Week Five: Earth s Interior/Structure

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 Why? It has to do with how Earth formed

Accretion and Differentiation http://www.nature.com/nature/journal/v473/n7348/fig_tab/473460a_f1.html http://ircamera.as.arizona.edu/natsci102/lectures/earth.htm

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

There are two ways to describe the structure of the Earth.

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

Continental crust Two Types of Crust Oceanic crust Thick: 40 km thick Thin: 5-8 km thick 20 km 70 km

Two Types of Crust Continental crust Lower density: 2.7 g/cm³ Oceanic crust Higher Density: 3.2 g/cm³ Remember Density is like packing a suitcase

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 20 km 70 km Lithosphere (100km)

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) Moho

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/cm 3 Two parts Asthenosphere or upper mantle Mesosphere (lower mantle)

Asthenosphere 20 km 70 km Lithosphere (100km) 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/cm 3 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

The Layers of the Earth

Interior of the Earth Crust Mantle Core Lithosphere Asthenosphere Mesosphere Outer Core D Inner Core Moho

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.

Seismic Waves!

Example: Running on stuff Concrete Sand Seismic Waves

Example: Running on stuff Concrete Sand Seismic Waves Earthquake waves run faster on stuff that is more dense (ex. Solids vs. liquids).

Seismic waves P waves Travels through liquids as well as solids In all materials, P waves travel faster than do S waves S waves Seismic Waves (cont.) 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=2kboqfs0n me

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.