Science Behind the Changing Earth

The advancement and knowledge on the plate tectonics theory was a very important asset to biogeography, it provided a foundation for our understanding of our ever-evolving planet. The theory provided our understanding on the landmasses that inhabit this world as well as the ocean basins. Paleontologists refer to the total of Earth’s dynamic as TECO events. This chapter describes more in-depth about the Earth’s dynamic structure and how plate tectonics affected biotas.

Starting with the geological timescale, we use a timescale to date history the groups consist of eons, supereons, periods, era, ages, and epochs. Geologists identified the different layers of sedimentary origin and the different fossils within them which helped create the periods of history. The fossils were reliable for the correlations in species and lifestyles for that time as well as showing the scientists how the environment at the time was. They refer to this as index fossils or guide fossils. Carolus Linnaeus in the 18th century noted that the Earth was only a few thousand years old at the time. However, in the 19th century, Alfred Russel Wallace estimated that the Earth was about 400 million years old. In the 20th century, radioactive material was developed and was more reliable. The radioactive material is unstable and as they are used, they decay releasing atomic particles. The rate of decay of each element is known as the half-life. By examining the rate of decay, the estimated age of the Earth can be calculated. By using this product the Earth’s estimated age was 4.6 gigannums, billions of years. Another useful tool to date the Earth is radiocarbon dating or organic material. There’s also tree-ring dating and luminescence dating to estimate the time since burial. Luminescence dating relies on the principle that when exposed to light stimulation and the amount of luminescence released by each quartz. It represents a function of total radiation-exposed at the surface and the amount of time elapsed since the burial.

As expressed in chapters before, the plate tectonics theory explains the origin and the destruction of the Earth’s plates and their movements. In the early 19th century, Charles Lyell noticed fossils found in Europe indicated that there was once a tropical climate in that region and that the Earth had cycles of different climate change. Lyell hypothesized that the Earth’s shell was rigid and at times rotated independently, compared to Benjamin Franklin’s hypothesis that the shell was soft and floated within the fluid. Lyell had a theory over the Earth’s seasons that at various times the continents were near the equator and at other times, the continents were closer to the poles. On the other hand, Antonio Snider-Pellegrini suggested a geometric fit of the coastlines referring to the Earth at one point having one supercontinent. Alfred Lothar Wegener, a German meteorologist, developed a theory of continental drift in 1910 after a trip to Greenland. He suggested that the east coast of South America fits exactly against the west coast of Africa. Over time many scientists ignored his theory, others wanted to dive further. Over a decade, a fourth edition of the theory came out and is widely used still today. The edition contains more information about plate tectonics but still has Wegener’s ideas and theories over the topic. There are about six conclusions about how the landmasses moved.

Stratigraphic, paleoclimate, and paleontological evidence helped support Wegener’s theory of continental drift. Stratigraphic evidence included topographic features and fossil deposits were found along the connections that Wegener hypothesized. The paleoclimate evidence showed that all the continents in the Southern Hemisphere have late Paleozoic glacial deposits. As the glaciers move, they leave scratches that located the direction they moved. Many glaciers are in warmer latitudes and have even risen out of the sea. In 1969, D.H. Elliot and E. H. Colbert discovered tetrapod fossils in Antarctica that belonged to Lystrosaurus, a reptile mammal-like creature that was also found in southern Africa and southern India. Many other fossils were found throughout the glacial movement. Discoveries were made about the ocean basins and rock magnetism after World War II that made scientists look over Wegener’s theory again. When Wegener first created his theory, there wasn’t much information or research about the structure of the ocean floor. Before World War II broke out, oceanographers were just starting their research; ocean topography was their main goal. During the war, Harry Hammond Hess discovered flat-topped submarine volcanoes also known as guyots. Peaked submarine volcanoes are referred to as seamounts. After the war ended, many discoveries were made by deep-sediment piston corers and explosive charges. They discovered ancient foundations being dated back to the Archean eon, which was about 4 billion years ago. In this eon, when the plates were moving downward, the continental crust was being formed by basaltic material melted and formed small protocontinents.

