Chimiche Quarry

The Chimiche ignimbrite quarry contains two rock types, the Granadilla Pumice and the Granadilla Ignimbrite, with a total thickness of approximately 18 metres, at the main quarry face.

The Granadilla Pumice is a loosely consolidated pyroclastic fall deposit, consisting of pumice lapilli, with occasional smaller grey lithic fragments and no ash. It is the product of gravity settling from a plinian eruption cloud. The ash has been winnowed away, but the heavier pumice (juvenile) and lithic (non-juvenile) clasts have fallen to earth.

The Granadilla Ignimbrite lies on top of the Granadilla pumice. The pumice lapilli have the same phonolitic composition as the lower unit and the same grey lithic fragments are present, but also ash. This is the product of a pyroclastic flow, following the collapse of a plinian eruption column.

This is a major stop, in which we examine the relative sizes of pumice and lithic clasts, grading in the Granadilla Pumice, the density of pumice lapilli and of the ignimbrite, the height of the quarry wall and the percentage of pumice lithics and ash.

A 360x180 degree panorama is on Flickr and 360cities.net.

The Granadilla Formation is dated at 600 ± 7 ka, at the end of a major eruptive cycle of Las Cañadas, followed by a 300 ka gap before the next set of pyroclastic rocks in the Bandas del Sur Group.

Because these rocks feature in our coursework section, we study them in detail in order to fully understand their formation and significance. The following exercises help students to explain and justify a mode of formation for each deposit.

Ex 05 - Point count to determine proportions of pumice, lithics and ash

To classify the Granadilla Pumice and Granadilla Ignimbrite, plotting the results on a triangular diagram to show the proportion of lithics, pumice and ash.

Ex 06 - Calculation of thickness using clinometer

Estimate the thickness of the deposits at this location and contrast this with the thickness at Tajao quarry, allowing a rough estimate of the change in thickness with distance from Las Cañadas.

Ex 07 - Measurement of pumice and lithic particle sizes

Some authors suggest that the pumice should be inversely graded and the lithics normally graded, due to their differing air-resistance as they fall from the eruption column.

Ex 08 - Density of pumice and lithics

The relative density of the pumice and lithic fragments is important in determining the rate at which they fall from the eruption column.

Ex 09 - Testing of air-resistance hypothesis for falling pumice and lithics

Authors suggest that the small, dense lithic fragments and large, low density pumice fragments will both fall at the same rate as they settle out of an eruption cloud. It should be possible to test this theory practically in the field and by calculation using the data collected in exercises Ex 05 to Ex 08.

A careful examination of the base of the Granadilla Ignimbrite in the roadside cutting, should also enable students to evaluate the Sparkes et al, (1973) model for Ignimbrite formation and contrast this with the Branney and Kokelaar, (1992, 2003) model for deposition progressively from a sustained pyroclastic density current (one event with fluctuations in the properties of the flow leading to variations in the deposit).