THE QUESTIONS:
1.) In front of you there is a dry stone wall, what do you notice about the textural nature of the rock it’s built from? Describe its characteristics.
2.) Do you think this stone in the dry stone wall has come from the Mt Elephant lava flow? Can you give 2 reasons why you think this is so?
Then post a photo of you at the location with Mount Elephant in the background & the dry stone wall in the foreground in your log, (please do not show the subject of the questions in your photo). Of course, if you do not want to appear in the photo, a personal item in the photo is enough proof of your presence. You may log the cache as soon as you submit your answers to us via messenger.
Logs without accompanying answers sent or without a photo uploaded may be deleted without notice.
THE LESSON:
In the near distance you can see Mount Elephant….it is a breached Scoria cone. It’s eruption cycle was primarily pyroclastic tephra but here we are more concerned with what may have followed the gaseous and volatile early eruptions. Over the amazing dry stone fences we can see what looks like ridges of volcanic rock. These are blocky lava flows that came from the lower slopes of the scoria cone of Mount Elephant.
*Picture: How a Scoria cone functions
Formation and Life Cycle of a Scoria Cone Volcano
Scoria cones form when gas-charged lava violently ejects from a vent under pressure. They tend to shoot straight up into the air in a fire fountain (just like we are seeing with Mount Kilauea in Hawaii right now). The frothy lava then breaks into small fragments that cool and solidify as they fall back to the ground. The formation process may be divided into four main stages, while number 5 has the possibility of occurring at any time of the process:
- Initial Eruption: Gas-rich magma rises to the surface, fragmenting as it escapes, creating ash and cinders. A low-rimmed scoria ring begins to form around the erupting vent.
- Cone Building: Pyroclastic material accumulates around the vent, a talus slope begins to form outside the rim, building the cone structure.
- Crater Formation: Continued explosive activity creates a crater at the summit. During this stage slumping and blasts can destroy the original rim.
- Talus Buildup: a buildup of talus occurs beyond the zone where tephra falls to the surface (the ballistic zone).
- Lava Flow: Lava may breach the cone, or erupt from the base, flowing out to form surrounding lava fields. During the waning stage of a cinder cone eruption, the magma tends to be gas-depleted. It is not volatile and does not fountain. By this stage lava oozes quietly from beneath the base of the cone. This occurs because the uncemented material of the cone is too weak to support the pressure exerted by molten rock as it rises toward the surface through the central vent. The molten lava is denser than the bubble-rich tephra, so it often pushes out along the bottom of the cone, lifting the less dense scoria like corks on water, and advances outward, creating a lava flow apron around the cone's base. When the eruption ends, a symmetrical cone of cinders sits at the center of a surrounding pad of lava. If the crater is fully breached, the remaining walls form an amphitheater or horseshoe shape around the vent.
It is step 5. of the Scoria Cone life cycle sequence we are concerned with here.
Once the bulk of the gasses have been released, the eruption begins to produce large flows of runny lava. These flows typically emerge from either fissures at the base of the volcano or breaches of the crater wall. The lava flow we can see in front of us seems to have been a basal flow, coming from a vent down at the base or from underneath the cone.
A basal lava flow occurs because the loose uncemented scoria structure of the built up cone is too weak to support the pressure exerted by molten rock as it rises toward the surface through the central vent to the summit crater. Instead it tends to leak like a sieve from low down the sides or the base. Because this later lava contains fewer gas bubbles, it is denser than the gas bubble-rich scoria. While the lava flow from under the cone can also be vesicular in nature, it typically has less intense vesiculation, with the most pronounced vesicular zones often restricted to the uppermost layers of the flow. The lava flow often pushes out along the bottom of the scoria cone lifting the light weight scoria cone up, the cone effectively ends up floating like a cork on its own lava flow! The lava flow advances outward around the cone's base. When the eruption ends, the cone sits on the center of a surrounding pad of lava. If the crater is fully breached, the remaining walls form an amphitheater or horseshoe shape around the vent. Most Scoria cones have a vent at the base or side, these basal vents are sometimes called bocas or boccas (Spanish or Italian respectively for “mouth”). Essentially the scoria cone can end up floating on its own lava lake, a bit like a cork! The lava produced in these flows, while much denser than scoria still does tend to produce lava with dissolved gases….thus the cooled rock tends to be vesicular (honeycombed) as the trapped gases created holes in the cooled basalt.
Example Photo for your Log
Resources:
https://www.nps.gov/articles/000/cinder-cones.htm
https://sciencenotes.org/cinder-cone-volcano-formation-characteristics-eruption/