Structure of Earth

• Earth made up of three layers.

1. Core (barysphere)

• Radius is about 3470km
• Made up of very dense rocks
• Further divided into ---- inner core which is solid and molten outer core
• Core has extremely high temperature, 3000 to 6000 degree Celsius
• Core has high pressure

1. Mantle (also known as 'cloak')

• About 2900km thick
• Top layer known as the asthenosphere
• Rocks are less dense than those of the core but denser than those of the crust.
• Lower temperature compared to the core (1000 to 2000 degree Celsius)
• Lower pressure than that of the core

1. Crust (lithosphere)

• Outer layer
• Thickness varies from 5 to 40 kilometers
• Made up of lighter rocks
• Part of the crust nearer the mantle has a temperature of 400 degree Celsius while the surface has a temperature of about 10 degree Celsius
• May be distinguished into two layers

• Lower layer ---- sima/oceanic crust

1. Consist of darker, denser basaltic rocks
2. More continuous and forms ocean floors and bases of continents

• Upper layer-- sial/ continental crust

1. Consists of lighter, granitic rocks which are more acidic
2. Discontinuous and forms the continents.

• Crust is thought to be broken into several plates (crustal plates)

Crustal movements

(Plate tectonics)

1. Continental drift theory

• Developed by Alfred Wegener in the early 1900's

• There was a single continent known as Pangaea (supercontinent)
• 200 million years ago, it began to break up into 2 land masses called Laurasia and Gondwanaland, which continued to break up and drift apart until they reached their present position.
• Surrounded by a large ocean (Panthalassa)

Does his theory make sense?

Yes. (Evidence)

• Coastlines of South America and Africa can fit together like a jigsaw puzzle.
• Identical fossils (bits of rocks and dead organisms) found in rocks in South America and Africa.
• Mountain chains of Europe, Africa and north and South America show many distinct similarities but are separated by the Atlantic Ocean.

No. (Limitations)

• Why did Pangaea break up?
• How did it break up?
• How did the continents move apart?

1. Plate tectonics theory

• Modifies the old theory of continental drift
• Earth is described as a restless planet with plates drifting across the surface
• It helps as it is a more satisfactory explanation that sheds light on the following

1. Internal forces shaping the Earth's surface
2. Crustal movements (folding and faulting) and volcanic activity
3. Formation of major landforms
4. Occurrence of some major natural hazards

What does the theory postulate (put forward) about crustal plates?

• Earth's crust is made up of 7 large slabs or major crustal plates and several minor ones

1. North American plate
2. South American plate
3. Indo-Australian plate
4. African plate
5. Eurasian plate
6. Antarctic plate

• Oceanic plates that lie beneath the oceans: ------

1. Pacific plate
2. Nazca plate (minor)

What causes plate movements and instability?

• Caused by complex convectional currents of magma in the asthenosphere
• These currents, driven by heat energy released by radioactivity in the mantle, drag the plates along.
• These movements are irregular

How do these plates move?

1. Plates split and move apart.

• Allow new crustal materials (magma) to rise to the ocean floor ----- mid ocean ridges
• Zone of divergence because new crust is created it is considered a constructive plate boundary.

1. Plates collide

Collision of continental plate and oceanic plate

• Denser oceanic crust slips beneath lighter continental crust ----zone of subduction
• The oceanic crust is subducted into the asthenosphere and melts into the magma
• Destructive plate boundary

1. Ocean trench is formed

• The Marianas /Philippines Trench
• Caused by the subduction of the Pacific Plate

1. Volcanoes / volcanic islands

• Crustal plate is melted to form magma during subduction
• Magma rises through the fault lines
• Some of it flows in a violent eruption
• Forms a chain of volcanic islands across the plate boundary
• Philippine islands

Collision of 2 crustal plates

• Because they are of equal density, neither sinks
• Plates edges are compressed
• Fold mountains results
• Collision of Indo-Australian plate and Eurasian plate ---- Himalayas

3. Plates scrape and slide past each other

• Conservative plate boundary
• Causes faults and earthquakes

• San Francisco, California, USA