By the 1950s geologists began to interpret mid-oceanic ridges as zones, oceanic trenches, and the Ring of Fire. The Ring of Fire surrounded the Pacific Ocean in a belt-like fashion of volcanism and earthquake activity. It was hypothesized that the trenches in the ocean were a direct cause of the geological events. Hugo Benioff was the first to provide evidence for the hypothesis. He plotted positions and depths of earthquakes epicenters in the region of the trenches. Benioff concluded that the closer the epicenter is to the trenches are less lethal than the epicenters that are farther away from the trenches. The epicenters are along the zone positioned at a 45-degree angle behind a trench. The earthquakes are cold and rigid and are known as Benioff zones. To describe the spreading of the seafloor, paleomagnetism provides more evidence for this. In the early 1950s, P.M.S. Blackett invented a magnetometer to determine the continental orientation throughout history. The magnetometer was used to show how the British Isles rotated 34-degrees clockwise since the Triassic period. This concluded that Europe and North America have once been joined together. During the 20th century, Bernard Brunhes discovered magnetic reversals which are changes in orientation of the Earth’s magnetic field. Shifts in the Earth’s magnetic field also can be associated with climate change (cooling). Frederick Vine and Drummond Matthews discovered three properties of the ocean floors. First, the basaltic rocks that are located the midoceanic ridges have normal magnetic properties. Second, the widths of alternating magnetic stripes on opposite sides are usually symmetrical and the stripes are parallel to each other along the axis of the ridge. Lastly, the stripes of any ocean closely match that of others and they correspond to reversal timetables from terrestrial lava flows.

The theory of plate tectonics is still a widely researched topic today. The lateral movements of the plate come from the underlying mantle and its core, where the intense heat moves the plates. Geologists have developed a wonderful understanding of plate movements, interactions, and related events such as volcanic activity and earthquakes. The current model of plate tectonics illustrates three forces that can be responsible for the movements this includes, ridge push, mantle drag, and slab pull. Plate boundaries can develop in three different forms, divergent boundaries, convergent boundaries, and transform boundaries. Most early supporters of the continental drift theory concluded that it was prior to the Mesozoic period and all the landmasses were joined together as the supercontinent we all know as Pangea, which took up one-third of the Earth’s surface. After the separation of Pangea, there were then two supercontinents; Laurasia, the northern supercontinent and Gondwana, the southern supercontinent. Gondwana included today’s South America, Madagascar, India, Africa, Australia, and more. It began during the Precambrian supereon and not completed until the Cambrian period. The breakup of Pangea began about 200 million years ago at the end of the Triassic period. As the continents drifted towards new climates and regions, it resulted in reduced genetic flow and allowed rapid speciation. Pangea was initiated when the Turgai Sea started to expand southward splitting the northern regions of Eurasia from North America. Pangea separated into three landmasses that continued to break up into smaller continents throughout the Jurassic to mid-Cretaceous periods.

Throughout the rest of the chapter, the book goes into discussing the tectonic development of marine basins and island chains. The marine basins were developed forming one ocean during the time when Pangea was joined together. As Pangea split, the ocean floor began to fragment as well. Epeiric seas are formed when the sea levels rise and oceans flood the continental plates. They usually act as barriers to terrestrial organisms. Panthalassa once covered two-thirds of the planet which after the breakage, lead to the formation of the Pacific ocean which contains hotspots and triple junctions. A hotspot has produced volcanic activity in the Hawaiian Islands. The Hawaiian Islands include the submerged Emperor Seamount chain. Volcanic eruptions can be formed by triple junctions; this is when the boundaries of three plates meet. These islands, Azorean, Nubian, and Atlantic plates are developed near the mid-Atlantic ridge.