Planet earth can be thought of as one large magnet, with magnetic field lines branching out from the southern hemisphere up towards the northern hemisphere. To understand how the earth naturally creates a magnetic field it is important to note that the earth is comprised of a variety of heavy metals, both liquid and solid. Each metal has different densities within the earth, with heavier metals predominantly being found closer to the earth's centre and lighter metals being found closer to the earth's surface. Looking at planet earth as an infant (4.5 - 4 billion years ago) it was essentially a hot ball of liquid rock, which is when a lot of the heavy metals were able to sink towards the earth's core, with lighter materials being pushed closer to the surface. In fact, the earth's inner core is comprised almost entirely of solid iron and its outer core is comprised mostly of freely flowing liquid iron and nickel. The reason that the inner core is solid is due to the immense pressure (3,000,000 atmospheres) and heat (5,200 °C) that forces the iron atoms into a solid atomic structure. The interactions between the liquid outer core and solid inner core is what's responsible for the formation of the magnetic field.

Electromagnetism refers to the interaction between particles that have an electric charge and is an important concept to give a further understanding of the earth's magnetic field. When a charged particle (electron) moves, it creates a resultant magnetic field. Referring to the image below, this concept can be visualized:

To visualize how a magnetic field can be formed you can imagine a directional flow of electricity (electrons moving through a TV wire) that creates a curved magnetic field around the wire. To reference this in different scenarios the "right hand rule" can be used where your thumb represents the flow of electricity and the other four fingers represent the direction of the magnetic field

The interactions between the earth's inner and outer core is what causes a magnetic field to be formed. More specifically, liquid metal that is flowing around the solid core can sometimes sink/descend into the solid centre. Simultaneously, solid metal within the inner core can be ejected to the liquid outter core. This creates a convection pattern within the earth's core, where charged liquid metal is continuously spiraling/swirling upwards and downwards in a circular pattern. Following the above diagram, whenever there is a flow of charged particles, a magnetic field is created, therefore, the cyclic flow of charged liquid metal within the core is what's responsible for the formation of magnetic field lines. Each individual magnetic field created within the core can be cumulatively added to create one large magnetic field around the earth. However, it is important to note that the magnetic field is not perfectly consistent and can become distorted in certain areas.

The blue lines in the image indicate the direction of flow of the liquid metal in the outter core. The white lines with arrows indicate the direction of the magnetic field lines based on the flow of liquid metal.

According to geologic records, there is evidence that earth has undergone magnetic field reversals, where the magnetic north and south poles have flipped. By measuring the magnetic fields within rocks of different ages, a record of earth's changing magnetic field direction can be created. More specifically, as magma/lava cools into solidified volcanic rock, the atoms within it will orient themselves in the direction of the earth's magnetic field during that time, keeping a geologic record of earth's magnetic field (refer to mid-oceanic ridge section for more information). Based on this record, the earth has undergone 183 magnetic field reversals over the last 83 million years. Satistically, these reversals are random, with the last reversal occuring 780,000 years ago (Brunhes-Matuyama reversal). It is suggested that magnetic reversals occur due to instability in the convection of the molten metal within the earth's core that eventually cause a reversal in the magnetic field. However, the exact process in which this occurs is unclear. Geologic records of magnetic reversals are challenging to come by, since reversals occur on quick timescales (∼22,000 years) relative to the earth's age. Therefore, there would need to be high geologic activity (e.g. volcanic eruptions) to obtain a detailed summary of the reversal.