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Volcanic bombs [Flores - Azores] EarthCache

Hidden : 09/13/2012
4 out of 5
3 out of 5

Size: Size:   other (other)

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Geocache Description:

THE "BASICS" -Flores Volcano (Stratovolcano) - Azores

Flores Island is the westernmost island in the Azores. It is located on the North American tectonic plate, and is separated from the rest of the archipelago by the mid-Atlantic Ridge.

The island is aligned north–south (parallel to the Mid Atlantic Ridge) and nearly perpendicular to the alignment of the other Azorean islands.

Caldeira Funda de Lajes tuff ring formed about 3150 years ago. It produced a lava flow which extended NW and reached the coast at Faja Grande.

The evolution of Flores island over the past 1 million years can be divided into four stages.

Stage 1 (1.0 to 0.55 Ma)

Explosive eruptions formed the initial island. Tectono-volcanic uplift took place, raising the island by 100 m.

Stage 2 (0.55 to 0.4 Ma)

An eruption of large volumes of lava from three eruptive centres was followed by a long period of volcanic quiescence (0.5 to 0.4 Ma).

Stage 3 (0.4 to 0.2 Ma)

The last major eruptive episode of this island occurred during this period. There were no important vertical tectonic movements during this stage.

Stage 4 (0.2 Ma until the present)

Strombolian and phreatomagmatic eruptions occurred about 3000 years ago.

(Flores Volcano Eruptions - 950 BC ± 100, 1200 BC ± 100.)


A volcanic bomb is a mass of molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lavaduring an eruption. They cool into solid fragments before they reach the ground. Because volcanic bombs cool after they leave the volcano, they do not have grains making them extrusive igneous rocks.

Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight. Bombs can be extremely large; the 1935 eruption of Mount Asama in Japan expelled bombs measuring 5–6 m in diameter up to 600 m from the vent. Volcanic bombs are a significant volcanic hazard, and can cause severe injuries and death to people in an eruption zone.

Volcanic bombs are known to occasionally explode from internal gas pressure as they cool, but contrary to some claims in popular culture explosions are rare; in most cases most of the damage they cause is from impact.

Types of bombs

Bombs are named according to their shape, which is determined by the fluidity of the magma from which they are formed.

Ribbon or cylindrical bombs form from highly to moderately fluid magma, ejected as irregular strings and blobs. The strings break up into small segments which fall to the ground intact and look like ribbons. Hence, the name- ribbon bombs. These bombs are circular or flattened in cross section, are fluted along their length, and have tabular vesicles.

Spherical bombs also form from high to moderately fluid magma. In the case of spherical bombs, surface tension plays a major role in pulling the ejecta into spheres.

Spindle, fusiform, or almond/rotational bombs are formed by the same processes as spherical bombs, though the major difference being the partial nature of the spherical shape. Spinning during flight leaves these bombs looking elongated or almond shape, the spinning theory behind these bombs' development has also given them the name 'fusiform bombs'. Spindle bombs are characterised by longitudinal fluting, one side slightly smoother and broader than the other. This smooth side represents the underside of the bomb as it fell through the air.

Cow pie bombs are formed when highly fluid magma falls from moderate height (so the bombs do not solidify before impact) which are still liquid when they strike the ground. They consequently flatten or splash and form irregular roundish disks which resemble cow-dung.
Bread-crust bombs are formed if the outside of the lava bombs solidify during their flights. They may develop cracked outer surfaces as the interiors continue to expand.
Cored bombs are bombs that have rinds of lava enclosing a core of previously consolidated lava. The core consists of accessory fragments of an earlier eruption, accidental fragments of country rock or in rare cases bits of lava formed earlier during the same eruption.

About the earthcache

Tasks to perform on GZ:

a) How many volcanic bombs can you see standing on the top of the islet?

b) What is the color of them?

c) Can you identify the type of rock material?

d) Can you identify the type of volcanic bomb you are seeing?

e) How did this volcanic bombs get on top of the islet?

f) Finally, how big are they? Can you estimate an average measure in meters?

Also, and despite being optional , please take a photo in the GZ near the islet with your GPS and show it with your log.

Send the responses via my geocaching profile and enjoy your visit.

Send e-mail responses and image to netuseraz (at) hotmail (dot) com before performing the log cache.

I will give feedback to authorize the log.

Additional Hints (No hints available.)