The Origin of Earth's Structure – A Level Geology OCR Revision

The Origin of Earth’s Structure

The nebular hypothesis suggests that the solar system was formed from a cloud of gas and dust.
This hypothetical cloud, known as the solar nebula, collapsed under the force of gravity, causing it to spin and form a disk.
The solar protostar in the middle eventually ignited to become the Sun.

The distribution of elements within the Earth was largely influenced by their atomic properties.
Elements with higher melting points and atomic weights (such as iron and nickel) gravitated towards the core.
While those with lower densities and melting points (for example, silicon, oxygen, etc.) formed the mantle and crust.
As a result, the Earth has a layered structure, consisting of a dense, metal core; a mantle made largely of silicate minerals; and a thin, light crust on the surface.
The maximum density gradient is located at the core-mantle boundary, and the minimum gradient is at the mantle-crust boundary.

The Earth’s crust is divided into several different plates that float on semi-fluid asthenosphere, leading to the formation of tectonic plates.
Plate tectonic theory emerged in the early 20th century, largely championing the idea that different geological phenomena can be attributed to the movements of these plates.
Geological activity occurs mostly at plate boundaries where plates diverge, converge or slide past each other.
Observations such as seafloor spreading, the global earthquake and volcano distribution and the apparent fit of continents provided evidence for plate tectonics.

The Earth’s crust consists mainly of crystalline rock such as granite and basalt.
The continental crust largely made up of granite, while the oceanic crust primarily consists of basalt.
The significant difference in density between these two rock types leads to differing plate behaviours at convergent boundaries.
The lighter, less dense continental crust will usually ride overtop the oceanic crust, leading to the creation of subduction zones and volcanic arcs.

The geologic time scale is a system used by geologists to segment the Earth’s long history into manageable parts.
This includes eons, eras, periods, epochs, and ages for referencing events in the Earth’s history.
The time scale was primarily established through the relative ages of rock layers and the fossils they contain, but it’s now buttressed by absolute ages determined with radiometric dating methods.