A Glimpse of the Past

Humans have learned to use rocks for many different purposes in life, such as for tools, jewelry, and even counter tops, but what intrigues me is the observation and inference of rocks that tell a story of the past. Even after taking multiple geology classes at NAU, I have only recently learned about one important characteristic of volcanic deposits that provides information on how an eruption occurred thousands of years ago; a block sag.  The geologic definition of a “block” is defined as an angular piece of lava (larger than 64mm) that was ballistically ejected from a volcanic vent. On the other hand, a block sag is a depressed or indented section of rock strata that was created by a ballistically ejected angular fragment of a volcano during an explosive eruption. The blocks that we’ve seen here on Santorini have had a wide variety of sizes, ranging up to two meters. We found an impressive two meter block on the edge of Cape Akrotiri, which is on the southwest tip of the island. (Observation Point 9 on the map below)





-The above image is a two meter block that we located on the edge of  Cape Akrotiri, which is on the southwestern tip of the island.

Here on Santorini, our class has identified block sags on multiple parts of the island and have made inferences about them that have given us clues to different aspects of the devastating Minoan Eruption, which occurred over 3400 years ago. Our class mainly focused on inferring information from block sags that gave us clues to the eruption style, exactly where the the volcanic vent was located, and even the amount of water that entered the vent from the Aegean Sea during all five different phases of the explosive eruption.(Druitt 2014)


-Even though the block is no longer present, a block sag can be seen in the center of the above image.  This picture is also a good example of the deposits from the first and second phases of the eruption (discussed below).  The second phase ash layer takes up most of the image, and is where the block sag is located.   The first phase layer of pumice (which has a rocky appearance in the image) is just below the ash layer, and is seen towards the bottom of the image.

Block sags can be found in phase one, two, and three of the ultra-plinian (extremely explosive) Minoan eruption that we have been learning about and will be studying for a total of three weeks. The zero phase of the eruption was a thin layer of ash that warned the Minoan people of the catastrophic event that was about to occur.  The first phase of the eruption caused the buoyant and porous volcanic rock, called pumice, to be deposited up to 7 meters thick on the southeast side of Santorini. The second phase consisted of multiple surges of ash and other volcanic material that spread outward from the vent and were then deposited on top of the first phase pumice layer. The third phase of the eruption was highly explosive and consisted of ash, pumice and lithic fragments that were deposited as a quickly traveling pyroclastic flow moved across the first two layers.(Druitt 2014) See Carly Stefano’s blog post, “A Geologic Lesson for the Little Ones” for a more detailed description of the 5 phases.

During the progression of the eruption, the volcanic vent widened, allowing more water to enter the vent and mix with the magma. This caused an increase in explosiveness in the second and third phases, which sent a larger amount of blocks and lithic fragments into the air. As the blocks landed in the first, second and third phases, their force and pressure was large enough to break through the water-saturated ash and pumice, causing the nearly horizontal layers to be pulled down along the path of the submerged block, creating multiple blog sags.(Druitt 2014) Observing a block sag in a volcanic deposit implies that the environment of the rock bed had to be cool and wet in order for the block to penetrate the layer. Coming to this conclusion about the environment of the rock strata strengthens the idea that an increased amount of seawater entered the volcanic vent at this time.

The analysis of a sag by a trained eye can also give us a solid idea of where the block came from and more specifically, where the volcanic vent was located before it was completely filled with water. Usually, one side of the sag is steeper than the other, implying that the block protruded the surface of the rock layer at an angle. As a result, the greater the difference in steepness of the two sides of the sag, the lower the angle of ejection from the vent. (see diagram)


-The top drawing in the above picture shows a block sag that implies that the block was ejected from the volcanic vent at an angle. The second drawing shows the opposite; a block that was ejected from the vent almost vertically.

Blocks were also ballistically ejected from the vent during the fourth phase of the eruption, but were not able to penetrate the massive hot and dry pyroclastic-deposited layer, known as ignimbrite. Therefore, there are no block sags present in this fourth layer of volcanic deposit. (Druitt 2014) Observing this, we can come to the conclusion that minimal water entered the vent during this phase.

Block sags are just one of thousands of geologic features that help humans decipher the past and predict the future. Proper knowledge, analyzation and interpretation of rock stratigraphy in an area can give us a timeline of the past that we would have never concluded before.

Therefore, “rock” on…


4 thoughts on “A Glimpse of the Past”

  1. Rock on big bro! I now have a complete understanding of a block sag, and it’s significance in “telling a story of the past”. I really like the angle you chose to elaborate on about the subject. It’s an Interesting way to think about block sags:-) You’re a great writer!

  2. Joe – well done and well composed. You can ‘edit’ the image in your post and add your figure captions there so they display at a different font style and size. Having the figure captions in the text is a little confusing for the reader. Block sags are found in phases 2, 3, and 4. Phase 1 is pumice fall (dry)….no sags. When citing references the format should be ……. (Druitt, 2014). (which the period after the citation and not before).

    Referring back to Scott’s post about the hot and dry pyroclastic flows of phase 4. There are no block sags in the hot flow (south and southeast Thera) the there are a few in the cold flow (north and northeast).

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