Download Composition Physical Properties

Composition
Physical
Properties
Summary
The Earth is a layered planet
The layers represent changes in composition and physical properties
The compositional layers are the Crust, Mantle and Core
The physical layers are the Lithosphere, Asthenosphere, Mesosphere,
Outer Core and Inner Core
Nearly all of this is known as the result of indirect observations, mostly
through interpretation of seismic waves generated by earthquakes
The Crust is divided into Oceanic crust, which is thinner, more dense,
richer in iron and magnesium (Mafic) minerals and relatively young;
And Continental crust, which is thicker, less dense, richer in silicon and
aluminum (Felsic) minerals and relatively young;
Summary (continued)
The Mantle is composed of dense, mafic silicates (Peridotites)
The Core is composed of iron
There is a solid inner core and an outer liquid core that spins generating
a magnetic field
The speed and refraction of seismic waves is generally used to interpret
the inner structure and composition of the Earth
There are also a few samples of rocks from the mantle in volcanics,
kimberlite pipes, and in oceanic crust
Aside from seismic studies, we also can explore the subsurface using
studies of gravitational and magnetic anomalies
And finally, due to Isostasy continental mountains are inferred to have
deep roots.
The Interior of the Earth
• The interior of the Earth must be studied indirectly
– Examples of upper mantle fragments brought up by volcanic
eruptions and kimberlite pipes, or scraped off onto
continents by subducting oceanic plates
– Deepest drillhole reached about 12 km, but did not reach
the mantle
• Geophysics is the branch of geology that studies the
interior of the Earth
http://www.youtube.com/watch?v=19fMs633Td4&feature=related
http://www.youtube.com/watch?v=FW-TkpvKPl0&feature=related
The Mohole Project.
Finally abandoned after reaching 12 km
Journey to the Center of the Earth,
Jules Verne
Earth’s Internal Structure
• The solid Earth has a layered structure
– Layers defined by composition and
physical properties
– Compositional layers
• crust - mantle - core
– Physical layers
• lithosphere - asthenosphere - mesosphere outer core - inner core
Composition
Physical
Properties
Compositional Layers
• Crust
– Outermost compositional layer
– Definite change in composition at the
base of the crust
– Crust may be divided into 2 types
• Continental crust
• Oceanic crust
Compositional Layers
• Crust
– Continental crust
• Thicker than oceanic crust - up to 90 km
• Less dense - 2.7 g/cm3
• Strongly deformed
• Contains portions that are very old; up to
3.8 Billion years
Compositional Layers
• Crust
– Oceanic crust
• Thinner than continental crust - about 8 km
• More dense - 3.0 g/cm3
• Comparatively undeformed, i.e. no folded
and faulted mountains
• Much younger - < 200 million years old
• Composed of basalt, (contains olivine)
Crustal Properties
Crust
Density
Composition Thickness
continental ~2.8 g/cm3
Felsic
Thick:
20-70 km
~3.2 g/cm3
Mafic
Thin:
2-10 km
oceanic
Age
Old:
up to
4 Byrs
Young:
<200 Mys
Compositional Layers
• Mantle
– Largest layer in the Earth
• 2900 km thick
• 82% by volume
• 68% by mass
– Composed of silicate rocks with abundant
iron and magnesium: Mafic
• Density ranges from 3.2 to 5 g/cm3
• Fragments found in some volcanic rocks,
kimberlite pipes, and oceanic rocks scraped off
onto the continent during subduction
Compositional Layers
• Core (inner and outer)
– Central mass about 7000km in diameter
– Average density of 10.8 g/cm3
– 16% by volume, 32% of mass
– Composition
• Density of Earth, composition of meteorites
and the Earth’s magnetic field requires
largely Metallic Iron plus other minor
elements, e.g. Sulfur, Silicon, Nickel, etc.
Physical Layers
• Lithosphere
– The rock layer
– Crust + upper portion of the mantle
– Solid & rigid
– Thickness ranges from 10 km beneath
oceans to 300 km in continental areas
Physical Layers
• Asthenosphere
– Upper layer in the mantle
– Temperature and pressure combine to allow
rock to partially melt
• Rocks are soft and plastic
• They flow and are easily deformed
• Results in a low velocity zone for seismic waves
– Boundary with lithosphere is defined by
mechanical properties, not composition
Physical Layers
• Mesosphere
– The region between the asthenosphere
and the core
– Higher pressure offsets higher
temperatures
– Rocks gain rigidity and mechanical
strength
Physical Layers
• Outer Core
– ~2270 km thick
– Liquid, flows
– Flow creates magnetic field
• Inner Core
– ~1200 km thick
– Solid
The Earth’s Magnetic Field
How you can use
seismic waves to
explore the interior
of the Earth
No refraction in
homogeneous materials
Fig. 11-4a, p. 341
Refraction in heterogeneous
materials
Fig. 11-4b, p. 341
Seismic waves in a homogeneous planet
Seismic waves in a differentiated planet
Fig. 11-4c, p. 341
Fig. 11-10, p. 346
Fig. 11-9c, p. 345
Fig. 11-9b, p. 345
Seismic Structure of the Earth
• Seismic wave velocities vary with depth
–Variation with depth is not smooth
–Discontinuities at certain depths represent
discrete changes in structure, chemistry and
phase (liquid/solid) of minerals in the mantle
Seismic Structure of the Earth
• Mohorovicic Discontinuity (Moho)
– First discovered by Andrija Mohorovicic
– Occurs between 5 and 70 km deep
– Represents the base of the crust, i.e. the
crust/mantle boundary
– Compositional change from feldspar rich to
olivine rich rocks causes a significant increase
in seismic velocities
– Causes refracted seismic waves
These waves travel are refracted here and
here.
