Manual de Plantas de Costa Rica | The physical environment | 75 
Lava flows, tuff and pumice deposits, fumaroles, crater lakes, and other artifacts of 
Quaternary vulcanism may harbor unusual florulas, and even endemic plant species, 
such as Axonopus volcanicus (Poaceae) on Volcan Rincon de La Vieja. 
Sedimentary rocks of Quaternary (Pleistocene) age, often derived from mudflows 
or river or lake sediments, occur at various sites in Costa Rica: in the northern Carib- 
bean lowlands (Rio San Juan slope), near Liberia, near Esparza, in the vicinity of Pal- 
mares and Turrtcares, and in the upper Rio Reventaz6n basin (including the Cartago 
valley). Some of these strata feature diatomites. In the General Valley, extensive, dis- 
sected Pleistocene terraces derived mainly from the Cordillera de Talamanca are rich in 
aluminum ore (and therefore of possible floristic interest). Quaternary travertine de- 
posits crop out in various localities, especially at Rio Chiquito (between Volcan Mi- 
ravalles and Volcan Tenorio) and in the vicinity of Cerro Pelado (near Cajfias). 
More recent (Holocene) alluvial and marine sediments cover older rocks in exten- 
sive portions of the country, including the Caribbean plains and along the Rio San Juan 
(Limon Basin), the Talamanca coastal region, the Rio Tempisque basin, and the lower 
basins of the Rios Barranca, Parrita, Grande de Térraba, Sierpe, and Coto Colorado. 
Geological processes continue to alter the Costa Rican landscape (and create new 
habitat) right up to the present day. Several Costa Rican volcanoes (Rinc6n de La Vieja, 
Poas, and Irazt) have erupted during the past half century, showering ash and dust over 
the adjacent terrain. Since reawakening in 1968, Volcan Arenal has emitted not only ash 
and dust, but also sizeable blocks of rock and new lava flows. Rivers continue to erode 
and deposit, and change their courses during heavy rains. Earthquakes may also play 
an important role. For example, the large earthquake of 22 April 1991, centered near 
Lim6n, exposed some 35,000 m? of living coral reefs (Tournon & Alvarado, 1997), 
augmenting the area of calciferous rock available for eventual colonization by adapted 
plant species. 
Soils 
A map depicting the distribution of various soil types in Costa Rica is reproduced in 
both Vasquez M. (1983) and Gémez P. (1986). The latter source also presents a detailed 
classification and descriptions of soils occurring in Costa Rica, with a lengthy and in- 
teresting discourse on the factors affecting soil formation in the tropics, from which 
most of the following is abstracted. 
The taxonomy of soil types is complicated and arcane, and the specifics are tan- 
gential to this account. See Sollins et al. (1994) for a cogent discussion of this topic. 
According to Gémez P. (1986), eight orders of soils and 55 principal associations are 
recorded from Costa Rica. However, soils tend to vary greatly over short distances in 
Costa Rica, and it is generally not possible to characterize large areas as dominated by 
a single type. 
