Lesson 1 - Understanding the Pieces

The plate tectonics theory is a theory that explains the different processes that cause the formation of different geologic features and phenomena. In the 1600s, many ideas about how the continents were connected to one another were being studied. The fact that Africa South America fit themselves like pieces of a jigsaw puzzle became an evidence that these two landmasses were once part of a big whole chunk of landmass.

In 1915, Alfred Wegener, a German meteorologist and geophysicist, hypothesized and suggested that the landmasses of Earth were once merged into a supercontinent known Pangaea. About 250 million years ago, this supercontinent began to break apart and formed smaller fragments of landmasses.

The plate tectonics theory states that Earth’s lithosphere is composed of fragments or plates that move around and interact with one another. The term “tectonic” (from the Latin word tectonicus or the Greek word tektonikos, which means “to build”) has been coined to give rise to the idea of the formation or sculpting of Earth’s lithosphere.

Scientists studied the ocean floor to look for pieces of evidence that would explain continental drift. In 1968, through extensive studies, the plate tectonics theory unfolded.

The lithosphere is the outermost layer of Earth composed of the upper part of the mantle and the crust. It extends to about 80 km deep. The crust is made of various types of rocks and sediments floating on top of the malleable upper mantle that extends from the surface to about 50 km deep. It has a density of around 2.6 g/cm³.

Earth’s crust has two types: continental and oceanic. The continental crust is thicker and less dense, which makes it “float” over a denser and relatively thinner oceanic crust.

The plate tectonics theory suggests that the lithosphere is divided into fragments known as plates. These plates are divided into large plates and small plates. The seven major plates are the North American, South American, Eurasian, Pacific, Indo-Australian, African, and Antarctic Plates. The intermediate plates include the Philippine, Caribbean, Cocos, Nazca, Scotia, Juan de Fuca, and Arabian Plates.

Each of these plates moves in a slow and constant motion. The Pacific Plate, for instance moves at a rate of approximately 8 cm (3 inches) per year, whereas the North American plate is moving at approximately 3 cm (1 inch) per year.

The strengths of earthquakes range from shallow to destructive. These earthquakes can be due to collisions, rifting, or slipping between or among the plates. Around 90% of earthquakes are felt along the plate margins Earthquakes felt in areas within the plate are mostly due to stress buildup in some continental rifting zones or due to the transfer of energy from other regions.

Earthquakes release different types of seismic waves. The primary and secondary waves travel within the interior of Earth, and are responsible for the detection on the epicenter of an earthquake. Waves such as long surface waves and Rayleigh waves are surface waves.

In determining the epicenter of an earthquake, the difference in arrival time between the primary waves and the secondary waves is shown in figure 1.5a, which comes from a seismograph wave in seismic recording stations. With the aid of an S-P graph, such as the graph shown in figure 1.5b, it will tell us the distance of the earthquake’s focus to the station. If three recorded measurements were done at different stations, the triangulation method will be used to find its epicenter.

Charles Wyville Thomson, a Scottish marine zoologist, led the Challenger expedition in 1872. In this expedition, he and the rest of the members of the expedition discovered the Mid-Atlantic Ocean Ridge, a long oceanic ridge in the Atlantic Ocean, which is formed by plates that are pulling apart.

Spewing magma from the underwater region of plates that are moving apart creates and forms new materials on both sides of the diverging plates. Some volcanoes are formed from the subduction (when a plate goes underneath another), resulting from the collision of two plates. When one plate tends to move back into Earth due to its higher density than the other to which it collides, it goes down the mantle. When this happens, due to high temperature and pressure, these plates melt, and the melted rock moves up through the other plate to reach the surface, becoming a part of the volcano. Over time, the magma builds a series of volcanoes known as a volcano arc. Most of these volcano arcs are found around the region of what is called the Pacific Ring of Fire.

Harry H. Hess is an American geologist who served in the military and became the major proponent of seafloor spreading. He was the captain in charge of the expedition and research about the ocean floor under the US Navy troop that was used during World War 2. He discovered that the ocean floor is not flat or smooth; instead, it is rough and full of land features such as underwater mountains and trenches. From what he discovered, he realizes that the ocean floor is drifting apart, causing these ocean floor features.