Exploring the inner and outer shells of earth

Chapters 2-3-4

Additional reading: MAR discovery, pdf, class web site

Learning Objectives:Earth structure, Plate tectonics and Ocean floor 

Difference between oceanic and continental crust.

Understand the processes that are continuously changing Earth’s surface as lithospheric plates move relative to one another. Identify the role of oceanic ridges, transform faults and deep-sea trenches in defining the edges of lithospheric plates. Understand the importance of asthenospheric thermal convection in plate tectonics and the resulting compression or tensional forces at the plate boundaries. Explain the distribution of magnetic anomaly stripes, seismicity, and volcanism in terms of the concept of global plate tectonics. Spreading rates of ocean basins.

• Layered system (like an onion, concentric regions)~ differentiation of mineral material

Earth’s Structure

Classification according to chemical compositionEarth’s Structure (cont’d.)

4 concentric regions of mineral material:

1. crust2. mantle3. outer core - molten4. inner core - solid

1. Crust Two types:continental granite – composed of silicates rich in Na, K & Al

ocean basalt – composed of silicates rich in Ca, Mg & Fe• represents 0.4% of Earth’s mass• extends down to 75 km

Classification according to chemical compositionEarth’s Structure (cont’d.)

2. Mantle Three parts:uppermost/middle/innermost• Composed of Mg-Fe silicates• represents 68% of Earth’s mass• extends down from base of

crust to ~2,900 km

Classification according to chemical compositionEarth’s Structure (cont’d.)

3. Core Two parts:Outer Inner• Composed of Fe & Ni• Represents 28% of Earth’s

mass• Extends down from base of

mantle ~ 6400km

Classification according to chemical compositionEarth’s Structure (cont’d.)

1. lithosphere - rigid outer shell (crust & uppermost mantle)• 100 - 150km thick• does not change shape

(factor in temperature and pressure)Classification according to physical properties

Earth’s Structure (cont’d.)

4 concentric regions:

2. Asthenosphere - soft, flows over geologic time under the weight of the lithosphere (small fraction of middle mantle)• lithosphere ‘floats on top’• zone where magma formed• 200 – 350km thick• easily deformed, can be pushed down by overlying lithosphere –

“plastic” – tar or asphalt

Earth’s Structure (cont’d.)

3. Mesosphere - rigid but not as hard as lithosphere• higher temp than asthenosphere, but not molten because of

compression pressure• 4950km thick

Classification according to physical propertiesEarth’s Structure (cont’d.)

4. Core - outer is molten, inner is solid

Classification according to physical propertiesEarth’s Structure (cont’d.)

Earth consists of a series of concentric layers or spheres which differ in chemistry and physical properties.

ChemicalLayers Physical

Layers

Physical state is determined by the combined effects of pressure and temperature.

• Increasing pressure raises the melting point of a material.

• Increasing temperature provides additional energy to the atoms and molecules of matter allowing them to move farther apart, eventually causing the material to melt.

• Both pressure and temperature increase toward the center of the Earth, but at variable rates.

The Oceans of the World

How do we learn about the innerstructure of the planet?

Seism

ic w

aves

Seismology is the study of elastic waves that travel through the earth

Two main wave types:

Shear waves (S-waves): travel by shearing medium they pass through. S-waves can travel only through solids since particles need to be bonded to each other to propagate wave;

Compression waves (P-waves): travel by squeezing and expanding medium they travel through. They can travel through both solids and liquids (e.g., sound waves);

Seismic waves travelalong the quickestroute, generallythrough the planetaryinterior to the seismicstations, changingspeed every timematerial propertieschange.

Earthquake are an incredible source of seismic waves

Reconstructing the internal structure of the planet

• Crust & upper mantle: large increases in seismic velocities and density

• Mantle: Gradual velocity and density increase

• Outer core: dramatic density increase and no S-waves

• Inner core: Jump in density and P-wave velocity, S-waves return

solid

solid,more dense

Liquid Iron

solid Iron

• Combining information from many seismic waves we can get a detailed internal structure of the earth.

