Homework – Volcanoes II

01:460:201 Earthquakes and Volcanoes Fall 2018

Homework – Volcanoes II

Please submit the assignment no later than 11:59 pm EST on Thursday, October 18th. The total value of this homework is 15 points of your final grade.

Results of your work should be submitted through the Assignments tab on Canvas as one single Microsoft Power Point (.ppt or .pptx) or PDF document, no other formats will be opened and graded.

Isopleths from Poás Lapilli

(Was it windy when Poás erupted 25 thousand years ago?)

Isopleth is a contour line drawn on a map through all points having the same numerical value. It is a tool to help visualize the spatial variation of a quantitative dataset (e.g. population density, average snowfall, air temperature, price of gas etc.) Become familiar with this terminology by looking at:


Lapilli are small rocks thrown out from a volcanic vent during the eruption. They are typically erupted upwards, and then fall on a ballistic trajectory. Typically, larger lapilli fall closer to the vent from which they are erupted. With no other contributing factors, lapilli should be flying in all directions from a vent, with lapilli of similar sizes reaching similar distances. However, lapilli are also small enough to have their flight trajectories affected by the wind. Consequently, size distribution of lapilli in a layer produced by a single eruption will record the direction and strength of the wind at the time.

In this project you will construct a lapilli size distribution map for a layer that was erupted from the Poás volcano (Costa Rica) 25 thousand years ago.

You will use the size (diameter in mm) of the lapilli from different places around the Poás Volcano to get information about:

  1. the location of the vent that produced the lapilli; and
  2. the direction of wind during the eruption.

You will also make an assessment of volcanic hazard for population centers that are situated around Poás for an eruption like this happening today.


Follow the instructions below to obtain isopleths (lines of equal value) for the Poás Lapilli layer.

  1. Using the table provided (page 3), write the lapilli size next to its corresponding location on the PowerPoint slide with a map [2 pts]. “Outcrop” is a technical term for a place where a sample of rock was collected.

 NOTE: Draw and write over the map in the PowerPoint slide. You cannot edit the map itself. Don’t forget to SAVE your edits!

  1. Identify localities showing the same range of deposit thickness. It may help to color-code similar values by changing text color (e.g., blue for 0 to 10, orange for 50 to 60 etc.). Draw lines on the map corresponding to approximate positions of the following values: 60, 50, 40, 30, 20 [5 pts].

An example to the right shows isopleths corresponding to 100 and 110 mm values. Isopleth for 110 mm encloses only one point, while the isopleth for 100 mm encloses that same point, and also points with values above 100.

  1. Give a different color to each isopleth, and annotate them in the legend of the map [2 pts].


Use the map you have constructed to answer the following questions:

  1. Where do the lapilli come from? Determine the location of the magmatic vent based on your isopleths and mark it on the map with a big red cross [2 pts].
  2. Is there a preferential direction in which lapilli flew during the eruption?

If yes – what was the likely direction of the wind at the time of the eruption? [2 pts]

(e.g. “from south to north”)

  1. If an eruption like the one that produced the Poás lapilli layer were to happen today, which towns are going to be most affected? [2 pts]

You may use the provided template Power Point file (SU18-VolcanoHW2-map.pptx) to make the map and answer the three questions. The breakdown of points is marked throughout the instruction; please make sure that you completed all of the required tasks before submitting.

Sample Location Lapilli Size (mm)
Sample Location Lapilli Size (mm)
Sample Location Lapilli Size (mm)
ATN-8 61
ATN-19 44
CBT-02 20
ATN-5 64
ATN-64 42
CBT-03 21
ATN-9 62
ATN-18 43
CBT-04 20
ATN-4 58
P-137 45
CBT-05 22
ATN-3 57
ATN-66 47
CBT-06 25
ATN-2 54
ATP-06 35
CBT-07 26
ATN-1 55
ATN-33 34
CBT-08 27
ATN-13 52
ATP-55 34
CBT-09 25
ATN-68 51
ATP-12 32
ATG-03 17
ATN-37 53
N-08 33
ATG-24 18
P-81 54
SR-43 34
ATG-02 14
ATP-7 56
ATP-03 34
ATG-06 15
ATN-6 57
P-60 35
ATG-08 17
ATN-44 49
ATG-04 38
ATG-09 9
PO-1 48
P-21 37
AGG-14 7
PO-2 47
P-16 35
PO-4 47
CBT-01 25

Table 1. Size of lapilli in different outcrops around Poas volcano

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