Basics of Geology
Structure of the Earth
The Earth is composed of different layers, each with its own unique characteristics and composition. From a geological perspective, Earth can be divided into three main layers: the core, the mantle, and the crust (Stanley, 1999).
The study of Earth’s deeper layers is based mainly on indirect methods, as the deepest human-made borehole (the Kola Superdeep Borehole in Russia) reached just over 12 kilometers. The drillings continued from 1965 to 1995, as the Russians aimed to reach a depth of 15 kilometers but temperatures increased more than expected the deeper they got. Drilling deeper would have required more advanced equipment.
However, through the study of seismic waves (earthquake waves), rock samples, and Earth’s magnetic field, scientists can study the planet’s internal structure and composition.
Crust
The outermost layer of the Earth is the crust, which is divided into continental and oceanic crust. Continental crust is thicker (averaging 35–70 km) and lighter, composed mainly of granitic rocks. Oceanic crust is thinner (about 5–10 km), denser, and primarily made up of basaltic rocks (Gradstein et al., 2020).
The crust and the uppermost part of the mantle together form the lithosphere, which is divided into several tectonic plates. The movement of these plates causes earthquakes, volcanic eruptions, and plays a major role in Earth’s geological processes (Stanley, 1999)
Mantle
The mantle lies beneath the crust and surrounds the core, forming the largest portion of Earth’s volume. It consists mainly of minerals containing silicon, oxygen, magnesium, and iron (Stanley, 1999). The mantle is mostly solid but moves very slowly through convection currents. These currents are the main driving force behind tectonic plate movement and contribute to the formation of volcanoes and earthquakes (Gradstein et al., 2020).
The mantle is divided into the upper and lower mantle. The upper mantle includes the asthenosphere, a partially molten layer that allows tectonic plates to move (Stanley, 1999).
Core
The innermost part of the Earth is the core, which is divided into two layers: the solid inner core and the liquid outer core. The inner core lies at a depth of about 5,150 kilometers and consists mainly of iron and nickel (Dalrymple, 2001). It remains solid due to immense pressure, despite extremely high temperatures (around 5,000-6,000 °C) (Jacobsen, 2001). The outer core is liquid, and its flowing motion generates Earth’s magnetic field (Stanley, 1999).
Plate tectonics
Plate tectonics is a geological theory that explains the structure of Earth’s crust and tectonic phenomena such as the formation of mountains, earthquakes, and volcanoes (Stanley, 1999).
This is a relatively recent theory, as it wasn’t widely accepted until the late 1960s. Alfred Wegener proposed the initial theory of continental drift and movement in 1912. He was also the first to suggest that the current continents originated from an ancient supercontinent, Pangaea.
Wegener’s ideas gained traction in the 1960s when Harry Hess, J. Tuzo Wilson, Jason Morgan, and Xavier Le Pichon advanced the theory through their own research. Before this theory, people could only guess why phenomena like earthquakes and volcanoes happen.
Principle of plate tectonics
Earth’s lithosphere is divided into several rigid plates that move slowly relative to each other due to the slow convection currents in the mantle (Stanley, 1999). The speed of plate movement ranges from a few millimeters to several centimeters per year (Gradstein et al., 2020).
Oceanic plates are typically about 50–100 km thick. Of this, about 5–10 km consists of oceanic crust (basalt and gabbro), and the rest is made up of solid upper mantle material (Stanley, 1999).
Continental plates are generally thicker, ranging from approximately 100–250 km. The actual continental crust (granite and sedimentary rocks) is 30–70 km thick, underlain by solid upper mantle (Gradstein et al., 2020; Stanley, 1999).
According to plate tectonics, these plates can collide, diverge, or slide past each other, significantly shaping Earth’s surface features (Stanley, 1999; Gradstein et al., 2020).
Plate boundaries
Different geological phenomena occur at the boundaries of tectonic plates. Divergent boundaries happens when plates move away from each other, and form new oceanic crust in these regions (Stanley, 1999). Convergent boundaries happens when plates collide. One plate may be forced beneath the other (subduction). This process creates volcanoes, deep ocean trenches, and high mountain ranges such as the Andes or the Himalayas (Gradstein et al., 2020). Transform boundaries happen when plates slide horizontally past one another, causing powerful earthquakes (Stanley, 1999).
Current configuration of tectonic plates
Today, Earth’s crust is divided into seven major and several smaller tectonic plates. The major plates are Eurasian plate, North American plate, South American plate, Pacific plate, African plate, Indo-Australian plate and Antarctic plate (Gradstein et al., 2020).
Sources
Dalrymple, G. B. (2001). The age of the Earth. Stanford University Press.
Gradstein, F. M., Ogg, J. G., Schmitz, M., & Ogg, G. (2020). Geologic Time Scale 2020. Elsevier.
Jacobsen, S. B. (2001). The Hadean Earth. Science, 293(5530), 1673-1677.
Stanley, S. M. (1999). Earth System History. W.H. Freeman & Company.