A Level Geology WJEC

The structure of the Earth and plate tectonics

Structure of the Earth

• The Earth is composed of four main layers: the inner core, the outer core, the mantle, and the crust.
• Each layer has unique properties, the inner core is a solid ball of metal, primarily iron and nickel. The outer core, which borders the inner core, is mostly composed of liquid iron and nickel.
• The mantle, the thickest layer, is composed of hot, semi-solid rock, and it drives plate tectonics through convection currents.
• The crust is the outermost layer, a thin shell where life exists. There are two types of crust: the denser, thinner oceanic crust, and the less dense, thicker continental crust.
• The boundary between the mantle and the crust is called the Mohorovičić discontinuity (Moho), and differences in seismic wave speeds demarcate this boundary.

Plate Tectonics

• The Earth's lithosphere (the crust and uppermost mantle) is broken into several large and small sections known as tectonic plates, which float on the semi-fluid part of the mantle, called the asthenosphere.
• There are two types of tectonic plates: oceanic plates which underlie the oceans and continental plates which underlie the continents and their shallow seabed.
• Plate boundary is the region where two tectonic plates are in contact. Plate boundaries may be divergent, convergent, or transform based on their motion relative to each other.
• Divergent boundaries occur when two tectonic plates move away from each other. Emergence of new crust is a common phenomena here.
• Convergent boundaries are formed when tectonic plates move towards each other and one plate is forced beneath the other in a process called subduction.
• At Transform boundaries, tectonic plates slide past each other horizontally.
• Tectonic plate movement is driven by convection currents in the Earth's mantle caused by the heat from the core.
• Tectonic plate movement causes earthquakes, volcanic activity, the creation of mountain ranges and oceanic trenches, which has a huge impact on the geographical features on the Earth's surface.
• The modern theory of plate tectonics has evolved from the early continental drift hypothesis proposed by Alfred Wegener and the concept of seafloor spreading.

Understanding these topics and fundamental geological principles will help explain the various physical phenomena around us and will support further studies of Earth's systems, resources, and history.