Even though they travel farther, they arrive first.
Therefore they have traveled faster
Seismic Structure of the Earth
• Low-velocity zone
– Layer from ~100 to 250 km deep
– Seismic velocities usually increase with depth
– In the low velocity zone velocity slows by ~ 6%
– Caused by partially molten mantle that slows
seismic waves
– May form a slippery layer that the overlying
crust slides upon
Internal structure of the Earth
Movement in the Earth
• In the core
– 3-D models and magnetic field suggest flowing
molten iron with likely internal convection
resulting in occasional chaotic reversals
• In the mantle
– Investigations show a complex convection system
occurring in the entire mantle system, including an
active D” layer at the core/mantle boundary
The ULVZ and the
D” layer
Convection in the Earth
The Ultra Low Velocity Zone (ULVZ) and the D” layer;
the stormy layer at the Core/Mantle boundary
Source of mantle plumes and hot spots?
The Core
•
Core composition inferred from its calculated
density, physical and electro-magnetic properties,
and composition of meteorites
– Iron metal (liquid in outer core and solid in
inner core) best fits observed properties
– Iron is the only metal common in
meteorites
•
Core-mantle boundary (D” layer) is marked by great
changes in seismic velocity, density and temperature
– Hot core may melt lowermost mantle or
react chemically to form iron silicates in this
seismic wave ultralow-velocity zone (ULVZ)
Meteorites record
the composition of
the early solar
system
~4.6 billion years old
Three types of meteorites
Iron (mostly iron, some nickel and other metals
Stony (most common; silicate minerals: plagioclase, olivine, pyroxene)
Stony-iron (mixed composition)
One unusual type is a carbonaceous chondrite, which can contain up
to 5% organic carbon, i.e. hydrocarbons, amino acids.
Heat Within the Earth
•
Geothermal gradient - temperature increase with depth
into the Earth
– Tapers off sharply beneath lithosphere
– Due to steady pressure increase with depth,
increased temperatures produce little melt
(mostly within asthenosphere) except in
the outer core
•
Heat flow - the gradual loss of heat through Earth’s
surface
– Major heat sources include original heat
(from accretion and compression as Earth
formed) and radioactive decay
– Locally higher where magma is near surface
– Same magnitude, but with different sources,
in the oceanic (from mantle) and continental
crust (radioactive decay within the crust)
Fig. 11-1a, p. 336
Fig. 11-1b, p. 336
Fig. 11-7, p. 343
Fig. 11-8a, p. 344
Fig. 11-8b, p. 344
Fig. 11-11, p. 347
Fig. 11-11a, p. 347
Fig. 11-11b, p. 347
Gravitational
Anomalies
Dense and
therefore
exerting more
gravitational
attraction
Gravitational
Anomalies
Common way to
explore for faults
and therefore
water in desert
areas such as the
southwest US
Gravitational
Anomalies
Common way to
explore for salt
domes and
therefore oil in
the Gulf Coast
area
Fig. 11-14a, p. 350
Fig. 11-14b, p. 350
Fig. 11-14, p. 350
The mass of the volume of water displaced
is equal to the total mass of the iceberg
10% of the mass
10% of the mass
90% of the mass
90% of the mass
Fig. 11-15b, p. 350
Fig. 11-15a, p. 350
Fig. 11-16a, p. 351
Fig. 11-16b, p. 351
Fig. 11-16c, p. 351
Fig. 11-16, p. 351
Fig. 11-17, p. 351
Fig. 11-17a, p. 351
Fig. 11-17b, p. 351
Fig. 11-17c, p. 351
Fig. 11-17a, p. 351
Fig. 11-18, p. 352
Fig. 11-19, p. 352
Magnetic
Anomalies
Fig. 11-20a, p. 353
Magnetic
Anomalies
Fig. 11-20b, p. 353
Magnetic
Anomalies
Fig. 11-20c, p. 353
Earth’s Internal Structure
•
•
•
•
Seismic waves have been used to determine the
three main zones within the Earth: the crust, mantle
and core
The crust is the outer layer of rock that forms a thin
skin on Earth’s surface
The mantle is a thick shell of dense rock that
separates the crust above from the core below
The core is the metallic central zone of the Earth