Figure 1. The rugged volcanic terrain at Bosque de Paz, as seen from the Sendero Catarata Fátima. The upper flanks of Poas are lost in the cloud.

Figure 2. Hand specimen of one of the porphyritic andesite samples: in each case, part resides with the author, the bulk is at the laboratory at Bosque de Paz.

Figure 3. A basaltic hornblende phenocryst in feldspar-rich matrix, showing 120° amphibole cleavage traces and dark oxidized rim.
Magnification 50x, long-axis field of view 1.7 mm, seen in plane-polarized, transmitted light.

---

"Rock of the Month #147, posted for September 2013" ---

The Samples

In January 2010, three samples of volcanic rock were retrieved from limited outcrop in the cloud forest of the Bosque de Paz biological reserve, on the west flank of the Poas stratovolcano in the central highlands of Costa Rica. The rock is quite fresh, with only a thin film of surface oxidation. The three samples are closely similar, probably representing a single lava flow that extends at least 400 metres downstream from the vertical headwall at the valley head, the site of the Catarata Fátima waterfall.

The three samples were thin-sectioned and are all quite similar, displaying abundant glassy white plagioclase feldspar and lesser black, prismatic hornblende phenocrysts in a very fine-grained blue-grey to grey to mauve groundmass, with minor limonitic alteration on fractures. The description here is taken from Wilson (2011).

The rock is a porphyritic andesite with plagioclase feldspar plus lesser amphibole, orthopyroxene, clinopyroxene and magnetite phenocrysts and microphenocrysts, in a fine-grained groundmass exhibiting trachytic fabric (a directive texture in which small crystallites acquire parallel alignment in the magma as it flows downhill). The iron-titanium oxides are primary magnetite, variably altered to hematite, comprising 6-7 vol.% of the rock, in microphenocrysts and fine (<0.03 mm) grains in the matrix, The hand specimens are moderately magnetic, magnetic susceptibility averaging 12-18x10^-3 SI units. The phenocryst feldspar is twinned and in some cases zoned, optically estimated composition An_44-50 (calcic andesine). The amphibole is basaltic hornblende (also known as lamprobolite, technically an oxyhornblende). Feldspar and hornblende are visible to the naked eye, generally 1-4 mm in length. Other minerals are colourless orthopyroxene, clinopyroxene, and traces of turbid apatite (phosphate) and sulphides (pyrrhotite and chalcopyrite).

The Reserve and the Volcano

The ecological reserve of Bosque de Paz is located on the northern rim of the Central Valley of Costa Rica, nestled on the western flank of Poas, a large stratovolcano. This private reserve lies in Alajuela province, a straight-line distance of some 70 km N.N.W. of the capital city, San José. Bosque de Paz is noted for forest ecology, including many species of birds, mammals, and especially orchids. It offers a small and comfortable base for exploration and research (see Bosque de Paz web site for details). The 7-km trail system in the reserve is a gem. Volcan Poas and the Bosque de Paz reserve are located at approximately 10°12'N, 84°15'W.

Costa Rica boasts 11 national parks and 24 or more official protected areas, a major achievement for a small country with an area of just 51,100 km². Poas volcano and Bosque de Paz are part of the central valley and highlands, north and east of the capital.

A detailed, poetical review of Costa Rica's principal volcanoes by Guillermo Alvarado-Induni is available in English (Alvarado, 1993; Alvarado-Induni, 2005). There are more than 200 volcanic centres in the country, most dated at less than 3 million years. One third of these are in the Central Range, including Poas (Alvarado, 1993, pp.90-115; Alvarado-Induni, 2005, pp.148-186). The most significant eruptive activity at Poas in modern times occurred in 1953-1955. More recently, the hot, acidic crater lake dropped in level over a 2-year period up to April 1989, to reveal molten sulphur lakes in part of the former lake bottom. The lake bed may have heated up with removal of the overlying water, melting crystalline native sulphur in the lake sediments.

