By David A. Neill
The definitive treatise on the geology of Ecuador, and the only modern treatment that covers the whole country, is that of Sauer (1965, in Spanish; the German version of the same work is Sauer, 1971; see a slightly simplified version of Sauer's map on left back endpaper). An earlier work by Wolf (1892; reprinted in 1975 with commentaries by modern authors) contains a wealth of information on Ecuadorian geography as well as geology. More recent geological studies in Ecuador have mostly dealt with Quaternary volcanism (Hall, 1977; Clapperton, 1993) and stratigraphy of the petroleum fields in the Amazonian region (Campbell, 1970). Following is a brief synopsis of the geological history of Ecuador, with emphasis on events and features that bear on present-day distribution of the flora.
During the Jurassic period, about 150 million years before present, South America was still joined to Africa, forming the subcontinent of West Gondwanaland. At that time, coniferous forests were present in Ecuador, with trees related to the modern-day Araucaria forests of southern South America; evidence for this are the fossilized tree trunks of the petrified forest in the Puyango valley of Loja and El Oro provinces.
By the mid-Cretaceous period, about 100 million years ago, South America had separated from Africa and begun to drift westward. This period corresponded with the origin and early diversification of the angiosperms on all continents (Raven & Axelrod, 1974). During most of the following 100 million years, South America was an island continent. The angiosperm flora evolved therefore in relative isolation from other land areas. Connections with North America via island chains permitted migration of floristic elements between the two continents long before the final closure of the Panama land bridge in the Pliocene, only about 3 million years ago (Gentry, 1982a) or perhaps as recently as 1.8 million years ago (Keller et al., 1989).
During the mid-Cretaceous, the region that is now Amazonian Ecuador was an embayment of the Pacific Ocean. The core areas of the South American continent, the Guayana and Brazilian Shields, were to the east, and the precursor of today's upper Amazon River flowed westward, eroding the shield areas and depositing sediments in the embayment. Marine deposits in the embayment at this time produced the limestones of the Napo formation, as well as the petroleum deposits that are presently being exploited in Amazonian Ecuador (Campbell, 1970).
Marine deposits of limestone and shale were also made during the Cretaceous and early Tertiary periods in the area that is now western Ecuador. Igneous rocks were also formed in the region during these times, particularly submarine pillow lavas alternating with marine sediments. These formations were later uplifted to form the present-day coastal range.
The westward tectonic movement of the South American plate, and the collision of the leading edge of the South American plate with the Pacific plates, resulted in the upthrust of continental rock that has formed the Andes along the entire western edge of South America. The southern Andes of Bolivia, Chile, and Argentina are the oldest, with considerable uplift during the early Tertiary period about 50 million years ago, but the northern Andes of Colombia and Ecuador are relatively younger, with the major uplift begun in the Miocene, about 25 million years ago.
The base of the Eastern Cordillera of the Ecuadorian Andes is mostly Precambrian metamorphic rock composed of crystalline schists, while the base of the Western Cordillera is mostly Cretaceous volcanic and pyroclastic rock. The inter-Andean corridor between the two cordilleras is a graben, a zone where tectonic uplift did not occur. Intense volcanic activity during the Tertiary, on top of the older, uplifted rocks of both cordilleras, began to build up the Andes to greater heights. Intrusions of granitic rock, known as batholiths, took place in some zones of both cordilleras.
The third cordillera of the sub-Andean region in eastern Ecuador was also uplifted by tectonic forces. The Cordillera Galeras, in Napo province, is formed mostly of the Cretaceous limestone Napo formation. The Cordillera de Cutucú, farther to the south in Morona-Santiago province, is made partly of the same Napo formation, but also contains older Jurassic sedimentary rocks. The stratigraphy of the Cordillera del Cóndor, along the southeast border of Ecuador with Peru, is poorly known, but includes shales, limestones, and sandstones of Mesozoic and Tertiary age.
The basal Precambrian rock of the Eastern Cordillera is exposed in some areas, such as the Llanganates region east of Ambato. Cerro Hermoso, a peak in the Llanganates over 4,600 m high, is a geological anomaly of the Eastern Cordillera. It is composed of a block of Cretaceous limestone, probably the same Napo formation as the Cordillera Galeras, that was uplifted, faulted, and thrust from the east on top of the basal metamorphic rock (Kennerley & Bromley, 1971). Cerro Hermoso is therefore the only high peak in the main Andean chain in Ecuador that is not of volcanic origin.
During the mid- to late Tertiary (25–2.5 million years ago) intensive volcanic activity on top of the uplifted basal rocks of both the Western and Eastern cordilleras built up the Andes to greater heights. By the end of the Tertiary, volcanic activity ceased in the Andes of southern Ecuador. Along both cordilleras of the northern and central Ecuadorian Andes, however, intensive volcanic activity continued throughout the Quaternary period, during the past 2.5 million years. This activity produced the avenue of volcanoes that we see today—the two lines of high peaks along the Western and Eastern cordilleras from Chiles volcano on the Colombian border, south to Chimborazo on the west and Sangay on the east. Quaternary volcanic activity also produced the Reventador and Sumaco volcanoes, east of the main Andes. Throughout the Andes of northern and central Ecuador thick layers of ash from the Quaternary eruptions were deposited. Volcanic ash was also deposited on the Pacific and Amazon plains west and east of the Andes, up to about 50 km from the base of the mountains.
The Galápagos Islands, like other volcanic, oceanic archipelagos such as the Hawaiian Islands, were formed from eruptions of magma issuing through a hot spot or weak point in the earth's oceanic crust. The Galápagos are geologically very young, with much of their formation having occurred during the last 1 million years, although some areas of the Galápagos are as old as 3 million years (van der Werff, 1978).
The distribution of plant species in relation to different geologic substrates is a topic that has not received much attention by botanists in Ecuador. There appears to be some evidence for floristic differences associated with substrates in areas where calcareous limestone and more acidic volcanic rocks occur together (D. Neill, pers. obs.). For example, in the deciduous forests of the Cordillera de Chongón near Guayaquil, stands dominated by Ceiba trichistandra are common on limestone substrate, while Cavanillesia platanifolia is more common on adjacent substrates of volcanic cherts. In Napo province, the lower montane rain forest atop the limestone massif of the Cordillera Galeras at 1,500 m elevation is floristically distinct from nearby forest at the same elevation on the slopes of the Sumaco volcano. In the same sub-Andean region of Napo, rheophytes such as Matelea rivularis and Phragmipedium pearcei are found along streams with calcareous rocks but not with volcanic lavas.