Igneous rocks.
Igneous minerals form as atoms combine from a cooling magma. In fire-borne,
pyroclastic rocks, the igneous constituents may largely be of glass. When
a frothy magma ascends through the neck of a volcano, the magma chills
into glass. Therefore, at the surface of the volcanic neck volcanic glass
full of bubbles is formed (pumice). However, this rock is broken to pieces
and air-borne as the volcano explodes. A gray cloud of broken pieces of
glass (glass shards) forms a volcanic ash that rains to form pyroclastic
materials. Upon falling on the ground, the ash might be welded to produce
tuff. Rock fragments and pumice fragments will be part of the materials
ejected along with glass shards, and these will be part of the pyroclastic
record. When the volcano is not very explosive it might eject liquid melt
that forms gravely (lappili) rock that fall on the sides of the volcano.
On the other hand, the magma might flow gently on the surface to produce lava rock such as dark basalt, or light colored rhyolite or intermediate colored andesite. Andesite and rhyolite generally form volcanic domes. The volcanic arc of Costa Rica is dominated by andesitic volcanic rocks (flows, and pyroclastics). Basalt does not form domes. Typically basalt flows readily, unless it is ponded in a local depression. In such ponds, as the lava cools, it forms columnar joints. These joints are polygonal sided in cross-section, and the columns might be straight or curved. Basaltic lava might flow under a body of water and produce pillowed (ellipsoidal) structures. The pillows are fractures , and fractures are healed by crytallization of calcite, zeolite, and or quartz.
Sedimentary rocks:
When sediments (particles, grains, or minerals) are naturally glued
together sedimentary rocks are formed. The sedimentary particles are derived
from a variety of environments to produce different sedimentary rocks.
A mineral called calcite precipitates from water. Marine organisms die and their shells become grains. Calcite and shells can be naturally glued to form a rock type called limestone.
Chert is formed from the precipitation of silicon dioxide in a marine setting.
All rock types can be weathered, eroded, and transported to a place where the sediments settle down. When these are naturally glued clastic sedimentary rocks are formed. There are many types of clastic sedimentary rocks. They are classified in different ways. When classified on grain size they are called:
1. Conglomerate . The size of the grains in the rock is > 2
mm in diameter (gravel) .
2. Sandstone. The grain size ranges from .063 to 2 mm. The
grains are visible to the necked eye.
Sandstone is further subdivided into graywacke, orthquartzite and
arkose, depending on the type and amount of grains:
Graywacke is composed of small pieces of rock fragments and quartz. Where
the grains are dominantly of broken pieces of volcanic rocks, it is called
a volcaniclastic rock. The sandstones of Costa Rica are graywacke.
Orthoquratzite is composed dominantly of quartz. The sandstone at
Lookout Mountain, Tennessee, is of this type.
Arkose contains potassic feldspar and quartz, and is formed close
to the source rock.
3. Shale. This is mud rock composed of clay minerals, with
sizes <.063 mm in diameter
Due to underwater slumping, a mud slurry flows down slope driven by gravity (turbidity current) only to settle at the toe of the slope. Here clastic grains settle down with the coarsest first followed by medium-sized grained and then by fine- grained particle. In this way a sedimentary bed shows graded bedding or size-grading. Such size-graded beds are called turbidites.
Metamorphic rocks.
There are many types of this rock. They are produced as a rock interacts with new conditions. In Costa Rica this interaction involves with hot fluids (hydrothermal fluids). Thus, hydrothermal metamorphism changse the minerals of the rock.
Most profoundly, a rock changes it nature when that rock is placed in
areas of increased pressure and temperature. Such metamorphism has altered
the rocks of the Carolina Terrane.
Layered earth
The earth is layered into the core, the mantle, and the crust. The
earth belches from time to time and produces volcanoes with associated
gases including water vapor. The vapor cools to form oceans, lakes, and
other bodies of water over millions of years of course. The atmosphere,
the water layer (hydrosphere), and DNA-bearing compounds that replicate
and evolve are essentially produced by the belching of the earth.
We conclude that the earth is layered, from its center outward, into
the core, the mantle, the crust, and the atmosphere.
The crust is roughly 10 to 50 km thick. If we include the topmost
mantle until we get about a 100 km thick layer of rock, that rock layer
is called the lithosphere. It turns out that the earth has a lithosphere
beneath which there is a partly molten zone known as the LVL (low velocity
layer, or asthenosphere, Fig. 4)
In a way, the earth is like an egg with the eggshell corresponding
to the lithosphere. If we crack the egg, its shell will have solid fragments
that are separated by fractures. Similarly, the earth's lithosphere is
broken into solid fragments that are called plates, bounded by fractures
that are called plate boundaries.
