# How far can an andesitic lava flow, and what is the velocity?

For an effusive volcanic activity, how far an andesitic lava can flow? What is the velocity?

Eruption volume 0.2 km3 with 25 degree slope.

• you need to give more information like the volume per hr and the angle of slope. Apr 12, 2019 at 5:20
• I don't know the volume per hr. Eruption amout 0.2 cubic km amount with 25 degree. Apr 12, 2019 at 20:26

Andesitic lava is viscous, compared to basaltic lava. Andesitic lava flows at a rate of a few kilometers per hour, whereas basaltic lava can flow up to 10 km/h. Because of their viscosity, andesitic lavas "rarely extend beyound 8 km from their vents".

Viscosity of andesite lava and its implications for possible drain-back processes in the 2011 eruption of the Shinmoedake volcano, Japan

Pulsatory andesite lava flow at Bagana Volcano

Volcanoes, Magma, and Volcanic Eruptions - Tulane University

• Would you please give your references? tThen i will able to read more from there. Apr 13, 2019 at 21:01
– Fred
Apr 14, 2019 at 3:07

During my PhD, I listed many dacite and rhyolite flows, but also included a few andesite flows for comparison. Here are a few interesting examples, first of andesite flows that have been observed while active (for which we have velocity measurements), then of past, frozen flows (for which we have the length only).

### Active flows

• Lonquimay volcano, 1988–1990

0.23 km3 of lava, emplaced over 13 months, reaching a distance of 10.2 km from the vent, according to Naranjo et al. (1992). The article contains nice data of lava flow advance (Table 3), allowing for calculating velocities. The velocity ranged from ~$$10^{-2}$$ m/s at the beginning of the eruption to ~$$10^{-4}$$ m/s at the end, and the slope from 1 to 6°. You can already see that velocity does not depend on slope, I will come back to this later on.

• Kizimen volcano, 2011–2012

0.30 km3 of lava, emplaced over ~17 months, reaching a distance of 3.5 km from the vent, according to Auer et al. (2018). Their Table 1 shows that the slope was much steeper than at Lonquimay (10$$-$$26°), but the flow is not even half as long, with a similar magma volume. Velocity was in the order of ~$$10^{-4}$$ m/s (Table 1). Note that this flow is borderline andesitic, involving magma mingling between two components:

Dark and light bands show a range of whole rock compositions between 62 wt% SiO2 for the dark silica-rich andesitic endmember and 65 wt% SiO2 for the light dacitic endmember

• Sinabung volcano, 2014–2015

0.10 km3 of lava, emplaced over a year, reaching a distance of ~3 km from the vent. There is a whole issue of JVGR on this eruption. This flow reached the same length as the Kizimen one, despite being less voluminous (hence it was "thinner", with a max thickness of ~100 m, versus > 200 m at Kizimen). The slope was variable: 33° for the first 1 km, 15° for 1–2 km, and 9° for the last 0.5 km. The velocity was ~$$10^{-3}$$ m/s at the beginning of the eruption, and ~$$10^{-5}$$ m/s at the end, but again this is not due to the change in slope (see below).

### Frozen flows

• Gusey et al. (2018) describe two andesite lava flows that are among the longest known, with lengths of 52 and 74 km. This gives you an idea of the upper bound. Of course you need more volume to reach such lengths: minimal volume estimates are 2.5 and 6.6 km3, respectively.

• Chevrel et al. (2016) describe several voluminous andesite flows at El Metate volcano. The largest, named F6, is 15 km long, with a volume of 2.2 km3. This paper is also a nice example of how you can infer dynamics parameters of frozen flows. For F6, they inferred a velocity in the order of ~$$10^{-4}$$ m/s.

• Yet another example is the 6.5 km long Oturere andesite flow, at Tongariro volcano, where Stevens (2002) calculated a volume of 0.37 km3.

### Factors controlling lava flow length

As you can see from these examples, andesite effusive eruptions with similar magma volumes can yield lava flows of different lengths. But the main factor controlling lava flow length is not the slope$$-$$it would be if lava was a Newtonian fluid, which it isn't. Walker (1973) Lengths of lava flows summarizes it well:

The principal factor influencing the length of a lava flow is the rate of effusion. With a high rate the lava flows rapidly from the source and tends to form an extensive and far-reaching flow which is simple in character (i.e. made of a single flow unit). With a low rate the lava tends to pile up layer upon layer to form a local accumulation of limited lateral extent near the source, and this accumulation strongly compound in character (i.e. divisible into flow units).

You can see the correlation between effusion rate and flow length on his Figure 4. Now, the reality is a bit more complex, and much work have been done since, but his assertion still stands true. At Lonquimay and Sinabung for instance, you'll see in the references that changes in effusion rate has been linked to changes in lava flow velocity (and, ultimately, to final length).