Seismic Velocity and Density

Other planets?

Courtesy: K. Satake, unpublished

Sea level: RISE FALL

Earthquakes produce waves in the ocean

World Seismicity (1898-2003)

Mw ≥ 6.0Mw ≥ 7.7

Many large earthquakes occur along subduction zonesMost “Great” earthquakes are subduction mega-thrust events

The Physiography of the North Atlantic Ocean Floor

continental margins

deep oceanbasins midoceanic

ridges

Mid Atlantic Ridge – new findings:

• http://www.noc.soton.ac.uk/gg/classroom@sea/JC007/about.html

• http://www.sciencedaily.com/releases/2007/03/070301103112.htm

http://www.noc.soton.ac.uk/gg/classroom@sea/JC007/background.html

MAR discovery - http://alpha.es.umb.edu/faculty/af/intro_ocean.htm#Unit1

Type of continental margins

Continental margins are the submerged edges of the continents and consist of massive wedges of sediment eroded from the land and deposited along the continental edge. The Continental Margin can be divided into three parts: the Continental shelf, the Continental slope, and the Continental rise.

Passive Continental Margin

• Midoceanic Ridge Province consists of a continuous submarine mountain range that covers about one third of the ocean floor and extends for about 60,000 km around the Earth.

Midocean Ridge

• Deep Ocean Province is between the continental margins and the midoceanic ridge and includes a variety of features from mountainous to flat plains: Abyssal plains, Abyssal hills, Seamounts, and Deep sea trenches.

Deep Ocean Basin

Hydrothermal Vents

03_13a

This three-dimensional rendition of a bathymetric map shows Patton Seamount, a Gulf of Alaska seamount we visited in 1999, with two smaller seamounts in the foreground. Deep areas are blue, and shallow areas are red.

WHY DO LAND AND OCEAN EXIST?

Continents and ocean basins differ in composition, elevation and physiographic features.

• Elevation of Earth’s surface displays a bimodal distribution with about 29% above sea level and much of the remainder at a depth of 4 to 5 kilometers below sea level.

• Continental crust is mainly composed of granite, a light colored, lower density, igneous rock rich in aluminum, silicon and oxygen.

• Oceanic crust is composed of basalt, a dark colored, higher density, volcanic rock rich in silicon, oxygen and magnesium.

2-3Geologic Differences between Continents and Ocean Basins

Geologic Differences between Continents and Ocean Basins

WHY DO LAND AND OCEAN EXIST?

OCEANIC CRUST = THIN AND DENSER

CONTINENTAL CRUST = THICK AND LEIGTHER

• Continents are thick (30 to 40 km), have low density and rise high above the supporting mantle rocks.

• Sea floor is thin (4 to 10 km), has greater density and does not rise as high above the mantle.

Oceanic Crust Versus Continental Crust

IsostacyPrinciple that dictates how different parts of the lithosphere stand in relation to each other in the vertical direction

• Continental crust less dense (granitic) therefore rises higher relative to ocean crust (basaltic)

• Continents move up and down depending on weight on top (i.e. from glaciers - ‘isostatic rebound’)

~ Continents pop up after glaciers melt

~ Canada and Scandinavia rising at a rate of 1m/100yrs because the glaciers are receding

Isostasy refers to the balance of an object “floating” upon a fluid medium. Height of the mass above and below the surface of the medium is controlled by the thickness of the mass and its density (similar to ice floating in water).

http://atlas.geo.cornell.edu/education/student/isostasy.html

http://woodshole.er.usgs.gov/operations/modeling/movies/fli/stellrise.flc

altimetry Satellites in orbit around the planet use radar altimetry to measure the height of the sea level (accuracy of 2 cm).http://www.ecco-group.org/animations_iter21/TP_ps21.mpeg

Model of the shape of the Earth

http://www.esri.com/news/arcuser/0703/geoid1of3.html

geoid: The equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level (MSL)

Why ocean bathymetry?

Ocean Circulation and ClimateDeep ocean mixing and pathways

Social impacts:Tsunami

Social impacts:Gas and oil extraction