Mapping lava flows – using color to work out relative ages

Mapping lava flows using satellite images is a wonderful activity that you can do in your classroom.   In some locations, the color of the flows due to the weathering of the lava and the amount of revegetation can be a great exercise that students can use not only to develop spatial skills but as a basis for inclass scientific debate.

Mapping lava flows

Below is a screen shot of a Google Earth image of flows from the Mauna Ulu eruptions (1969-1974 and older) on Kilauea volcano on the Big Island of Hawaii.   You can get more information about this eruption here.

The distance across the image is approximately one kilometer.   The slope is high at the top and low at the bottom, meaning all the flows go from top to bottom.  The image contains a landscape that has multiple lava flows of multiple ages but of all of one rock type – basalt.

One added complication is that basalt can flow in two forms – a relatively smooth surfaced flow called pahoehoe and a very rough surface flow called a’a.   In a satellite image, a young pahoehoe flow will appear as a silvery-black color and an a’a flow as a deep chocolate brown.

As both of these types of flows age, the rock type tends to lighten in color through mid browns to light brown.  On top of these colors is the color of the vegetation with the oldest flows being completely covered and appearing bright green through to moderately aged flows that are a mix of green and brown.

So let’s look at the same image and identify these flow features:

A – a mid aged flow.  Some early vegetation, but mostly we see the weathering of the flow.  It is an A’a flow.

B – an old remnant flow heavily vegetated.

C – a young pahoehoe flow.

D – a very young a’a flow.

So on face value, it looks like the order of these (from oldest to youngest) is B-A-C-D and that would probably be a great guess.

But here is the question I would ask students.  How can you work out with just the image which is younger – C or D?  This could produce a lively discussion as they hunt for evidence to back their argument.   If they appear to be floundering, I would suggest that they look for evidence for one flowing over the other…and may provide them a zoomed in image like this:

I think the evidence points to the darker a’a as being younger as it LOOKS like has flowed over the pahoehoe….but it may be that in other places on this image that the opposite is true.  Let them decide based on the evidence they can find.

The next stage is for the students to draw on a printed image (or use a computer drawing program that you can upload the image) around each of the flow types, to produce a flow age map.

Nothing adds more to these mapping lava flow activities than you being able to show students what these look like on the ground (ground-truthing) and in the activity below, you can do just that by joining us in Hawaii on one of our teacher experiences….I have shared some images below. (Check them out here).

Here is a photo of the young pahoehoe flow and the a’a flow taken close to C.   It is a little more obvious on the ground that the a’a flowed over the pahoehoe as described above.  Also, you get a better appreciation of the slope of the site.

Here is a photo standing near C and looking towards B.  You can see the richly vegetated C area.

This is a photo showing two pahoehoe flows.  You can tell the age due to weathering color change.  This was taken near A.

I hope that you can use this Mapping lava flow activity in your classroom.  And why not make it really special – come and join us in Hawaii.  🙂