Transcript Downloadable Structure PP

Structure and Composition of the
Earth
1.0 mile = 1.6 km
0.6 miles = 1.0 km
This means that 1000
km is about 600 miles
Figure 1.7
1. The Crust
a. Continental Crust
- Sedimentary rocks atop metamorphic and
plutonic rocks
Sedimentary rocks of the Grand Canyon
The
Crust
Continental crust is composed primarily of 8 elements
(elements are listed in the periodic table of elements):
Silicon (Si)*
Oxygen (O)*
Aluminum (Al)*
Iron (Fe)
Magnesium (Mg)
Calcium (Ca)
Potassium (K)
Sodium (Na)
* - most abundant elements in continental crust
Continental crust is the least dense layer of
the earth (compared to ocean crust, the
mantle, and the core). This is why
continental crust is the uppermost layer and
the other, more-dense layers are below it.
Density of continental crust = 2.7 g/cm3
Continental crust has a thick “root”: it is thicker under
mountains and thinner under lowlands (like an iceberg).
This is due to isostasy (flotational balance).
Continental crust is about 30-70 km thick (18 to 42 miles).
b. Ocean crust
-Thinner than continental crust
- About 5 to 10 km thick (3-6 miles)
Figure 2.6, page 33
The ocean crust is mainly composed of pillow basalts
from submarine eruptions at mid-ocean ridges (M.O.Rs.)
with gabbro at depth. The ocean crust is overlain by a
thin layer of sediment.
Pillow basalts
http://www.pmel.noaa.gov/vents/marianas/movies/rof2006/480
x360_Quicktime/NWRota_brimstone12_480.mov
Note that Iceland sits atop the mid-Atlantic Ridge, which results in extensive volcanic
eruptions of basaltic lava which cools to form the igneous rock basalt.
Iceland eruption, 1973
Age of the seafloor
Note that the age gets older away from the mid-ocean ridges and is youngest at the ridge
2. The Mantle
- the layer below the crust
- divided into 2 parts: the upper mantle and the lower mantle
a.The Upper Mantle
-
from the base of the crust to about 700 km depth
less Si, O, Al than above layers
more Fe and Mg than above layers
density = 3.3 to 5.5 g/cm3
b. The Lower Mantle
-
from the base of the upper mantle to about 2900 km depth
less Si, O, Al than above layers
more Fe and Mg than above layers
density = 5.5 to 8.0 g/cm3
3. The Core
- rich in iron (Fe)
- most dense layer of earth (10-13 g/cm3)
- divided into 2 parts, the outer core and inner core
a.The Outer Core
-
from 2900 km to 5100 km depth
S-waves do not travel through the outer core
S-waves do not travel through liquids
therefore, the outer core is molten and behaves like a liquid
it is believed that flow of the molten outer core generates the
earth’s magnetic field
Fig 2.8
a.The Inner Core
- the inner core is a solid (it’s hotter than the outer core, but the
high pressure makes it solid
- from 5100 km depth to:
≈ 6354 km at the poles
≈ 6378 km at the equator
* spin of the earth results in an equitorial bulge
Fig 2.9
Evidence from Seismic
Waves
• Seismic waves or vibrations from a large
earthquake (or underground nuclear test) will
pass through the entire Earth
• Seismic reflection - the return of some waves to
the surface after bouncing off a rock layer
boundary
– Sharp boundary between two materials
of different densities will reflect seismic
waves
• Seismic refraction - bending of seismic waves as
they pass from one material to another having
different seismic wave velocities
Earth’s Magnetic Field
• A magnetic field (region of magnetic force) surrounds the
Earth
– Field has north and south magnetic poles
– Earth’s magnetic field is what a compass detects
– Recorded by magnetic minerals (e.g., magnetite) in
igneous rocks as they cool below their Curie Point
• Magnetic reversals - times when the
poles of Earth’s
magnetic field switch
– Recorded in magnetic minerals
– Occurred many times; timing appears chaotic
– After next reversal, a compass needle will point
toward the south magnetic pole
• Paleomagnetism - the study of ancient magnetic fields in
rocks
– allows reconstruction of plate motions over time