidation of iron compounds there are developed colorful features known as paint pots. Fumaroles are fractures that convey steam from below.

To date we have no confirmatory evidence that the geyser basins are fault controlled. Many of the smaller thermal areas are localized along the margins of lava flows. The larger basins may be depressions where hydrothermally altered rock has been easily removed by later erosion. The basins are now paved with a cover of glacial gravel that serves as the porous aquifer to feed the thousands of hot springs and geysers.

FIG. 8. Cross Section of View of a Typical Geyser Basin. Depth and temperature data based on Fenner (1936).

During the hours immediately following the main earthquake shock the chief concern was for the safety of the people; few had time to consider what might be happening in the geyser basins. Had the earthquake occurred during daylight one could probably have seen more geysers and hot springs in action than at any time since the establishment of the park.

On the morning of August 18, Park Naturalist George D. Marler made a reconnaissance of the Upper, Midway and Lower Basins. A rapid inventory revealed that 298 geysers and hot springs had erupted and of this total 160 had no previous record of eruptive activity. Some of the less accessible thermal areas showed signs of increased activity and some geysers dormant for years were rejuvenated. There was an obvious need for a thorough inventory of Yellowstone's thermal features.

PLATE 28. Sylvan Springs showing increased thermal activity after the earthquake. National Park Service photo (59-1090)

The National Park Service approved a project known as the "Emergency Interpretive Study of Earthquake Phenomena, Yellowstone National Park," and under the guidance of Chief Park Naturalist Robert N. McIntyre, Park Naturalists began the laborious accumulation of data on temperature, discharge and physical change.

Temperatures were recorded by Taylor Maximum Registering Fahrenheit Thermometers and discharge was computed by using a large caliber hose and graduated pail to determine the flow in gallons per minute. The criteria of physical change included: 1) evidence of unusual eruptions, such as broken and dislocated sinter rims, eroded soil nearby, dead or dying vegetation adjacent to the feature, and increased erosion of the drainage channels; 2) a large rise or fall in temperature compared with recent records; 3) a change in water level as evidenced by flooded margins or dried out algae above present water level; and 4) recent cracks through sinter or soil in the immediate area. All major features were documented by photographs permitting a comparison of the 1959 status with that of earlier years. The study required pack trips into remote areas and with early snows and sub-zero weather, working conditions were at times hazardous.

From these observations there has resulted a vast accumulation of data that are now being evaluated. A few of the more pronounced changes believed to be of interest to park visitors are summarized in the pages to follow.

Yellowstone's major thermal resources are concentrated on the western side of the park along the Firehole and Gibbon River drainage. Smaller areas are found at Shoshone Lake, Heart Lake, West Thumb, Hayden Valley and other remote areas seldom seen by man.

Observations to date show that the greatest physical changes occurred on the west side, particularly in the Upper, Midway and Lower Basins. In these areas there is evidence of violent reactions immediately following the shock and as time progressed it become apparent that many more subtle and delayed responses were being felt. Now, eight months after the disturbance many features are gradually returning to their former state.

Lower Basin:

We are fortunate in having a Park Naturalist's eyewitness account of the simultaneous eruption of three major geysers immediately following the main shock. Fountain, Morning and Clepsydra went into action and continued erupting all through the 18th at which time Fountain Geyser ceased. Periodic eruptions of Morning Geyser continued until September first and Clepsydra, eight months later is still in action.

Their pre-earthquake pattern had been a chain reaction started by Morning, followed by Fountain and terminated by Clepsydra. At times they have erupted independently.

"Earthquake Geyser," a new feature west of Fountain Geyser, was born of the tremors. For several days it erupted at frequent intervals and then gradually declined.

Some springs acquired new water supplies at the expense of others and Gentian Pool ebbed 41 inches and has taken seven months to resume its former status.

PLATE 29. Clepsydra Geyser. Lower Geyser Basin, still in eruption eight months after the earthquake. National Park Service photo (59-846)

PLATE 30. Gentian Pool, Lower Geyser Basin, showing drop in water level to expose thin ledges of geyserite that grew over the surface of the pool. National Park Service photo (59-989)

Celestine, Silex and Leather Pools became violently active and for a two week period were ejecting muddy water.

