[Geysers] East Sentinel report

[Barrett Southworth]

On the 25th, I spent 14 hours watching East Sentinel. I’ve noticed the 
behavior of East Sentinel can be quite varied and different. I don’t 
know how many people know this, but there are definitely two different 
types of minor eruptions to East Sentinel. There was a 12 minute 
duration at 3:28 and a 20 to 50 second duration major at 9:43.

As a note I don’t know about the behavior in these terms for the vent 
facing away from the river.

I chatted with Will for a bit so I think this type of minor is the 
default one that is normally seen and does not lead to major eruptions. 
I’ll call it a Push minor the two majors that I witnessed both had a 
different type of minor leading up to them.

Push minor – A push minor attempts to push when the water height reaches 
maximum and boils heavily at water low height. Typically the vent facing 
towards the river has a highly variable strength eruption that is 
occurring separately from the pushes. The vent facing towards the river 
also typically never stops at the end of a minor duration. Not to 
mention occasional surges in strength for the minor eruption.

Alternate minor – An alternate minor has very different behavior. 
Typically at the end of the near 20 minutes minor duration all activity 
stops from the vent facing towards the river, some water may actually 
drain into the vent. The stop in activity doesn’t last long, just long 
enough for the water level in the pool to drop to its low height. With 
the first part of its eruption, the alternate minor pushes water out 
during the high boiling period, this last for 3 to 5 minutes and is what 
started the two majors I witnessed eruptions. The other 15 to 17 minutes 
the alternate minor is sending out water in two different ways at a 
mostly constant strength. The alternate minor alternates between these 
two ways during that time. The first way it is erupt is water goes 
mostly vertical, there is splashing on the side with the geyserite, but 
there is no major splashing/turbulence on its own with the water side. 
The other is a low height bust across the length of the pool. Successive 
alternates have stronger vertical and sideways eruptions, height for the 
nearly pure vertical increases steadily. At what I’d say is about 7.5 
feet to 10 feet above mid-water level is when the chance for the next 
vent stop push to start a major occurs.

Observations of the three majors I’ve seen this year.
On June 27th while waiting for Riverside and visiting Fan&Mortar for a 
bit I heard water splashing in the river. I did not go down to East 
Sentinel but went up to the half-way point of the bridge, East Sentinel 
is visible from there and I noticed the water jets extending across the 
river. I made notes on the location of the geyser so I could figure out 
its name later. Not knowing how rare it was I went back to Riverside to 
wait for its Eruption.

July 25th, I had my reasons for staying up all night to wander Upper 
Geyser Basin.

The 3:28 Eruption, before its sequence of minors started I was wandering 
in between East Sentinel, Fan & Mortar and Riverside. Upon witnessing 
the start of a minor at 1:12 am I decided I’d just stay and watch East 
Sentinel. I got extremely lucky since it led to a major eruption, at the 
time I did not know that it would. There were seven Alternate minor 
eruptions that led to a major. After one hour gurgling water noises 
underground started, being herd from the dry vent on the geyser. This 
led to the 3:28 major eruption. I can’t be certain but I think both 
vents joined into the play, but I am certain of the river one. The 
duration in total was 12 minutes, it had a brief pause in the middle for 
about 30 seconds. During this twelve minutes, the vents stopped and the 
water drained well down below normal minor eruption levels, it then 
immediately resumed the major finishing out the duration. At the end of 
the duration there was another water drain like the first, that was 
simply refilled nearly immediately and indicated an end to eruptive 
activity for the moment at 3:40.

The 9:43 eruption, at seven A.M. intending to wander over to Artemisia I 
stopped by East Sentinel and witnessed the Alternate minor again as well 
as the water gurgling start. I decided to stay around and wait. If the 
Gurgling sound only starts after about an hour then I arrived at the end 
of the third minor leading to the brief major. It had eight more minors 
before this led to another major eruption by East Sentinel, the duration 
though was only 20 to 50 seconds, I don’t have an exact time. It was 
just a little longer than however long it took me to call in the time 
over FRS, have someone ask could you repeat that, and me repeating it.
Post short duration behavior, immediately upon the end of the short 
duration major, alternate minors resumed, with this next sequence 
actually being stronger than the one that started the major. This went 
on till around 11:20, at which point East Sentinel did something weird, 
the duration of alternate minors reduced to 2 minutes. The push phase 
lasts about 30 seconds and the eruptive phase about 1 minute 30 seconds. 
After about 30 minutes of this, the last one actually become a normal 20 
minute duration Push minor. After 2 push minors, East Sentinel once 
again went back to Alternate minors, with 10 more short duration 
alternate minors, that led to a permanent status of Push minors. The 
push minors continued on, at 2:30 pm they increased noticeably in energy 
beyond the normal increase in strength, with the sound of heavy deep 
boiling being heard. I stayed until 6 pm, being too hungry to hang out 
at E Sentinel after 11 hours straight. There had been two more increases 
in energy like the first during that time. E Sentinel’s push minors 
pushes were also getting very big. But nothing led to an eruption and 
they were already bigger than the two alternate minor pushes that led to 
majors. I am not certain if there was a major by E Sentinel later that 
day, but I would suspect that Push minors do not lead to major eruptions.

There aren’t enough observations to be certain about anything but things 
I think are likely/possible.
1. There are indications of a major eruption, 2-5 hours before it 
occurs, in a way similar to Giant Hot periods, and Fan & Mortar event 
cycles, these are alternate minors.
2. Push minors never lead to a major, or don’t do it without a massive 
period of minors preceding them.
3. Alternate minors fail to lead to a major eruption if they turn into 
push minors.
4. As long as alternates don’t morph into push minors, then a series of 
major eruptions is possible.

Hopefully the rest of this season, several people will keep a watch out 
for alternate minors and stick around for a few hours and see what they do.