The Forces That Shape the Earth & Plate Tectonics

Part 1 NotesThe Forces That Shape the Earth

• Geologist – scientist who studies forces that make and shape the Earth.

• Constructive forces – forces that shape the surface by building up mountains and landmasses.

• Destructive forces – forces that slowly wear away mountains and other features on the surface.

Structure of the Earth

• The Earth is made up of 3 main layers:– Core– Mantle– Crust

Inner core

Outer core

Mantle

Crust

The Crust• This is where we live!

• The Earth’s crust is made of:

Continental Crust

- thick (10-70km)- buoyant (less dense than oceanic crust) - mostly old

Oceanic Crust

- thin (~7 km)- dense (sinks under continental crust)- Young & old

Mantle Layers

• Lithosphere: The rigid, stiffer part of the outer mantle and the crust. The lithosphere 'floats' on the asthenosphere, like ice on water.

• Asthenosphere: The thick semi-liquid part of the outer mantle. 

• http://pubs.usgs.gov/gip/dynamic/inside.html

CORE– a) Outer core – layer of molten/liquid

metal that surrounds the inner core. Convection currents here creates Earth’s magnetic field-giant bar magnet

– b) Inner core – dense solid ball of metal

• Composition: nickel, iron. The core is thought to be composed of nickel and iron alloy. The outer core is liquid while the inner core is solid.

• Tremendous pressure, produced by the weight of the overlying rocks is believed to crowd the atoms tightly together and prevent the liquid state.

How do we know what the Earth is made of?

• Geophysical surveys: seismic, gravity, magnetics, electrical, geodesy– Acquisition: land, air, sea and satellite

– Geological surveys: fieldwork, boreholes, mines

Rock Cycle

• Igneous Rocks-melting is the process that creates the magma and lava that can form this kind of rock

• Metamorphic Rocks-formed from any rock undergoing intense heat & pressure

• Sedimentary Rocks-form from the processes of weathering, erosion & deposition

Processes:

•Weathering-breaks down the rocks

•Erosion is the movement of the weathered pieces of rock-sediments

•Deposition is the settling out and layering of sediments

Sedimentary rocks

• Grand Canyon• Youngest rocks at the

top• Rocks and fossils

older as you go down• Why is that?

Unstable Earth• The Earths crust is

subjected to huge forces

• Very large forces can fracture the rock

• This can be seen as fault lines in the rock layers

Tilted and Folded• Sedimentary rocks

are often found tilted• They can also be

folded by the huge forces

• Sometimes the rock layers can even be turned upside down

• Part 2 Notes

Key Questions:

What is Plate Tectonics?What are the different theories and evidence that support plate tectonics?

• If you look at a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle.

Alfred Wegener• Some people thought the

Earth was shrinking and cracking

• Some people did not understand and thought the continents floated around in the oceans (continental drift)

• Continental drift – Wegener’s hypothesis that the continents slowly move across the Earth’s surface.

• Wegener could not explain the force that pulls or pushes the continents so his hypothesis was rejected.

Evidence for Continental DriftLandforms – continents can be pieced together like a jigsaw puzzle and features like mountain ranges matchFossils – similar reptile and plant fossils found in places now separated by oceansClimate – tropical plant fossils in now polar climates, continental glacier evidence found in South Africa

Sea Floor Spreading• Sea-floor spreading –

the process by which molten material adds new oceanic crust to the ocean floor.

• In 1960, Harry Hess proposed that the ocean floors move like conveyor belts carrying the continents with them.

• mid-ocean ridge – the undersea mountain chain where new ocean floor is produced.

Evidence for Sea-floor Spreading

• New molten material in erupting along the mid-ocean ridge.

• When Earth’s magnetic poles reversed the occurrence was recorded in the newly formed rock at the mid-oceanic ridge.

• Using drilling samples scientists have determined that rocks are youngest in the center of the mid-ocean ridge and get older the farther you move from the ridge.

 

        

Sea Floor Spreading & Pole Reversal Evidence Website

http://oceanexplorer.noaa.gov/edu/learning/player/lesson02/

l2la2.htm

World Plates-Part 3

Plate Tectonics-Part 3

Theory of Plate Tectonics•In 1965, J. Tuzo Wilson proposed that the lithosphere is broken into separate sections called plates.•Plate – A section of the lithosphere that slowly moves over the asthenosphere carrying pieces of continental and oceanic crust.•Plate tectonics - The theory that pieces of Earth’s lithosphere are in constant motion, driven by convection currents in the mantle. This explains the formation, movement and subduction of Earth’s plates.

