Geologic Influence on the Terrestrial Ecology of Costa Rica (Final)

This discussion topic submitted by James Essex ( Essexjj@miamioh.edu) at 2:02 am on 5/17/00. Additions were last made on Wednesday, May 7, 2014.


Introduction
Many mechanisms combine to ultimately control the variables that affect the development of the Earth's biosphere. Every ecosystem is controlled, somewhat, by local, regional, and global geologic processes. Geologic processes affect ecological systems at different magnitudes of intensity and at varied spatial and temporal scales. This paper will identify the geologic processes affecting the ecology of Costa Rica and consider the impact of all processes on the geologic signature.

The approach in this paper is purely speculative because of an incomplete literature search and no means of observation and/or data collection. Specifically, this paper will address effects on ecosystems and species diversity caused by: 1) short duration, high intensity geologic events and 2) gradual, low intensity geologic processes that may be unobservable at short time scales.

Earthquakes and volcanic eruptions are short duration (seconds to days), high intensity events impacting local ecological conditions. Short duration, high intensity events also trigger landslides, fire, and effect forest fragmentation, species diversity, drainage patterns and topography. Continental drift and Milankovitch cycles are gradual (thousands of years), low intensity processes that impact local, regional, and global variables controlling ecological conditions. The gradual processes change geographic position and/or orbital characteristics and affect available solar insolation, patterns of atmospheric and ocean circulation, and sea level.


Plate Tectonic Theory
The Earth is a heterogeneous collection of matter that takes the shape of an oblate spheroid. Earth scientists have studied and labeled the Earth based on composition and density differences that exist from the core to the crust. The Earth's interior starting with the inner core composes 1.7% of the mass. The inner core is solid and hypothesized to be completely detached from the mantle. The outer core composes 30.8% of Earth's mass and believed to be an electrically conductive layer that is responsible for the Earth's magnetic field. The composition of the outer core, based on density estimations, is suspected to consist of about 10% sulfur and oxygen. The D" layer resides at a depth of 2700-2890 km, is 200 km thick, and composes 3% of Earth's mass. The layer is based on seismic discontinuities that represent a density change at that depth. The lower mantle, 49.2% of Earth's mass, is composed mainly of silicon, magnesium, and oxygen. The upper mantle, 10.3% of Earth's mass exists at depths as little as 6 km from the surface. The Oceanic crust is .099% of Earth's mass, generated by volcanic processes at the ocean floor. The continental crust is, .374% of Earth's mass, has a mean depth of 25 km. The crust is composed mostly of silicon, is floats on the mantle due to a lower density. The outer, rigid crust of Earth is called the lithosphere and the hot liquid mantle the asthenosphere [http:photo.net].

Isaac Newton postulated that the Earth once hot and cooling has caused the surface to become rigid and contraction to cause the rise of mountains. In 1596, a Flemish geographer Abraham Ortelius observing the shapes of Africa and South America, proposed that the continents were once assembled together. Antonio Snider-Pelligrini (1858) attributed the jigsaw shapes to the Biblical flood. The American Frank Taylor (1908) was the first to observe the similarities of rock types on either side of the Atlantic. The German meteorologist, Alfred Wegener (1915) compiled the evidence of others and postulated a "supercontinent" he deemed Pangea. He speculated the motion of the crust was related to the rotation of the Earth [Press, 1998].

When observation and technology caught up with pure speculation sometime around 1950 scientists used the shape of the continents, rock and fossil evidence, paleomagnetics, and ocean sediments to prove the plates constantly rearrange themselves on the Earth's surface. Plate tectonics and seafloor spreading became widely accepted after Harry Hess and Robert Dietz, using world war II sonar data collected during enemy submarine hunts, explained seafloor spreading as a driving force of plate motions. The discovery of new crust being created paved the way for the discovery that old oceanic crust is destroyed. Subduction zones are places where dense oceanic crust is pushed under buoyant continental crust and remelted in the mantle. Subduction, melting and release of volatiles explain volcanic chains arcs that exist all over Earth. Only since the 1980s have reconstructions, based on significant amounts of data, shown the paths of the continents and the ages of the ocean basins.

Current plate motion is measured using very long baseline interferometry and satellite data that has divided the Earth into rigid segments. The major plates include the North American, Eurasian, Arabian, Anotolian, African, Indian, Phillipine, Pacific, Anarctic, Nazca, South American, Cocos, and Caribbean. The boundaries between all plates are subduction zones, spreading ridges, collisional, or shear zones. The relative motions of plates often cause interaction between the huge segments of the Earth's crust. The forces behind the motion are enough to build mountains when equally buoyant pieces of the crust interact and cause subduction or destruction of plates if density differences exist.


