GEOL 333 #3 Earth Structure and Composition

I) Layers of Earth (Compositional distinct layers)

A) Crust = Thin outer layer that consists mostly of igneous rock (or metamorphic equivalent). Two types of crust:

1) Continental - underlies continents; upper part is ~ granite (silicic, i.e., rich in SiO2, relatively low density) and lower part is ~ basalt (mafic - less SiO2, more Mg and Fe, denser than granite). Average thickness = 35 km.

2) Oceanic - underlies oceans; basalt; usually ~7 km thick.

Crust has ~95% igneous (or metamorphosed igneous) rock and 5% sedimentary rock; at Earth's surface outcrops ~2/3 sedimentary and ~1/3 igneous. Crust is thin veneer of entire Earth (0.7% of Earth mass), but is most studied layer. Why?


Oxygen and silicon are most abundant elements in crust (Table 2.1) because most crustal rocks are composed largely of silicate minerals. 8 most abundant elements in Table 2.1 (down to K) = ~98% of mass of crust. Fig. 2.13 shows abundance of crustal minerals - silicates comprise ~92% with feldspars (plagioclase and K-feldspar), quartz, and pyroxene most abundant.

B) Mantle = Middle layer of rock, from base of crust to depth of 2,900 km. Largest layer (~68% of Earth mass). Upper part = ultramafic rock (peridotite - compared to basalt, richer in Mg and Fe-silicates - olivine and pyroxene, less SiO2, and denser). Lower part of mantle contains denser minerals (spinel and perovskite).

C) Core = Inner layer of iron/nickel alloy (mostly iron); from 2,900 km to center of Earth. Second largest layer (~31% of Earth mass). Outer part of core is liquid, inner part is solid. Circulation (driven by differences in temperature) in liquid part of Earth's core is source of magnetic field of Earth. Circulating metal produces electrical current, which results in magnetic field.

Iron, oxygen and silicon are most abundant elements in entire Earth (Table 2.2), reflecting Fe in core and ultramafic, Mg/Fe-silicate minerals in mantle.

II) Second kind of layering (based on mechanical properties - rock strength)

Rocks near surface are cool, strong, and brittle. This layer = lithosphere (= tectonic plate), includes crust and upper part of mantle (~100 km deep). With > depth (~100 - 300 km), it becomes hotter and rocks become weaker and deform plastically. This rock layer = asthenosphere, located entirely in mantle. Rock in asthenosphere is close to its melting temperature, consists of ~1% melt, 99% solid.

III) How do we know this information? (e.g., composition of Earth's layers, depths, liquid vs. solid)