Poas is classified as a stratovolcano, a large edifice with a high width: height ratio. The volcano is 2,700 m high and 20 km wide, and may have evolved in the past 50,000 years. This is a short span in geological time, well within the Quaternary era, the past two million years, a time which covers all the history of humans and their near ancestors. Most of this period is termed the Pleistocene epoch, while the past 10,000 years or so are referred to as the Holocene or Recent.

There is a high level of historic activity from the active crater, which is 1 km wide and 300 m deep, containing a hot lake. The lavas and dominant pyroclastic rocks are classified as calc-alkaline basaltic andesites and andesites (Thorpe et al., 1981). Poas is thus typical of volcanoes in island arcs and young continental margins. Gravity measurements reveal a negative Bouguer anomaly, within which is a closed positive anomaly 2 km wide in the area of the active crater. This led geophysicists to suggest that a cylinder of solid rock several km thick may underlie the crater, rather than a shallow magma chamber.

As noted above, the volcanic edifice is large, with an area of some 300 km², and its evolution displays a range of craters, crater lakes, and fumaroles. The volcanic mud is rich in sulphur and gypsum (calcium sulphate). A range of technical publications address aspects of the volcano's geology and geochemistry. These include volcanology and geophysical characteristics (Thorpe et al., 1981); molten sulphur lakes in the modern crater (Oppenheimer and Stevenson, 1989); volcanic gases; and studies of the chemistry and mineralogy of the volcanic rocks. The seven Aguas Zarcas cinder cones lie northwest of Poas, between Poas and Platanar, another stratovolcano. These rocks exhibit the chemistry of primitive basalts. The Benioff (subduction) zone, along which the Pacific ocean slides beneath Central America, lies some 125 km beneath the Earth’s surface.

Findings beneath the Microscope:

The samples described here are all of one lithology, an andesite with feldspar plus hornblende phenocrysts and minor pyroxene and oxide microphenocrysts. Just to the east, up on the massive dome of Poas itself, a more varied suite of volcanic rocks has been described. Chemically, the lava flows and pyroclastic layers (ash falls) are classified as dacites, andesites and basalts. As a global generalization (the author did not get the opportunity to visit the summit plateau of Poas in 2010), basalts tend to be darker than andesites, with fewer phenocrysts, the chemistry richer in elements such as iron, magnesium and calcium, while the feldspar contains more calcium and less sodium. The Poas basalts show features absent from our andesite, such as orthopyroxenes rimmed by clinopyroxene, and calcic (anorthite) plagioclase feldspar (An_93-95) megacrysts (Cigolini and Kudo, 1987), large crystals which speak of an earlier stage in the evolution of the lavas, perhaps involving contamination of the dominant magma type at depth.

References

Alvarado,GE (1993) Costa Rica: Land of Volcanoes. Gallo Pinto Press / Editorial Tecnologia de Costa Rica, Cartago, 1st Engl. edition, 181pp.

Alvarado-Induni,GE (2005) Costa Rica: Land of Volcanoes. Editorial Universidad Estatal a Distancia, EUNED, 2nd Engl. edition, 306pp.

Cigolini,C and Kudo,AM (1987) Basalt-andesite relationship, Poas volcano, Costa Rica: geochemical evidence for crustal contamination. EOS 68 no.44 (AGI Annual Meeting, San Francisco), 1526.

Oppenheimer,C and Stevenson,D (1989) Liquid sulphur lakes at Poas volcano. Nature 342, 790-793.

Thorpe,RS, Locke,CA, Brown,GC, Francis,PW and Randal,M (1981) Magma chamber beneath Poas volcano, Costa Rica. Quart.J.Geol.Soc. 138, 367-373.

Wilson,GC (2011) Mineralogy of samples from Bosque de Paz (Costa Rica) and Bocas del Toro (Panama). Turnstone Geol. Serv. Limited Report 2010-07P, vi+22pp.