The universe is all of space and the matter and energy within it. Elements not formed in the Big Bang (those heavier than lithium) are formed by stellar nucleosynthesis and supernova nucleosynthesis. Our Sun, together with over 300 billion other stars, makes up the Milky Way galaxy. Our Sun’s gravitational pull holds on to many objects that, together with the Sun, comprise the Solar System. The nebular theory explains the origins of the Solar System, which formed about 4.57 billion years ago. Through differentiation, protoplanets and large planetesimals developed internal layering. A planet is an object that orbits a star, is roughly spherical, and has “cleared its neighborhood of other objects”. A collision of a protoplanet with the Earth resulted in the Moon‘s formation. The Earth produces a magnetic field that deflects the solar wind.
The Earth System consists of the following spheres:
- The atmosphere is the mixture of gas known as air that surrounds the planet. It consists of 78% nitrogen, 21% oxygen, and 1% trace gases. The atmosphere is divided into layers named the troposphere, the stratosphere, the mesosphere, and the thermosphere. The exosphere marks the transition from air to space.
- The hydrosphere includes the oceans, surface water on land, and groundwater.
- The crysosphere is the frozen portion of the hydrosphere.
- The geosphere is the solid Earth. Land is where the surface of the geosphere is exposed to the atmosphere. The seafloor is the surface portion submerged beneath the oceans. The surface of the geosphere consists of organic chemicals, minerals, glass, melts, rocks, grains, sediment, metals, and volatiles. 93% of the Earth’s mass consists of iron, oxygen, silicon, and magnesium. Most of the rock in the geosphere consists of silicate minerals.
- The biosphere is the aggregate of all living organisms as well as the portion of the Earth in which living organisms exist. It overlaps with portions of the geosphere, hydrosphere, and atmosphere.
The Earth system stays in constant motion, driven by internal energy (heat stored or produced inside the Earth), external energy (much of which comes in the form of radiation from the Sun), and gravity.
The Earth has three principal layers – a not-so-dense crust, a denser solid mantle, and a very dense core. Pressure and temperature increase with depth in the Earth. The rate of change in temperature with depth is the geothermal gradient. The crust is only 0.1% to 1.0% of the Earth’s radius. Oceanic crust, which underlies the seafloor, is only 7 to 10 km thick. Continental crust generally has a thickness of 35 to 50 km. Oceanic crust and continental crust differ in composition. The mantle ranges in thickness from 2,820 to 2,890 km and is the largest part of the Earth in terms of volume. It is divided into the upper mantle and the lower mantle. The lower part of the upper mantle is the transition zone. Almost all of the mantle is solid rock but it is so hot that it’s soft enough to flow (though at a rate of less than 15 cm a year). Because of this softness, the mantle can undergo convection very slowly. The core consists of iron alloy (mostly iron and nickel). The core is divided into the outer core and the inner core. The outer core’s iron alloy exists as a liquid due to the high temperature. The liquid iron alloy’s rapid convective flow generates the Earth’s magnetic field. The inner core consists of a solid iron alloy. It is hotter than the outer core but remains solid because of the higher pressure.
An alternative way of thinking about Earth’s layers is in the context of the degree in which the material of each layer can flow. The lithosphere is the outer 100 to 150 km of the Earth, which behaves as a relatively rigid material. It consists of the crust plus the uppermost part of the mantle. The lithosphere lies on top of the asthenosphere, the portion of the mantle in which rock behaves like soft plastic and can flow.
—January 2021
—March 2023
Lance Jones 