Tectonic setting of Costa Rica
The country of Costa Rica lies on the Caribbean Plate, which moves at 9.2 cm per year in a southwest direction. The southwest margin of the Caribbean plate is overriding the Cocos Plate creating the Middle America trench that runs Northwest-Southeast (figure xx). Current thinking, suggests that terrestrial land of Central America was produced by the interaction between the Cocos and Caribbean plates [Meschede, 1998]. The subduction first produced an Island Arc system (85Ma- 20Ma); extruding lava and building land much like the volcanic hotspot of Hawaii. Land was also added as pieces of the sedimentary package, accumulated on top of the Cocos plate, was scraped off and piled up on the margin of the Caribbean plate. As material was added over millions of years, a volcanic range was produced. Shifts in the subduction angle of the plate at different periods moved the position of the arc of volcanoes either away from or towards the trench. Evidence suggests, absence of in place marine strata, that Costa Rica had a land area similar to today by 3 Ma [Johnston, 1997].

The Caribbean plate is bounded to the north and south by the North American and South American plates respectively. The interaction between these boundaries is minimal and difference in relative motion is taken up by shear zones at each boundary [Mechede, 1998].


Tectonic Expressions
Costa Rica is tectonically active; eruptions and earthquakes regularly occur due to the interaction of the Cocos and the Caribbean plates. The landscape of Costa Rica is dominated by extinct calderas and active volcanoes. The production of elevation is attributed to the volcanic centers that have extruded material over the last 120Ma [Meschede, 1998]. A less defined component of the topography is produced by uplift and subsidence events that occur due to earthquake activity. One of the major topographic features is the Andean-Sierra Madre chain that runs the length of Central America and includes the Cordillera de Tilaran of which Arenal is the major volcanic center.

Shifting volcanic centers through the last 85 Ma have continuously built peaks which are left to erode and collapse after the volcanic activity finds a new path of least resistance. The topography of Costa Rica is similar to the rest of Central America with 7 active volcanoes and 60 that lie dormant. Arenal one of the worlds most active volcanoes today but had been dormant for 4000 years then suddenly in 1968 it laid down four square miles of volcanic debris. The temporal punctuation of Arenal gives evidence for the time in which the landscape of Costa Rica was created [figure xx][www.volcano.si.edu].

Earthquakes occur daily throughout the country of Costa Rica [figure xx]. The forces related to the subduction of the Cocos plate has caused the plate to break up into large pieces, as the pieces jostle for position they build up strain which is released abruptly as an earthquake. The effect of earthquakes on the landscape includes subsidence, uplift, landslides and surface ruptures.


Other considerations
The Ecology of Costa Rica has many outside forcing factors including the orbital variation around the Sun, Solar output, atmospheric composition, ocean circulation, storm frequency/ intensity, sea level, and now human induced changes to land use. The art of separating the amount of influence each mechanism exerts on the ecology of Costa Rica would require an accurate real time model of the Earth system. Rough estimates will have to suffice and as for the effects of geology; some effects are apparent while others are blanketed by other processes.

Conclusion
Geologic processes laid the foundation for the ecosystem of Costa Rica and are major regulators on different temporal and spatial scales. The diversity and productivity of the rain forest is directly related to the atmospheric interaction with topography produced by volcanoes. A balance exists around active volcanic centers where extremely high intensity, explosive eruptions consistently redefine where the volcano ends and the ecology can begin. Changes to the radiation budget caused by gradual plate motion or orbital cycles, slowly effects plant productivity, which affects all forms of life in the ecosystem. The tentative conclusion based on intuitive speculation; the intense geologic filter placed on the ecosystem of Costa Rica promotes ecological flexibility, adaptation, and species diversity.

References
Meschede, M. and Frisch, W., 1998 A plate-tectonic model for the Mesozoic and Early Cenozoic history of the Caribbean plate. Tectonphysics 296 pg 269-291

Colombo, D. et al. 1997 Application of pattern recognition techniques to long-term earquake prediction in central Costa Rica. Engineering Geology 48 pg 7-18

Johnston, S. and Thorkelson, D. 1997 Cocos-Nazca slab window beneath Central America. Earth and Planetary Science Letters 146 pg 456-474

Press, F. and Sievier, R. Understanding Earth second Edition 1998

http://photo.net/cr/moon/the-land.html

http://www.volcano.si.edu/gvp/volcano/region14/costaric/arenal/var.html


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