Geography

Internal structures of the Earth

• The Crust:
• The outer most thin layer consisting of continental and oceanic crust
• It is made up of 2 types of materials: Sima (Silica and Magnesium) and Sila (Silica and Aluminum)
• It is broken up into 7 large plates and 7 small plates.
• The Mantel:
• A mixture of rock in the solid or molten state.
• It is very rich in iron and magnesium.
• The molten/ liquid portion is called magma.
• Slow movements in the mantel, due to convection currents, disturb the crust.
• It is approximately 2900 km thick.
• The Core:
• Innermost layer, it is about 3500 km deep and contains 2 sections.
• The core has the highest temperature.
• Consists of mainly iron and nickel.

Theory of Plate Tectonics

• Theory of plate Tectonics:
• The Earth's crust is broken up into plates.
• These plates fit together like a puzzle.
• Plate Tectonics
• The study of plates that make up the Earth's surface, and how their movement affect rocks and landforms.

Plate Margins/ Boundaries

• Plate Margins/ Boundaries:
• Where plates meet, lines of weakness are formed.
• Earthquakes and volcanoes are associated with these zones:
• Convergent
• Divergent
• Transform

Convergent/ Destructive Plate Boundaries

• When 2 plates are pushed together.
• The oceanic plate is pushed downwards by the adjoining plate and is absorb into the mantel.
• That area is know as the subduction zone.
• 75% of Earthquakes are at convergent boundaries.
• Volcanoes and Volcanic islands (St. Vincent) are formed here.
• An example is where the Caribbean plate meets the Cocos plate.
• The highest fold mountains are formed where 2 continental crust meet. (Himalayas)
• 
• Bildunterschrift: Convergent Plate Boundary

Divergent/ Constructive Plate Boundaries

• When 2 plates are pushed slowly apart.
• Molten rock or magma is forced slowly between them.
• When it hardens, it forms a strip of new crust.
• Great under water ridges are formed like this.
• There are divergent plate boundaries beneath the sea.
• The Mid- Atlantic Ridge is best known.
• Volcanoes, ocean ridges and faults are associated with this boundary.
• 
• Bildunterschrift: Divergent Plate Boundary

Transform Plate Boundaries

• When to plates move past each other without converging or diverging.
• Earthquakes occur at this boundary.
• No volcanoes are found here.
• Faulting may occur here.
• The Northern part of the Caribbean plate boundary is a Transform Boundary.
• 
• Bildunterschrift: Transform Plate Boundary

All Plate Boundaries

Volcanic Island Arc

• A chain of Volcanic islands.
• Formed at subduction zones.
• Can be seen where the North and South American Plate move West and are forced under the Caribbean plate.
• From Grenada to Saba is a volcanic arch.

Ocean Trenches and Ridges

• Formed at Convergent plate margins.
• A trench marks the position where the flexed subducting slab begins to descend.
• Trenches are parallel to Volcanic Archs and are about 200 km away.

Volcanoes

• Formed when magma rises and is able to penetrate the earth's surface.
• Most volcanoes are close to convergent and divergent plates.
• Some may form at other points of weakness in the surface of the earth.
• Volcanoes and other features like lava plateaus are formed over time by repeated eruptions.

Classification of Volcanoes

• Active Volcanoes
• Shows signs of one or more of the following: gases, steam, ash, rock, lava, etc.
• Example: Soufriere Hills volcano in Montserrat
• Dormant Volcano
• Currently inactive, may erupt in the future
• Example: Mount Fuji, Japan
• Extinct Volcano
• Not expected to erupt again
• Example: Mount Kenya, Africa

Types of Volcanos

• A volcanos shape and size depends on the type of lava and the way it erupted.
• The types of volcanos are:
• Composite Volcano
• Shield Volcano

Composite Volcanos

• Made up of alternate layers of viscous(thick) lava ash and cinder.
• Gases trapped in acid magma leads to violent eruption.
• This ejects ash and cinder which settle all around the crater.
• The eruption clears the vent, allowing lava to flow freely.
• The lava cools/ solidifies of the ash and cinders.
• Repeated eruptions like this build a composite volcano.
• Violent eruptions may cause cracks on the sides of the volcano.
• Lave may escape causing parasitic cones there.
• 
• Bildunterschrift: Composite Volcano

Shield Volcanos

• Formed by basic lava (low viscosity) which is thin and runny
• This lava type flows for long distances and takes longer to cool
• the crater is wide
• the sides of the volcano is broad and gentle
• Found at divergent plate boundaries
• 
• Bildunterschrift: Shield Volcano

Extrusive Volcanic Features

• Viscous Lava- Comes to the surface close to subduction zones. It is high in silica and is called acid lava. It cannot flow easily. Contains gas which cannot escape easily, so eruptions are explosive and violent. Caribbean Volcanos produce viscous lava
• Basic Lava- Comes to the surface at divergent plate boundaries. It has a low silica content and flows easily. Basalt Rock is formed when it solidifies. Volcanos in hawii