The Forces That Drive Rock

Sitting under the hot, blistering sun, my classmates and I drew and described the layers of a large rocky outcrop of what appeared to be the same material. All of it was a blinding tan/white and contained small to large pieces of dark rock. I would soon find out that there were small differences in each layer that signifies a change in how the volcano was erupting. As I found out more about this wall of rock we were attempting to recreate, I found I was most interested in how the ash, pumice and rock fragments came to be where they are. Two of the main ways the material was deposited are by pyroclastic surges and pyroclastic flows.

Pyroclastic by definition is “relating to, consisting of, or denoting fragments of rock erupted by a volcano[1].” Both pyroclastic surges and flows consist of hot gases, dust, ash, pumice, and lithic fragments, their difference lies in how the material is moved across the ground.

Ash is fine, very pulverized volcanic material under 2mm in size (human fingerprint lines are around 1mm in apart for scale). Pumice is a volcanic rock that consists of highly vesicular (has a lot of holes) volcanic glass[2]. Pumice can range in size from millimeters to meters, depending on the efficiency of the eruption. Lithic fragments result from blown up pieces of the rock that made up the vent of the volcano before it erupted[3]. These rocks were broken up by the powerful explosions that occur below them. As I stared up at the rock face and with this information in mind, I wondered, what types of forces moved these rocks here?

Figure 1: Lithic fragments and pumice surrounded by fine ash.

Although the rock face seemed monotonous, I began to notice small differences. The first difference I found was formed by what are called pyroclastic surges, which are lateral blasts that are accelerated across the ground by shocks(see Figure 2). Surges occur when water falls into the volcanic vent, causing violent explosions as the water flashes to steam from the heat. These surges are unstable and do not extend in a circle, rather, they rush out in fans. They do not always cover all of the surrounding area, but can cover a good portion of the surrounding land, and do so in bands that are easily seen in the rock face, once you know what to look for.


Figure 2: Diagram of a pyroclastic surge during a volcanic event.

Once my professor pointed out some differences in the layers, I could identify them. The first layer I picked out was formed by pyroclastic surges, which are evident within the rock layers because of alternating bands of ash and pumice. As seen in Figure 3, the pumice appears in light tan hues, while the ash looks almost completely white. This contrast displays that there was a lot of little changes in how efficiently the material was created. Fine ash signifies very efficient creation of the material, while the larger pumice pieces display that the volcano was not creating the pieces very efficiently. Once I found this difference, I was able to look up the length of the bed, and admire how many different surges occurred with each alternating band.


Figure 3: Ourcrop with bands of ash and pumice from a pyroclastic surge.

When I looked closely at the bed, I noticed that the pumice itself appears more rounded and are less vesicular(see Figure 4). This is a result of the turbulent flow that occurs in surges. Turbulent flow is a circular movement of material within the surge(see Figure 5). The result of this type of flow is erosive, and often carves out parts of the material below it, leading to some lense shaped beds.  I can picture the hot, turbulent surges ripping across the land, simultaneously eroding the material below and depositing ash and pumice on top.


Figure 4:(1) Rounded pumice pieces from pyroclastic surges. (2) More angular pumice pieces from a layer below from pumice fall.


Figure 5: Sketch of turbulent flow, the spiral displays the circular movement of material within the flow.

Towering above me and the layer created by pyroclastic surges was a layer created by pyroclastic flows. A pyroclastic flow is the unilateral movement of material across the ground, accelerated by gravity. Pyroclastic flows occur when the eruption column of a plinian style eruption collapses(see Figure 6). A plinian style eruption is when the volcano releases a large ash cloud that reaches high into the air, much like the way carbonation blows out of a soda bottle. Pyroclastic flows typically cover large amounts of land and can lead to extremely thick deposits.


Figure 6: Diagram of pyroclastic flows during a plinian volcanic event.

As with pyroclastic surges, pumice and ash are found within the layer. Unlike the surge, pyroclastic flows have little to no sorting, and includes a large amount of lithic fragments. This is because once the flow stops moving, all of the material is deposited at once, leaving random bits of material next to each other. The type of flow that deposited this is called laminar flow, unlike turbulent flow, laminar flow carries material in a uniform, lateral way (see Figure 7).


Figure 7: Sketch of (1) laminar flow, and (2) deposition of material once the flow stops moving.

I could pick out the pyroclastic flow because of how pandemonious the layer is, because of the nature of laminar flows, the material is deposited in a chaotic manner, with large lithic fragments existing right next to tiny pumice pieces. The material is also highly angular, meaning none of the corners have been rounded. This is because all of the material flows down with little resistance, and don’t bump against each other, which would cause them to become more round as in surges.


Figure 8: Pyroclastic flow deposit with weathering that is common with ash formations.

The outcrop in Figure 9 displays the layer with the evidence of surges(on the bottom) and the layer that was most likely deposited by pyroclastic flows(towards the top). Compared to when I first looked up at this intimidating rock face, I can now pick out the different layers with ease. I am still amazed at the processes that moved the tons of rock I saw before me. I can picture the turbulent flows of the surges, that pulse across the landscape and deposits material in thin layers. I can imagine the huge flow of material that caused the chaotic layer above the surges. Now I look at this huge outcrop of rock, and I see the power and forces that drove the rock to where it lies today.


Figure 9: Comparison of the pyroclastic surge layer and the pyroclastic flow layer.


[1] Pyroclastic Flow. (2011). Retrieved June 11, 2016, from

[2] The definition of ash. (n.d.). Retrieved June 11, 2016, from

[3] Pumice. (n.d.). Retrieved June 11, 2016, from


2 thoughts on “The Forces That Drive Rock”

  1. Hello Jessica,

    Informative post this week. Although your opening sentence creates a hang up, it sounds like you’re talking about the sun when you say “same material”. Revise your opening lines to better help the reader understand what your activity was. I can put together that you are talking about drawing the outcrop but the sentence makes it seem like you were drawing the hot, blistering sun.
    Other than this your post does a nice job of explaining varying geological processes. Not only this but you then connect these definitions to a specific portion of your trip. It was a bit tough to follow along with all your diagrams and definitions but seeing the final photo of the outcrop was worth the reading.
    For future posts, attempt to include more of a personal spin (narrative) to the information you are providing. I can see how it would be difficult in a post as factual as this, but try to connect with your readers like you did in your introduction and conclusion. As for this post, thanks for all the information. Your definitions and visuals assist outside readers in a way that helps them understand how a specific rock formation came to be. This is so cool as it allows your audience to understand the time and geological occurrences that are responsible for the makeup of our natural world.

    Happy Writing,

    Jose Martinez

    1. Jose,
      Thank you very much for your critique. One of the things I struggle with in blog writing is putting a personal spin on things. I just love the facts so much!

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