Chapter 2: Internal Structure of Earth and Plate Tectonics

Structure of Earth: Asthenosphere

- Below lithosphere. - Hot, slowly flowing layer of weak rock.

Structure of Earth: Lithosphere

- Cool, strong outermost layer of Earth.

Plate Tectonics and Hazards

- Divergent plate boundaries (Mid-Atlantic Ridge) exhibit earthquakes and volcanic eruptions. - Boundaries that slide past each other (San Andreas Fault) have earthquake hazards. - Convergent plate boundaries where one plate sinks (subduction zones) are home to explosive volcanoes and earthquake hazards. - Convergent plate boundaries where continents collide (Himalayas) have high topography and earthquakes.

Internal Structure of Earth:

- Earth is layered and dynamic. - Internal structure of Earth. - By composition and density. - By physical properties.

Paleomagnetism: Earth's magnetic field can be represented by dipole

- Forces extend from North to South Poles. - Caused by convection in the outer core.

Magnetic Stripes

- Geologists towed magnetometers along ocean floor. - Instruments that measure magnetic properties of rocks. - When mapped, the ocean floor had stripes. - Areas of "regular" and "irregular" magnetic fields. Stripes were parallel to oceanic ridges. - Sequences of stripe width patterns matched the sequences established by geologists on land.

A Detailed Look at Seafloor Spreading

- Global mid-ocean ridges were discovered in the 1950's - Seafloor spreading proposed by Harry H. Hess in 1962 Validity of seafloor spreading established by: - Identification and mapping of oceanic ridges - Dating of volcanic rocks on the floor of the ocean - Understanding and mapping of the paleomagnetic history of ocean basins

Plate Tectonics

- Large-scale geologic processes that deform Earth's lithosphere. - Produce landforms such as ocean basins, continents, and mountains. - Processes are driven by forces within Earth

Structure of Earth: Outer core

- Liquid - 2,000 km (1,243 mi.) in thickness. - Composition similar to inner core. - Density (10.7 g/cm3).

Structure of Earth: Crust

- Outer rock layer of Earth. - Density 2.8 g/cm3 Moho discontinuity. - Separates lighter crustal rocks from more dense mantle.

Rates of Plate Motion

- Plate motion is fast (geologically). - Plates move a few centimeters per year. - Movement may not be smooth or steady. - Plates can displace by several meters during a great earthquake.

Sinking Plates Generate Earthquakes

- Sinking ocean plates contain water and are relatively cold. - Plates come in contact with hot asthenosphere. - Water is drawn out of the descending plates to generate magma. - Magma rises to produce volcanoes. - Earthquakes occur along the path of the descending plate.

Structure of Earth: Inner core

- Solid - 1,300 km (808 mi.) in thickness. - High temperature. - Composed of iron (90 percent by weight) and other elements (sulfur, oxygen, and nickel).

Structure of Earth: Mantle

- Solid - 3,000 km (1,864 km) in thickness. - Composed of iron- and magnesium- rich silicate rocks. - Density 4.5 g/cm3.

How We Know About Internal Structure of Earth: Earthquakes cause seismic energy to move through Earth

- Some waves move through solid, but not liquids. - Some waves are reflected. - Some waves are refraction. - Information on wave movement gives a picture of inside of Earth.

What We Have Learned About Earth from Earthquakes: Where magma is generated in the asthenosphere.

- The existence of slabs of lithosphere that have apparently sunk deep into the mantle. - The extreme variability of lithospheric thickness, reflecting its composition, age, and history.

Seafloor Age cont...

- Using the magnetic anomalies, geologists can infer ages for the ocean rocks. - Seafloor is no older than 200 million years old.

Seafloor Spreading Is the Mechanism for Plate Tectonics

At mid-ocean ridges new crust is added to edges of lithospheric plates. - Continents are carried along with plates. - Crust is destroyed along other plate edges. - Subduction zones.

Continents versus Ocean Basins: Ocean crust is young (< 200 million years old)

Continental crust is older (up to several billion years old).

Mantle Convection drives Plate Tectonics

Earth's internal heat causes mantle rocks to heat up and become less dense. - Less dense mantle rises - Cool mantle rocks move back downward - Similar to pan of boiling water - but much slower!!

Paleomagnetism: Magnetic field has permanently magnetized some surface rocks at the time of their formation.

Iron-bearing minerals orient themselves parallel to the magnetic field at the critical temperature known as Curie Point

How We Know About Internal Structure of Earth:

Most of our knowledge of Earth's structure comes from seismology. - Study of earthquakes.

Plate Tectonics: Tectonics

Movement of Earth's crust.

Plate Tectonics: Plate Tectonics

Movement of discrete segments of Earth's crust in relation to one another.

Continents versus Ocean Basins: Crust is embedded on top of lithosphere.

Ocean crust is more dense than continental crust - Ocean crust is also thinner

Paleomagnetism

Paleomagnetism is the study of magnetism of such rocks

Magnetic Reversals

Some volcanic rocks show magnetism in opposite direction as today. - Earth's magnetic field has reverse. - Reversals are random. - Occur on average every few hundred thousand years.

Seafloor Age

Spreading at the mid-ocean ridges can explain stripe patterns. - Rising magma at ridge is extruded. - Cooling rocks are normally magnetized. - Magnetic field reverses and so does magnetization of rocks.

Hot Spots

Volcanic centers resulting from hot materials from deep in the mantle. - Materials move up through mantle and overlying plates. - Found under both oceanic and continental crust. - Plates move over hot spots creating a chain of island volcanoes. Seamounts are submarine volcanoes.