The solid Earth consists of distinct layers, each with a characteristic chemical composition. The most abundant crustal elements, in order of abundance, are oxygen, silicon, aluminium, sodium, magnesium, calcium, and iron. The mantle is rich in magnesium, iron, aluminium, silicon, and oxygen. The outer core contains iron and sulphur. The inner core is iron, with some nickel.
Earth compositional layering
- original position
- original sin
- origination
- origin of elements
- origin of life
- origin principle of taxation
- Origins Program
- Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer
- O-ring
- Orion
- Orion Arm
- Orion Association
- Orionid meteors
- Orion Molecular Clouds
- Orion Nebula
- Orion spacecraft
- Orion Spur
- Orion variable
- Oriskanyan
- Orlando, Vittorio Emanuele (1860–1952)
- Orléans, Duc d’
- ORM
- ornithine
- ornithine cycle
- Ornithischia