PLATE 31. Silex Spring. Lower Geyser Basin. in violent stage with boiling muddy water. National Park Service photo (59-823)

There were no immediate changes in the Fountain Paint Pots but by August 21 new mud pots began to form and more violent action cast mud beyond the guard rail and some of the vertical guard rail supports served as escape routes for steam thus converting them into miniature fumaroles.

New steam vents opened up in the parking area and because of the obvious changes and unpredictable pattern of activity, the old parking area was removed and a new one is being developed a short distance away. Geysers and hot springs had reclaimed a tract of land on which man had encroached.

PLATE 32. Fountain Paint Pots. Lower Geyser Basin, in state of post-earthquake rejuvenation casting mud beyond the guard rail. National Park Service photo (59-829)

PLATE 33. Former Fountain Paint Pot parking area, Lower Geyser Basin, showing post-earthquake encroachment of fumaroles and mud pots in the parking area. National Park Service photo (59-827)

In the Firehole Lake area a random network of fissures developed in the alluvial cover, some with minor displacement, and totaling 9,072 feet in length.

PLATE 34. Earthquake fissures in alluvial cover near Firehole Lake in Lower Geyser Basin. National Park Service photo (59-782)

The well-known Great Fountain, White Dome and Pink Dome Geysers departed radically from their normal eruption intervals.

Interested park visitors will soon find that the Lower Basin provides some of the more dramatic evidence of an earthquake's effect on hydrothermal features.

Midway Basin:

Most of the springs in this basin became turbid and many lowered their water levels. Turquoise Pool ebbed eight feet and the bowl of Grand Prismatic was slightly tilted so that the overflow shifted to a predominately easterly direction. The exact change in level is not known but it is estimated to be between one-half and one inch.

Biscuit Basin:

The stellar attraction in this area has always been Sapphire Pool, a feature that has enthralled thousands of people over the years. Its pre-earthquake activity consisted of small eruptions about every 20 minutes and from the boardwalk it was possible to see steam bubbles form, rise, and dome the surface with umbrella-like bursts while the water ebbed away in discharge channels ringed with globular growths of geyserite.

All is different now. On the morning of August 18 Sapphire had become a steady geyser with murky water rising eight feet in the air. During the night of August 21 there was a major eruption followed by even more violent ones in the months that followed. Blocks of geyserite with weights estimated from 50 to 100 pounds were torn from the rim and cast up to 50 feet from the crater.

PLATE 35. Sapphire Pool, Midway Geyser Basin, in post-earthquake eruptive stage. National Park Service photo (59-1125)

Some of the eruptions of Sapphire can definitely be associated with aftershocks as recorded on the seismograph at Butte, Montana. It would be interesting to learn if Sapphire is an "earthquake thermometer" triggered into action by tremors emanating from one epicenter.

Upper Basin:

The Grand Geyser erupted the night of August 17 and then went into a state of dormancy and was believed to be an earthquake fatality. However, later in the season it resumed limited activity and in time may regain its former status.

After forty years of dormancy the Cascade and Economic Geysers began periodic activity and Giantess went into an unprecedented eruptive phase that lasted for over 100 hours.

Other well known geysers such as the Daisy, Riverside, Castle, Grotto and Oblong began playing on shorter eruptive intervals that persisted for the balance of the year. The Giant Geyser has remained dormant.

World famous Morning Glory Pool ebbed six inches, became murky, and has taken seven months to resume its former status.

On the 18th of August it was noticed that Old Faithful was playing on a more erratic schedule with successive long and short intervals. Old Faithful's average eruption interval from June to August 17 was 61.8 minutes. For the last ten days of December two hundred and fifty-five eruption intervals averaged out to 67.4 minutes. Apparently there is a delayed response here to the tremors of last August. Whether it will continue or not time alone will tell.

Forceful eruption of previously quiet hot springs, erratic eruptive intervals of geysers, ebbed pools, and turbid water conditions are all to be expected when a geyser basin is shaken by earth tremors. These basins are topographically low and veneered with a cover of glacial gravels that provide the porous medium for ground water circulation. Earthquake vibrations have altered these circulation routes opening up new channels and closing off some of the old.

An analysis of temperature and discharge measurements on several hundred springs shows a post-earthquake temperature rise of about 6° Fahrenheit and an increased discharge of about ten per cent. How long these conditions will persist we of course have no way of knowing.