The Tectonic Cycle

From ZuluNotes - Free Leaving Cert Notes

Patterns and Processes in the Physical Environment.

Contents 1 Structure of the Earth 1.1 The Continents 1.2 Ocean Floor 2 Plate Tectonics 2.1 The Convectional Currents 2.2 Hess-Sea Floor Spreading 2.2.1 Proof of Sea Floor Spreading 2.2.2 Plate Margins 2.2.3 3 TYPES 3 Oceanic-Continental

Structure of the Earth

1. The Crust
2. The Mantle
3. The Core

The Continents

• Light Granite like rocks
• Average thickness 45km and 70km under mountain ranges.

Ocean Floor

• Heavy Basalt like rock.
• Average thickness of about 3-20km.

• The continents, ocean floors and the upper mantle form the LITOSPHERE. All rocks here are solid.

• The lower mantle (ASTHENOSPHERE) consists of rock in a molten/semi-molten state, that moves to form convection currents.
  • Made up of Nickel and Iron.
  • Hottest part of the earth.
  • Temperature greater than 4,000⁰.

Plate Tectonics

• The earths crust is made up of plates that float on the heavy molten rocks and move around due to convection currents.
• As the plates move so do the oceans and the continents that sit on top of them.

The Convectional Currents

• Drag Plates apart in Separation.
• Push plates together in collision.

High Mountains are formed where plates separate. Fold Mts form when plates collide.

Hess-Sea Floor Spreading

The Theory of sea floor spreading suggests that the ocean floors widen as new rock is formed along mid-ocean ridges where continents have separated.

Proof of Sea Floor Spreading

• Existence of Mid Ocean Ridges
• Varying ages of sea floors.

Plate Margins

• New Ocean Floor is created at these boundaries where plates separate.
• Old ocean floor at the opposite side of the globe is sucked back into the mantle and destroyed.

There are three different types of plate boundaries.

1. Constructive/Divergent plate boundaries.
2. Destructive Boundaries/Convergent plate boundaries.
3. Passive Boundaries/Conservative/Transverse.

Constructive Boundaries Convergent Boundaries. Passive

• A rising current of magma splits the plate in two.
• The cracks created are immediately filled in with magma from the mantle.
• This magma cools to form new crust and is called a constructive boundary as new land is formed! (constructed)
• If plates separate in one place they must collide in another, where this happens a Convergent plate boundary is present.

3 TYPES

1. Oceanic-Oceanic
2. Oceanic-Continental
3. Continental-Continental
   1. Two plates slide past each other.
   2. Resulting in Friction =Earthquakes.

Oceanic-Continental

• Continental Crust (granite) is much lighter than oceanic (basalt) therefore overrides it.(the oceanic is subducted)
• Plates collide heavier oceanic plate is subducted as it does it creates and oceanic trench.
• Stress causes the edge of the continents to be crumbled and lifted into fold mts.
• Molten rocks results in magma and volcanic activity.
• Earthquakes. Oceanic-Oceanic.
• When two oceanic plates combine the older heavier plate is subducted beneath the younger lighter plate. A TRENCH develops at the point of subduction between the two.
• The plate goes deep into the asthenosphere, it melts and the magma is released due to pressure and water, resulting in VOLCANIC ACTIVITY.
• A chain of offshore islands rises from the ocean floor(or island arc) JAPAN.
• Since crust is being destroyed it is called a DESTRUCTIVE plate boundary. Continental-Continental.
• When continental plates meet head on subduction does not occur, as they are made from very thick but light rocks.
• The lithosphere subducts but the plates remain afloat.
• Collision sweeps up and deforms any sediments that were on the sea floor.
• Intense pressure causes the crust to fracture and buckle leading to FOLD MOUNTAINS.
• Stress also results in EARTHQUAKES.

dissertation editing