Download File - earth science online

•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Week Five: Earth’s Interior/Structure
The Earth’s Interior/Structure
Cut a planet in half…
See layers
– Most dense material (metals) at bottom
– Medium density material (rocks) in middle
– Even less dense (liquids and gases) at top
Accretion and Differentiation
Earth Formed by Accretion
Accretion: colliding of material to make a larger mass.
Asteroids and comets accreted together to form our planet.
Then, Earth differentiated to form its layers
Differentiation: separation or to become specialized (in this case by density)
Denser elements and minerals fell toward the center of the Earth and the less
dense elements and minerals rose towards the surface.
Cut a planet in half…
See layers
– Most dense material (metals) at bottom
– Medium density material (rocks) in middle
– Even less dense (liquids and gases) at top
Structure of the Earth
Layers – by composition
– Crust
– Mantle
– Core
Structure of the Earth
Layers – by physical properties
– Lithosphere
– Asthenosphere
– Mesosphere
– Outer Core
– Inner Core
Compositional Layering
Crust: the outermost layer of the Earth
– composed of low density rock
Mantle: the layer of the Earth between the crust and the core
– Composed of high density rock
Core: extends from the bottom of the mantle to the center of the Earth
– Composed of higher density metal
Layering by Physical Properties
Depending on the temperature and depth, a particular Earth material may behave
like a brittle solid, deform in a plastic-like manner, or melt and become liquid.
The Crust
•
70 km
• Two Types of Crust
Oceanic crust
– Higher Density: 3.2 g/cm³
• Crust
• Two types of crust:
– Continental crust
• thicker (20 to 70 km thick, 12-43 miles)
• Lower density: 2.7 g/cm³
• Composed of igneous, metamorphic, and sedimentary rocks
– Oceanic crust
• thinner (5 to 8 km thick, 3-5miles).
• Higher Density: 3.2 g/cm³
• Composed mainly of the igneous rock basalt
• Continental Crust
• Continental Crustal Thicknesses
– a. Eastern US seaboard and Texas coast 30 km
– b. Appalachians and Interior plains 40 km
– c. Rocky Mountains
40 km
– d. Great Basin
30 km
– e. Sierra Nevada Mtns
50 km
– f. California Coast
20 km
• Lithosphere
• Lithosphere means “sphere of rock”
• Outermost, rigid layer of the Earth
• Made of the crust and the rigid uppermost part of the mantle
• Makes up the plates
• Outer approximately 100 km of the Earth
• Relatively cool and brittle.
• Mohorovicic Discontinuity (Moho)
• Boundary between the crust and mantle
• Discovered in 1909 by a Yugoslav seismologist, Andriaja Mohorovicic
• P and S waves speed up here (more on this later)
• Mantle
• Under the crust is the mantle, almost 2900 km thick (1800 miles).
• The mantle is made of dense, solid rock
• Average density is 3.3 g/cm3
• Two parts
• Asthenosphere or upper mantle
• Mesosphere (lower mantle)
Asthenosphere
• The Mantle (cont.)
• Asthenosphere
– Means “weak sphere”
– A critical and dynamic part of the upper mantle
– This zone extends from approximately 100 km to 250 km depth
– Near its melting temperature and is therefore hot and pliable
• Partly molten (2% melt) and capable of plastic flow
–
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Flows very slowly-at about the same rate your fingernails
grow
• Enables plates to move above it
– It is the site of magma generation
Mesosphere
Mesosphere
Mesosphere: the strong, lower part of the mantle, beneath the asthenosphere, that
extends down to the core.
Rocks are very hot and capable of very gradual flow
The Mantle (cont.)
The mantle convects
– Convection is slow, about 2 to 10 cm/yr
– Convection is the critical process that enables plate tectonics and all the
associated benefits
– Where convection currents are rising in the asthenosphere, the surface of
the Earth is bulged upward and may be rifted
• Oceanic ridges/rises are an example
• Hawaii is bulged up over the Hawaiian Hot Spot
The Mantle (cont.)
Where convection currents are descending, the crust is deeply depressed
– Sea floor trenches
D’’ Layer
Boundary between the mantle and outer core.
“Graveyard” of subducted lithosphere and “birthplace” of mantle plumes.
The Core
The Core
Composed mostly of iron; some nickel.
Average density is 11 g/cm3
Two layers:
– Inner Core
• Solid iron and Nickel
• Center of the Earth; surrounded by outer core
– Outer Core
• Liquid iron and nickel
• Lies beneath the mantle, surrounding the inner core
• Convective flow within generates Earth’s magnetic field
The Core
Responsible for Earth’s magnetic field
– Made of material that conducts electricity
– Core is mobile
How do we know what the interior is like?
Could try to drill a hole…
Deepest we have drilled is 12.3 km (7.6 miles - not even through the crust!)
Temperature and pressure too much.
– Example: Running on stuff
Concrete
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Sand
Example: Running on stuff
Concrete
Sand
Seismic Waves (cont.)
Seismic waves
– P waves
• Travels through liquids as well as solids
• In all materials, P waves travel faster than do S waves
– S waves
• Cannot travel through liquids
Seismic Waves (cont.)
Seismic waves
– Seismic waves refract (bend) as they pass from one material to another
– P & S wave shadow zones
– http://www.youtube.com/watch?v=2kBOqfS0nmE
– Seismic Waves (cont.)
Many earthquakes are big enough that their waves go all the way through earth
and can be recorded on the other side!
– 100-200 earthquakes a year that are 6’s or larger
– Help us to “see” into Earth like X-rays.
P and S wave Shadow Zones
When P waves get to the liquid outer core, their rays are refracted (bent), but they
still go through.
This creates a “shadow zone” where no direct P waves are recorded.
S waves can’t go though liquids, so a “shadow zone” exists for S waves.
P and S Wave Shadow Zones
Seismic waves alone can’t tell us what the Earth is made of.
Lab experiments help to figure out what it is actually made of.
Squeeze and heat minerals and rocks to see which ones “fit” with the seismic
velocities.
Diamond Press Experiments
Long story short…
Combining data from seismic waves and lab tests with minerals, we have a good
understanding of the interior of the Earth.