Neighboring plates might move toward each other and converge on each other. The boundary that demarcates neighboring converging plates is called a convergent boundary. Other types of boundaries include divergent and transform plate boundaries; but we will focus on the convergent type. When plates collide, a plate might be denser than its neighbor, and the denser plate might slide under (be subducted) and move into the LVL beneath the lighter plate (Fig. 5). Mind you, the LVL is partly molten because it is very hot. Thus, as the edge of a descending plate enters into the LVL it is cooked and ultimately it will melt. Molten rock, magma, ascends through the crust of the overriding plate and creates a volcanic edifice, a mountain chain or a cordillera. That is how the Andes Mountains are born. That is how the "Cordilleras" of Costa Rica are born. We are going to Costa Rica to see the whole deal. However, we should remind ourselves that plate movements are luckily very slow ( moving at about 1 to 10 cm per year), and so most of what we will see are materials that were produced over a number of years, perhaps. Still, we will see some action in the form of volcanic effusion of rocks, ash, and gases. We shall visit strombolian eruption at Arenal volcano, and gas emission at either Poas or Irazu. We might even be treated to an earthquake, wow!
Obviously one plate will not slide beneath another like a knife goes through butter, We are talking here about a rock layer (lithosphere) being pushed under another. It is therefore sensible to assume that there is going to be friction associated with storing and releasing of energy, as the mighty lithospheres negotiate the manner of their movement. When energy is released, it radiates in all directions and causes the shaking of planet earth. That is why we call it an earthquake. for all we know, earthquakes might be the way in which planets communicate with each other! If so the communication can not be by sound, because sound is very slow. Rather it should be the vibration that should be detected. At any rate, that is what seismographs and geophones detect.
Topography, Physiography, lithotectonic provinces:
It is an oceanic lithosphere that will descend beneath another oceanic lithosphere (to form an island arc), or beneath a continental lithosphere (to form an Andean type mountain chain). Regardless, a characteristic topography is developed above a descending lithosphere in a convergent setting (Fig. 6).
The topographic features are as follows.
A. Trench. A depression, called a trench is developed in the ocean at which the neighboring plates converge. See Figure 6 for a section view and Figure 2 for a map view. In the map view, the Middle America trench marks the location of this feature. The same trench is sometimes called the Central American Fault (Fig. 7)
B. Forearc Ridge. Landward from the trench is a region of low ridges called the forearc ridge (Fig. 6). In Costa Rica this corresponds the peninsular belt, on the Pacific side of the country (Fig 7).
The rocks of this belt are generally dark colored and include pillow basalts. Pillow basalts are ellipsoidal piles of basalt that are so shaped due to the flowing of lava under a body of water. Typically, these pillows are fractured by hot steam interaction and filled by white crystals. Occasionally, gravel, pebble, and cobble-sized rock pieces produce a rock called volcanic agglomerate. Lava pools that may be ponded in certain regions, might upon cooling develop columnar structures that geologists call columnar jointing. Yellowish green minerals in these rocks might be the mineral epidote, while deep green might be olivine. The dark minerals are pyroxene and amphibole. Black sands mark the beaches where these dark rock are the dominant rock type.
C. Forearc Basin. The forearc basin is farther landward (Figures 6 & 7). It is a depression and one that may be penetrated by the pacific ocean to form gulfs, such as the Nicoya Gulf (Fig. 1). Rivers flow down the axis of the depression and empty into the ocean. The Tempisque River does that. Other rivers flow both from the forearc ridge on the southwest and the volcanic arc on the northeast (Fig. 3).
Rocks of this basin include sedimentary deposits consisting of volcanic grains (volcaniclastic rocks), and rocks called graywackes that consist of grains of pieces of rock and minerals, limestones composed of calcite and fossils, and igneous rocks such as volcanic ash deposits, and other fireborne rocks or pyroclastics such as tuffs or lapilli tuffs. Pyroclastic rocks are mainly light colored to grayish. Some cinder cones and piles of lapilli are red.
Some rocks were deposited before the region was exhumed from the oceanic water. While under the ocean, some sedimentary turbidites were deposited slumping processes. Turbidites are marked by size grading with coarser grains at the bottom and fine grains at the top of beds. In a stacked sequence of beds, each bed will exhibit size grading .
D. Volcanic Arc. The volcanic arc is farther landward still.
In Figure 6, this is shown by the high topographic expression. Volcanoes
lie along the center of the volcanic chain (Figures 2 &7). But the
volcanic chain is limited to Northern Costa Rica, the Cordileera Central
( Gunacaste and Tilarran cordilleras). There are no volcanoes in Southern
Costa Rica in the Cordillera Talamanca.
The Rocks in the volcanic arc of northern Costa Rica are generally light colored volcanic rocks. They include, agglomerates, cinders, pyroclastic falls and flows. Minerals include dark phenocrysts of pyroxene in andesite or andesitic pyroclastics. In the Talamanca igneous rocks include gabbros, monzonites, and diorites.
