PlateTectonics

Plate Tectonics

The Plate Tectonics reef formation theory arose well after Darwin's theory in response to the increased knowledge of the earth's crust. Broken into many uneven pieces known as plates, the earth's crust is not a solid, unchanging mass, but instead, very dynamic. The movement of these plates against each other causes many geological events and phenomenon including the formation of mountain ranges, oceanic trenches, earthquakes, and volcanoes. The reef formation theory explains the growth, decline, and location of coral reefs by the interaction of the tectonic plates.

It is important to note that Darwin's theory and the plate tectonic theory are not incongruent. While Darwin connected the three classifications of reefs as stages of reef formation around a submerging island, plate tectonics explain how those islands came about and submerged. Reef formation theory also extends to explain the growth and decline of Darwin's reefs in response to the sea level changes caused by shifting plates, sea-floor spreading, and subduction. Healthy reefs rely on a certain level of sunlight. If the formations are slowly submerging, the corals may experience a period of upward growth to maintain consistent sunlight. If the submergence is too fast, the reef may die.

What are Plate Tectonics?

The earth's crust is broken into 9 large plates and many smaller ones. Plate tectonics are the movements of these plates towards (convergent), away (divergent), or against (transform) each other. File:/Users/smcshane/Documents/UNC Semester 2/CORALS/Tectonicplates.jpg.jpg

Convection Cells Very simply, the earth consists of a solid radiating core, a solid outer crust, and the asthenosphere, a layer of "hotter, softer, more malleable rock" between the core and crust. In this layer, rocks near to the earth's core are heated, becoming less dense. They slowly rise toward the earth's surface while the cooler rocks fall towards the core. This cycle creates slowly moving currents in the asthenosphere. These currents are the driving forces of the plates' movement, slowly shifting a few centimeters a year. File:/Users/smcshane/Documents/UNC Semester 2/CORALS/Convection("howplatesmove"2).gif.jpg

Convergent Plates Two plates that move towards each other are known as convergent. One plate dives under the other into the asthenosphere where the crust is melted and recycled. This "subduction zone" creates deep oceanic trenches. Often small islands are formed from escaping molten lava. File:/Users/smcshane/Documents/UNC Semester 2/CORALS/Convergence2(Plateboundaries).gif.jpg

Divergent Plates Two plates that move away from each other are known as divergent. As a gap forms between the plates, hot magma rises forming a ridge of new crust that spreads out into the sea floor. The Mid-Atlantic Ridge (running down the middle of the Atlantic Ocean) is the result of the divergence of the North and South American plates from the Eurasian and African plates and is the largest mountain range in the world. File:/Users/smcshane/Documents/UNC Semester 2/CORALS/EFPlateP3(http-::www.cotf.edu:ete:images:modules:msese:earthsysflr:EFPlateP3.gif).gif.jpg

Transform Plates Two plates sliding horizontally against each other are known as transform and are often known as faults. Earthquakes most commonly result from these boundaries.

Reef Formation Theory

Both the Caribbean and the Indo-Pacific are regions with significant tectonic activity. The plate tectonics theory explains the growth of ancient coral reefs in response to sea level changes. Ancient reefs submerge or emerge in regions of sea floor spreading or subduction. If the submergence is gradual, the reef may exhibit upward growth in efforts to maintain consistent sunlight.