Earth Structure and Composition Summary - Myweb @ CW Post

Earthʼs Origin, Layers, Bulk Composition, Tectonic Settings
Correlative readings from Hefferan & OʼBrien (2010) textbook listed in italic.
Earth Formation
The Earth formed ~4.6 b.y. ago from the solar nebula which contained largely H with
some He and H compounds, but also small amounts of “dust” of silicate and metallic
particles. The Earth and the other inner planets were formed principally from the
metallic and silicate materials because it was too hot in the inner solar nebula to hold
onto the volatile gaseous elements and compounds. Earth and the inner planets
have metallic, iron-dominated cores and rocky, silicate mantles. The metallic and
silicate materials were able to segregate by density because the early Earthʼs
interior was much hotter than today and was partly to largely molten.
Earthʼs Layers (textbook sections 1.2 & 1.3)
The crust: Mohorovicic (1909), studying seismic waves from an earthquake in eastern
Europe, discovered that there is an abrupt increase in seismic wave velocity at a
depth of around 50 km in Yugoslavia. This is called the Mohorovicic seismic
discontinuity or simply the Moho. The velocity increase is caused by a change from
less dense mafic through felsic igneous, metamorphic, and sedimentary rocks of the
crust to denser ultramafic rocks of the mantle. The depth to the continental Moho
varies from less than 20 km in rifted continental areas to over 70 km in mountain
belts like the Himalayas and Andes. The ocean crust is almost uniformly about 7 km
thick and composed almost entirely of mafic rock. The crust formed over billions of
years by melting and segregation of lower melting temperature and lower density
materials from the ultramafic mantle. The crust is the “scum of the Earth.”
The core: Gutenburg (1914) compiled seismic waves recorded at many locations
around the world from many distant earthquakes. From the pattern of the distances
at which seismic waves were and were not recorded he recognized that there was a
boundary between the solid mantle and a liquid core at a depth of about 2900 km
that stopped S waves (shear waves canʼt travel through liquid) and refracted P
(compression ) waves. Later, Lehman (1936) discovered a transition from liquid
back to (probably) solid at a depth of 5150 km based on an increase in P wave
velocity.
What is the core made of? Ans: mostly iron
it is high density
it is common in the solar system (one of the most abundant after H & He)
it will approximately account for the observed seismic velocity
it has magnetic properties that can account for Earthʼs magnetic field.
-90-95% iron, 5-9% nickel, ~1% S (like meteorites) or other light element (O, C)
Compositional Layers
crust: felsic to mafic silicates
mantle: ultramafic silicates and oxides
core: iron/nickel alloy

Mechanical Layers
lithosphere: strong, rigid crust plus uppermost mantle
asthenosphere: ~100-250 km, mantle near melting temp for that depth (pressure)
" weak, easily sheared, possibly/probably slightly molten (perhaps ~1%)
transition zone: ~410 km & ~660 km olivine changes to denser crystal structure
" olivine above 410 km
" spinel below 410 km
" perovskite below 660 km
lower mantle (also called mesosphere): 660 km - 2900 km, strong ultramafic rock
outer core: liquid iron/nickel
inner core: solid (or crystal mush) iron/nickel
Tectonic Settings (textbook section 1.4 & 1.5)
just a quickie review; detailed discussions of rock formation will come later
Divergent: Midocean Ridges:
two lithospheric plates separate
new crust forms by continuous formation and intrusion of mafic magma into fractures
ocean crust eventually receives a thin layer of ocean sediment (muds)
Convergent: Subduction Zones:
oceanic lithosphere sinks back down into the mantle at deep ocean trenches
mafic magma rises to form volcanic arc parallel to trench
sediments scraped off descending plate form accretionary wedge
Convergent: Continental Collisions:
continental crust is not dense enough to subduct - mountain building occurs
metamorphism of rock buried deep under mountain belt
mountains gradually weathered to produce sediments
Hotspot Tracks:
age-progressive chains of volcanoes (best seen on ocean crust)
more-or-less stationary plume of rising, hot mantle
melting as plume material nears base of lithosphere, produces mafic magma