Earth's Interior: Layers Defined by Chemical Composition

This lecture focuses specifically on the layers of the Earth as defined by their chemical composition, which refers to their molecular makeup.
Understanding the chemistry of the interior is vital for future topics in the course, though the approach taken here is qualitative; no chemical formulas or equations are required for solving.
The layers defined by composition are the Crust, the Mantle, and the Core.
These are distinct from layers defined by physical properties (how they behave when stressed), which include the lithosphere, asthenosphere, mesosphere, outer core, and inner core.

The fundamental building block for all rocks in the crust and the mantle is the silicon oxygen tetrahedron molecule.
The silicon oxygen tetrahedron consists of one silicon atom surrounded by four oxygen atoms.
Silicate rocks contain this molecule along with varying amounts of other ions, specifically cations (positively charged ions).
Iron ( $Fe$ ) is the most significant cation attracted to the negatively charged silicon oxygen molecule.
Magnesium ( $Mg$ ) is another relevant cation because it is atomically the same size as iron and can replace it in chemical structures, although iron is significantly more prevalent.

Rocks are categorized into four groups based on their iron and magnesium content, which also influences their physical appearance (color):

Felsic Rocks:
- Contain less than $31\%$ iron and magnesium content.
- Appear light-colored (pink, white) with few dark minerals.
- Primarily found near the Earth's surface.
Intermediate Rocks:
- Contain $32\%$ to $47\%$ iron and magnesium content.
Mafic Rocks:
- Contain $48\%$ to $54\%$ iron and magnesium content.
- Appear very dark or black in color.
- Synonymous with "basaltic" composition, named after the rock basalt.
Ultramafic Rocks:
- Contain greater than $55\%$ iron and magnesium content (more than half the rock is iron).
- Specifically define the composition of the Mantle.

Continental Crust:
- Thickness: Varies between $30\,km$ and $70\,km$ .
- Composition: Highly felsic near the surface, becoming more mafic with depth. Does not contain ultramafic rock.
- Rock Types: Composed of sedimentary, metamorphic, and igneous rocks.
- Example: The granites of Yosemite National Park and the Sierra Nevada Mountain Range (e.g., Half Dome) represent standard continental composition.
- Density: Least dense of all layers at approximately $2.7\,g\,cm^{-3}$ .
- Age: Contains the oldest known rocks, up to $3,800,000,000$ years old.
Oceanic Crust:
- Thickness: Much thinner, ranging from $3\,km$ to $10\,km$ (averaging $5\,km$ to $7\,km$ ).
- Composition: Homogeneous (the same throughout) and mafic (basaltic).
- Rock Types: Primarily igneous.
- Density: Approximately $3\,g\,cm^{-3}$ . This higher density relative to continental crust is critical for plate tectonics and subduction.
- Age: Relatively young, less than $200,000,000$ years old. Compared to the planet's age (> 4,000,000,000 years), this is a "drop in the bucket."
- Volcanism: $75\%$ of all planetary volcanism occurs underwater.
- Pillow Basalts: When lava erupts on the ocean floor, the cold water quenches it quickly while high pressure creates puffy, rounded shapes called pillow basalts.
- Example: The Juan De Fuca Ridge off the coast of Oregon is a site of research for oceanic crust composition.

The Mohawk discontinuity, or MOHO for short, is the official name for the boundary between the Earth's crust and the mantle.
Scientists identify the MOHO through seismic waves, imaging the Earth's interior much like an ultrasound images a baby in a womb.
The depth of the MOHO varies by location:
- Under mountain ranges (e.g., the Himalayas/Mount Everest or South American Andes), the crustal thickness and MOHO depth can reach $70\,km$ or more.
- Under flat continental areas, the crust is thinner.
- Under the ocean, the crust is thinnest, especially near Mid-Ocean Ridges.
Both the crust and the upper portion of the mantle at this boundary are brittle and hard. Together, the crust and the brittle mantle are known as the lithosphere.

The Mantle:
- Composition: Purely ultramalific (> 55\% iron/magnesium).
- Thickness: Much thicker than the crust by a huge measure; the comparison of an eggshell to an egg illustrates how thin the crust is relative to the internal layers.
- Density: Approximately $3.3\,g\,cm^{-3}$ .
The Core:
- Composition: Non-silicate. It is composed of approximately $90\%$ pure iron and nickel, with trace elements such as sulfur.
- Size: Approximately $3,486\,km$ in diameter.
- Density: Significantly higher at $11\,g\,cm^{-3}$ .
- Density Drivers:
  1. Atomic Structure: The iron atom is very small compared to the oxygen atom, but more massive. Therefore, iron occupies a smaller volume per unit of mass.
  2. Pressure: Tremendous confining pressure at the center of the Earth pushes atoms very close together.
- Reaching the Mantle/Core: Humans have not yet drilled into the mantle. The most likely site would be the ocean floor near a Mid-Ocean Ridge where the crust is thinnest, though the depth and water pressure make this a significant engineering challenge.