Convection Currents and the Mantle

Remember, heat is transferred through radiation, conduction, and convection.

Radiation – the transfer of heat by electromagnetic waves.Sunlight warms surface of Earth.

Convection Currents and the Mantle

Conduction – heat transfer by direct contact of particles of matter.

-Burning your hand when touching a stove

Convection Currents and the Mantle

Convection – heat transfer by the movement of heated fluid (liquid or gas). This is what makes tectonic plates move.

-Convection oven or heating soup -Wind in the atmosphere-Heat from the Earth’s core and the mantle itself causes convection currents in the mantle

Plate Tectonics• The Earth’s crust is divided into 12 major

plates which are moved in various directions. There are many minor plates as well.

• The plates are made of the thin, rigid lithosphere with the continental and oceanic crust attached to it.

• This plate motion causes them to collide, pull apart, or scrape against each other.

Plate Tectonics• Each type of interaction causes a

characteristic set of Earth structures or “tectonic” features.

• The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction.

• “Plates” of lithosphere are moved around by the underlying hot mantle convection cells

Rates of Movement

Plates move at amazingly slow rates, from one to ten centimeters a year. Fingernail growth speed!

Plate Movement• “Plates” of lithosphere are moved around by

the underlying hot mantle convection cells

Part 4: Plate Boundaries-Key Questions:

What are the three types of plate boundaries?

What kind of movement occurs at the different tectonic plate boundaries?

What crustal features result from the movement at these plate boundaries?

• Divergent

• Convergent

• Transform

Three types of plate boundary

• Spreading ridges~mid-ocean ridge on ocean floor• As plates move apart new material is erupted to fill the

gap; volcanic activity

Divergent Boundariesmid-ocean ridge & rift valleys

• Iceland has a divergent plate boundary running through its middle

Iceland: An example of continental rifting

Age of Oceanic Crust

Courtesy of www.ngdc.noaa.gov

• There are three types of convergent plate boundaries– Continent-continent collision– Continent-oceanic crust collision– Ocean-ocean collision

Convergent Boundaries

• Forms folded mountains, e.g. European Alps, Himalayas

Continent-Continent Collision

Himalayas

Continent-Oceanic Crust Collision

• Oceanic lithosphere subducts underneath the continental lithosphere

• Oceanic lithosphere heats and melts forming magma

• The magma rises forming volcanic mountains.

• Ex. The Andes (S. America)

Subduction Occurs

• When two oceanic plates collide, the older, denser oceanic plate sinks under the newer, less dense plate into the mantle forming a __________________ zone.

• The subducting plate is bent downward to form a very deep depression in the ocean floor called a ________________.

• The worlds deepest parts of the ocean are found along trenches. – E.g. The Marianas Trench is 11 km deep!

Ocean-Ocean Plate Collision

• Where plates slide past each other• Events & features that result at this boundary type: Faults & Earthquakes

Transform Boundaries

Above: View of the San Andreas transform fault

…what’s the connection?

Volcanoes and Plate Tectonics…

Volcanism & earthquakesare mostly focused at plate margins

Pacific Ring of Fire

- Subduction - Rifting - Hotspots

Volcanoes are formed by:

Pacific Ring of Fire

Hotspot volcanoes

• Hot mantle plumes breaching the surface in the middle of a tectonic plate

What are Hotspot Volcanoes?

Photo: Tom Pfeiffer / www.volcanodiscovery.com

The Hawaiian island chain are examples of hotspot volcanoes.

The tectonic plate moves over a fixed hotspot forming a chain of volcanoes.

The volcanoes get younger from one end to the other.

…what’s the connection?

Earthquakes and Plate Tectonics…

• As with volcanoes, earthquakes are not randomly distributed over the globe

At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur. Earthquakes occur in subduction zones from the displacement of magma.

Figure showing the distribution of earthquakes around the globe

Plate Tectonics Summary• The Earth is made up of 3 main layers (core,

mantle, crust)• On the surface of the Earth are tectonic

plates that slowly move around the globe• Plates are made of crust and upper mantle

(lithosphere)• There are 2 types of plate• There are 3 types of plate boundaries• Volcanoes and Earthquakes are closely

linked to the margins of the tectonic plates