2.2: Earth's Structure and the Origin of the Continents and Oceans

Forming Earth's Structure

Around 4.55 billion years ago, the brand new Earth was extraordinarily hot and mostly molten, resembling a ball of lava floating in space.

Figure \(\PageIndex{1}\): Earliest Earth.

As Earth cooled from its molten state, minerals crystallized and settled according to their densities - differentiation transformed a homogenous sphere of magma into a heterogeneous planet with a distinct structure. Metals and radioactive elements have the greatest density and sank to the center to make the core, while silicate compounds floated to the surface. Iron and magnesium-rich silica compounds of moderate density took up the space between. This resulted in three primary layers: the core, mantle, and crust. The terms lithosphere and asthenosphere are also used to describe the structure of Earth's outer layers. The lithosphere includes the solid uppermost mantle and crust, while the asthenosphere describes the semi-solid upper mantle below. These terms are particularly important in the context of plate tectonics which is discussed in detail elsewhere in this book. For now, consider it the theory that Earth’s rigid outer shell, the lithosphere, is broken up into individual pieces called plates, that move and interact with one another along plate boundaries.

Figure \(\PageIndex{2}\): The structure of Earth. The crust, mantle, and core are layers with distinct compositions. Lithosphere and asthenosphere have different physical properties.

Origin of the Continents

The first continental crust formed shortly after the earth's origin, but continents, as we know them today, and the plate tectonic processes that build and shape them did not happen until much later. Initially, crustal pieces were continually split apart by heat rising from Earth's very hot core. Earth's surface may have resembled the lava lakes we can see today within the crater of a few volcanoes.

Lava Lake Video

Watch the video from 1:31 to 2:03.

Origin of Earth’s Water

The origin of surface water on Earth is still uncertain. Explanations for its origin include volcanic outgassing, comets, and meteorites. The volcanic outgassing hypothesis for the origin of Earth’s water is that it originated from inside the planet, and emerged via tectonic processes as vapor associated with volcanic eruptions. Since all volcanic eruptions contain some water vapor, at times more than 1% of the volume, these alone could have created Earth’s surface water. Another likely source of water was from space. Comets are a mixture of dust and ice, with some or most of that ice being frozen water. Seemingly dry meteors can contain small but measurable amounts of water, usually trapped in their mineral structures.

Where Did Our Water Come From? Video

Watch the video from 4:47 to 10:03.

During heavy bombardment periods later in Earth’s history, its cooled surface was pummeled by comets and meteorites, which could be why so much water exists above ground. There isn’t a definitive answer for what process is the source of ocean water. Earth’s water isotopically matches water found in meteorites much better than that of comets. However, it is hard to know if Earth processes could have changed the water’s isotopic signature over the last 4-plus billion years. It is probably that all three sources contributed to the origin of Earth’s water.

Figure \(\PageIndex{3}\): Water vapor leaves comet 67P/Churyumov–Gerasimenko.

Key Terms

• asthenosphere - the layer beneath the lithosphere, which is in a plastic, semi-solid state
• lithosphere - the outermost layer of Earth that includes the crust and solid upper mantle; this solid, rock-layer, is broken up into individual pieces called plates
• plate tectonics - the theory that Earth’s rigid outer shell, the lithosphere, is broken up into individual pieces called plates, that move and interact with one another along plate boundaries, driving Earth’s constructive geologic processes