17.2: Events that shaped the Hadean

Differentiation of Earth’s Interior

What evidence do we have of Earth shaping events occurring in the Hadean? Planetary geologists theorize that toward the end of the accretionary stage in Earth formation, 4.5 – 4.6 Ga, Earth was pummeled by larger, planetesimal-sized space debris. Planetesimals were other “wanna-be” planets traveling in the same orbit as the developing Earth. They were massive in their own right, but the developing Earth was larger and able to withstand their impacts and remain intact. The energy of these massive collisions melted the surface of the young planet resulting in the formation of vast “magma oceans.” Energetic radioactive decay of unstable elements added to the heat production from inside the early Earth. This double whammy of interior and exterior heat generation may have melted the entire planet, or turned it into a thick, slushy mass of highly convective molten rock material.[1], [2]

This allowed the Earth, and other developing planets in our solar system, to go through a process of differentiation where the heaviest, siderophile (iron associated) elements migrated toward the central core while the lighter lithophile (lithosphere concentrated) elements rose toward the surface. Differentiation happened very quickly with respect to the immensity of geologic time – over the span of several tens of millions of years. Differentiation was not a wholesale turnover of the entire interior of the planet but more like a percolation of siderophile elements through the slushy magma ocean (see image below).

Figure \(\PageIndex{1}\): Hypothetical core-mantle differentiation processes: Percolation, diking, and diapirism. After Rubie et al. (2015). (CC BY-SA 4.0; By AlexInMetal – Own work, on Wikimedia.)

Analysis of seismic waves produced by earthquakes has helped us understand the density differences and relative composition of the materials that exist within the modern Earth at different depths. These seismic wave velocity analyses over almost the past 100 years have allowed us to see a very clear picture of Earth’s layered interior [4].

Figure \(\PageIndex{2}\): The radial structure of Earth’s interior. The diagram details the concentric shells within Earth and their approximate composition. These were established over the first half of the twentieth century from measurements of the travel times of seismic waves refracted and reflected inside Earth. Each abrupt shift in seismic wave velocity a phase change (liquid/semi-solid, solid), and a compositional change. (Modified from [5].)

Earth’s first crust

As differentiation proceeded, the light-element-enriched magma ocean at the surface was exposed to the chill of space and a thin, early crust began to form. Meteors, asteroids and comets continued their impact unabated, puncturing the earliest crust and allowing the magma, hidden just beneath, to flow again. The composition of this earliest crust was similar to very rare ultramafic rock komatiite, a volcanic rock largely composed of the mineral olivine. Komatiites are very rare in Earth’s rock record, almost entirely restricted to rock of Archean age.[6] The komatiites were largely derived from the very hot mantle of the young Earth, and pooled on the surface as lava. As Earth continued to cool and the mantle solidified, generation of highly ultramafic magma that has made its way to the surface has been rare.[7]

A quick review of magma composition

Differentiation produced our iron-dominated inner and outer core and silicate-dominated rock material of the mantle and ultimately, the crust. Silicates are minerals that are largely composed of the elements of silicon and oxygen bound together with other lithophile elements including aluminum, calcium, potassium, sodium, and magnesium. Not all of the iron migrated to the core. In the early Hadean, the Earth was continually being bombarded by space debris that contained all of Earth’s natural elements in varying percentages. We can think of the composition of the magma ocean as being silica (silicon and oxygen) dominated and relatively homogeneous. The first crust, composed of komatiite, represented this composition. This convecting ocean of silicate mush brought diapirs (rising teardrop shaped blobs) of deep magma ocean material toward the surface where it pooled and cooled in thick komatiite masses. These heavy, dense komatiite masses dripped back into the magma ocean for recycling. This remelting process further differentiated the early crust. Eddys of convecting magma near the surface partially melted the komatiite. This “partial melt” was more silica enriched because the minerals in the komatiite with the most silica would be first to melt (see discussion on Bowen’s reaction series to understand this process). This is referred to as an “evolved magma,” not primary, but remelted and more silica enriched. Silica enriched magmas are lower in density and more buoyant and can be envisioned as rising to the surface like ice cream in a rootbeer float. This evolution of magma leads to the varying compositions we recognize today in the classification of igneous rock. Notice where komatiite appears on the diagram below. As magma evolution proceeds, the composition becomes more silica enriched, creating magma that becomes increasingly more “felsic” in composition.

Figure \(\PageIndex{3}\): Igneous rock classification diagram.

Did I Get It? - The Hadean Quiz

Exercise \(\PageIndex{1}\)

The early Earth grew by adding space debris and planetesimals through the process known as

a. convection

b. differentiation

c. accretion

d. percolation

Answer

c. accretion

Exercise \(\PageIndex{2}\)

During the process of differentiation, the _________________ elements migrated toward Earth's core while the __________________ elements rose toward Earth's surface.

a. siderophile, lithophile

b. light, heavy

c. least dense, most dense

d. lithophile, siderophile

Answer

a. siderophile, lithophile

Exercise \(\PageIndex{3}\)

Earth's first crust had the composition of which type of igneous rock?

a. granite

b. komatiite

c. basalt

d. felsic

Answer

b. komatiite

Exercise \(\PageIndex{4}\)

What have scientists studied for more than 100 years to learn about the layered interior of Earth?

a. komatiites

b. rock from the core

c. images taken of Earth from space

d. seismic waves from earthquakes

Answer

d. seismic waves from earthquakes