E. Backarc Basin. The backarc basin lies behind the volcanic arc. In Costa Rica this corresponds to the Limon Basin (Figures 1 & 7).
In summary, the landmass of Costa Rica has four belts. From the Pacific to the Atlantic Ocean, they are: the forearc ridge (Peninsular Belt), forearc basin (the Tempisque-Terraba Basin), the Volcanic arc (Guancaste-CentraL, and Talamanca cordilleras), and backarc basin (Limon Basin).
We shall examine rocks in these belts. We should ask ourselves the following question. If we scraped off the topographic expression of Costa Rica and turned it into a flat land, can we distinguish the 4 belts that are described above:? Can the belts be distinguished by their rock attributes? If we can, then it should be possible to examine ancient regions that have been worn down to flat lands by millions of years of erosion. Therein lies the reasoning behind using Costa Rica as a modern analog for the ancient Carolina Terrane.
You see, while time and place are not interchangeable, we can boldly
state that what happened in a place a long time ago is similar to what
is happening currently in another place. This is just a restatement that
natural laws and laws of physics are time independent, and forms the basis
of the Uniformitarianism paradigm of geologists.
**
Plate tectonic reconstruction of Costa Rica.
There is more to the geology of Costa Rica than what we just touched
upon above. North and South America were juxtaposed to form Pangaea until
some 200 million years ago. The two continents split apart, and a part
of South America (the Chortis block) was tectonically sliced (Fig. 8) and
was appended to North America much later. By 80 million years ago, the
Chortis block was put in place to form the
Central American region from Mexico to Nicaragua. But Costa Rica and Panama
have an entirely different origin. There was no Costa Rica 150 million
years ago. The oldest rocks of Costa Rica are found in Santa Elena Peninsula,
and they are about 80 million years old ocean floor rocks. Most of Costa
Rican rocks are much younger, and they were formed under water. Costa
Rica started to be assembled when an oceanic lithosphere was subducted
beneath another. An under water volcanic edifice began to form. That
was taking place tens of kilometers west of the location modern Costa Rica
(Fig. 9). Eventually, the under water volcanic edifice emerged as an island.
That is why we say that Costa Rica is an island arc. That island arc (Costa
Rica along with Panama) was transported, by plate tectonic movement, and
fitted between North and South America. Let me emphasis the point by stating
that no dinosaur, however enterprising, could have set foot on Costa Rica.
Therefore, the Caribbean Plate (Fig. 10) is a young and restless region.
Figure 11 shows the tectonic movement of the Cocos Ridge in the last 8
million years (Ma).
The Cocos Ridge is subducted beneath Southern Costa Rica and it produce
the following effects.
1) It lifted the Talamanca to higher elevation so that the volcanic
rock cover was eroded away and deposited in the Caribbean Sea.
2) North and south Costa Rica are separated by a plate boundary called
CRTF (Central Costa Rica Transcurrent Fault , or ENZF (East Nicoya Fracture
Zone - Fig. 2). The fore arc may also be segmented into northern, central
and southern sectors depending on the degree of tilting of rocks, which
in turn is dependent on the buoyancy of the subducted plate (Fig. 2).
The ENZF is a transform fault similar to the San Andreas Fault of California.
In time northern Costa Rica will move farther into the Pacific Ocean. (
Of course this does not pay attention to any binding Security Council resolutions
of the United Nations.)
**
Notes on Recent Research on Costa Rican Geology:
Research in Costa Rica is approached from a variety of subdisciplines
of Earth Sciences including seismology, gravity methods, volcanology, geomorphology,
sedimentology, structure and tectonics, as a way of elucidating how an
"island arc" continues to evolve. Interesting discoveries include that
during strong earthquake (seismic) events beneath Peninsula de Nicoya the
Pacific Ocean recedes and exposes large parts of the beach. However, the
ocean gradually, over decades, reclaims part of that beach. The earthquake
is generated as the ocean floor (Cocos Plate) is subducted beneath northern
Costa Rica (Caribbean Plate). That suduction flexes the peninsula and
causes it to decline to the east toward the arc. Essentially, during such
coseismic deformation the northwest coast of Peninsula de Nicoya is uplifted,
which in relative terms is similar to the Pacific ocean receding. Due
perhaps to horizontal squishing of the underlying asthenosphere, the western
end of the flexed lithosphere gradually declines (by isostatic adjustment)
toward the Pacific Ocean. Thus, the lithosphere is warped into a gentle
anticline (Fig. 12). This warping produces a crest that serves a s a drainage
drive (fig. 2). Moreover, the peninsulas on the pacific shore have beach
terraces (Fig. 13), with older beaches lying more inland and at higher
elevations compared to younger ones. The older beaches generally decline
arcward on the southeast part of Peninsula de Nicoya, and oceanward on
northwest side of the peninsula.
The Appalachians are a mountain belt of Eastern USA. The Physiographic
Provinces of Southern Appalachians, from west to east are (Fig. 14)
Cumberland Plateau: bounded on the east by the Eastern Cumberland Escarpment
Valley and Ridge: bounded on the east by the Great Smoky Mountains
Blue Ridge: bounded on the east by the Brevard Fault Zone
Piedmont: bounded on the east by the Fall Line.
Within these Provinces lie the Hayesville Fault, and the Central Piedmont
Fault (Fig. 14 )
The Hayesville Fault divides the Blue Ridge Province into the Western
and Eastern Blue Ridge. It is believed to be a suture at which separate
terranes were welded by plate convergence. The terranes were North America
and an island called EBRIP (Eastern Blue Ridge and Inner Piedmont (Fig.
14 ). The Hayesville Fault is also called a Taconian Suture, referring
to the mountain building process (435 million year ago) that produced the
Taconic Mountains of eastern New York State.
The Central Piedmont Fault divides the piedmont into the Inner Piedmont and the Eastern Piedmont (Fig. 14 ). It is believed to be an Acadian suture that juxtaposed the IP Terrane with the rest of America (Fig ). It refers to a mountain building process (350 million years ago) that produced Acadia , an old French term for the Canadian Maritime Provinces,
Another suture is the Brunswick-East Coast Magnetic Anomaly (Fig. 14). This is the Place were North America and Africa were welded together during the Alleghanian Orogeny. The Allegheny Mountains of Pennsylvania are used for these orogeny.
The region east the Central Piedmont fault is called the Carolina Terrane.
It included the Inner Piedmont and the Coastal plains. The Carolina Terrane
is believed to be an island arc (Fig. 15).
Tectonic cycles
Continents assemble to form a large supercontinet. The latest supercontinent
is called Pangaea. Pangaea existed from 245-320 million years ago.
The supercontinet just before Pangaea is called Rodinia. It lasted from 700 to 900 million years ago. There were many other supercontinents before then. The main point to understand is that continents assemble, and they break up and drift away from each, only to reassemble at a later time. The time from the break up of a supercontinent to the formation of a subsequent one is called a tectonic cycle.
As Rodinia began to break up, the eastern part of North America was segmented into blocks. One block drifted away to a far off location separated by an ocean (the Iapetus-Theic Rheic). Another block (EBRIP) was merely separated by a marginal sea from the North American block (Fig. 15). In the Ocean east of the EBRIP island, there were other islands. One notable island was an island arc ( IP Terrane) that was formed due to a collision of neighboring ocean floors. The break up of Rodinia and the creation of these islands constitutes the early and middle phases of a tectonic cycle. In the late stage of the tectonic cycle large continents collide after the ocean floor that separated them is destroyed by subduction. However, before the continents collide, islands will be welded to the edges of continents. Thus, the islands next to North America, EPRIP and EP (Fig. 15) became welded before Africa and America collided to form Pangaea.
Each time a piece of land collides with another a mountain belt (an orogen) is formed. The first orogen is the Taconian; the Hayesville Fault (e.g. near Allatoona Lake on I-75 , Georgia) is the place where the landmasses were juxtaposed. The second orogen was the Acadian; the central Piedmont Fault is the place where the Carolina Terrane was juxtaposed with north America. The last orogen was the Allaghanian, The East Coast Magnetic Anomaly was where Africa and America were welded.
You see that Appalachians were formed by three mountain building episodes (orogenies), the Taconian (435 million years ago), Acadian (350 million years ago) and Alleghanian (280 million years ago). The Carolina Terrane was welded on to north America about 350 million years ago, during the Acadian orogeny. The Carolina Terrane was developed above an east dipping subduction zone, so that the forearc ridge rocks are close to the central Piedmont Fault.
Costa Rica & Carolina
We go to Costa Rica as a quick stop for seeing the geology of the Carolina Terrane.
The trip helps us gain an appreciation of the Uniformitarianism Principle. The ancients taught us that "The present is the key to the past". Indeed it is. However, how are we to fathom the meaning of this paradigm unless we extract relationships by simultaneously grappling with the modern and the ancient. As always, we need to realign our axioms and see if we can comprehend ideas that may be new to us. As we do so, we find that what was happening in a place a long time ago can be currently happening at another place. This is a restatement of Uniformitarianism; isn't it? Or, are we saying that natural laws and laws of physics are time independent?
Natural laws are obeyed and things do change. However, the rate and
amount of the change, in other words the response to the natural laws,
maybe modified by environmental conditions. In Geology we worry not only
about natural laws but also about the vicissitudes of responses.