THE STRUCTURE OF THE EARTH
• The earth is made up of 4 distinct layers:
• The inner core is in the centre of the earth and is the hottest part of the earth.
The inner core is solid. It is made up of iron and nickel with temperatures of
up to 5500°C. With its immense heat energy, the inner core is like the engine
room of the Earth.
The outer core is the layer surrounding the inner core. It is a liquid layer, also
made up of iron and nickel. It is still extremely hot here, with temperatures
similar to the inner core.
The mantle is the widest section of the earth. It has a diameter of
approximately 2900km. The mantle is made up of semi-molten rock called
magma. In the upper parts of the mantle the rock is hard, but lower down,
nearer the inner core, the rock is soft and beginning to melt.
The crust is the outer layer of the earth. It is a thin layer between 0-60km
thick. The crust is the solid rock layer upon which we live.
There are two different types of crust: continental crust, which carries land,
and oceanic crust, which carries water.
The diagram below shows the structure of the earth. In geography, taking a
slice through a structure to see inside is called a cross section.
CROSS SECTION OF THE
EARTH
TECTONIC PLATES
• Plates and plate boundaries
• The earth's crust is broken up into pieces. These
pieces are called plates. Heat rising and falling
inside the mantle creates convection currents.
The convection currents move the plates. The
movement of the plates, and the activity inside the
earth, is called plate tectonics.
• Plate tectonics cause earthquakes and volcanoes.
The point where two plates meet is called a plate
boundary. Earthquakes and volcanoes are most
likely to occur either on or near plate boundaries.
Subduction zone
• The San Andreas fault in Western
California
•
Convection currents
Conservative plate margins
• Conservative margins occur where two plates slide
horizontally past each other. There is no subduction zone
with one plate being destroyed beneath another nor is there
a constructive zone as at the Mid-Atlantic ridge. As the
two plates slide past each other, the high levels of friction
create large strains along the slippage zone faults.
Eventually the strain energy is released in an earthquake.
When this happens there is likely to be large scale
movement along the transform fault with substantial
damage to nearby buildings.
• The best known example of a conservative plate margin
can be found along the west coast of the USA where the
Nazca plate is moving in a North Westerly direction and
sliding past the North American Plate. The North
American plate is also moving in a North-Westerly
direction but not so fast as the Nazca plate so the two
appear to be moving in opposite directions. The fracture
zone between the two plates is the San Andreas Fault - a
huge fault running for hundreds of km along the
Californian coastal region from San Francisco to Los
Angeles. The region has a number of other substantial
faults running parallel to the major San Andreas fault - the
Hayward Fault runs almost parallel and to the east of the
San Andreas fault with the modern city built across the two
active fault planes. Earth tremors and earthquakes
regularly occur along these fault planes - in 1906 the city
of San Francisco was destroyed in a magnitude 8.2 quake
on the San Andreas fault; in 1989 substantial damage and
some 69 fatalities resulted from the magnitude 7.1 Loma
Prieta earthquake.
The 1989 Loma Prieta
earthquake
The 1906 San
Francisco
earthquake
Volcanism
• Processes which lead to the eruption of lava,
gases, and pyroclastic materials onto the surface
and into the atmosphere
• Active volcanoes An active volcano has erupted recently and is likely to erupt again.
• Dormant volcanoes A dormant volcano has not erupted in 2000 years.
• Extinct volcanoes An extinct volcano will never erupt again.
What are Volcanoes?
• Conical mountains formed
around a vent where lava,
gases, and pyroclastic
materials are erupted
– Variations in the type of lava
and other factors distinguish
three types
– Most have a central crater,
while calderas and fissures
are also common
Cone volcanoes:
•These are usually found at destructive boundaries.
•Cone volcanoes are tall and steep-sided.
•Cone volcanoes are formed by eruptions of thick, viscous (sticky) lava.
•The thick lava moves relatively slowly and hardens quickly to form new
rock - this explains the formation of a cone shape.
•Eruptions tend to be violent.
Shield volcanoes:
•These are usually found at constructive boundaries.
•They are low, with gently sloping sides.
•Shield volcanoes are formed by eruptions of thin, runny lava.
•Eruptions here tend to be frequent but relatively gentle.
Composite volcanoes:
•These volcanoes are composed (made up) of alternating layers of lava and
ash (other volcanoes just consist of lava).
•The eruptions from these volcanoes may be a pyroclastic flow rather than a
lava flow. A pyroclastic flow is a mixture of hot steam, ash, rock and dust.
•A pyroclastic flow can roll down the sides of a volcano at very high speeds
and with temperatures of over 400° C.
What is this guy doing?
Why is he dressed like this?
Crater Lake
Cinder Cone
Lava Types
Debris Avalanche, Mt. Adams
Pyroclastic Flow, Mt. St. Helens
Mt St Helens Eruption
Volcanic eruptions can have a devastating
effect on people and the environment.
However, unlike earthquakes, volcanoes can
also have a positive impact on an area. These
positive impacts can help to explain why people
choose to live near volcanoes.
MONTSERRAT
• Montserrat is a small island in the Caribbean. There is a volcanic area
located in the south of the island, called Soufriere Hills.
• The volcanic peak in this area is called Chances Peak, which had been
dormant for over 300 years. Then in 1995, the volcano began to give
off warning signs of an eruption (small earthquakes and eruptions of
dust and ash). Once Chances Peak had woken up it then remained
active for a period of 5 years. The most intense eruptions occurred in
1997.
• During this time, Montserrat was devastated by pyroclastic flows. The
small population of the island (11,000 people) was evacuated in 1995
to neighbouring islands. The evacuees became refugees.
• Despite the evacuations, 19 people were killed by the eruptions. This is
because a small group of people chose to stay behind on the island and
watch over their crops.
• Volcanic eruptions and lahars have destroyed large areas of
Montserrat. The capital, Plymouth, has been covered in layers of ash
and mud. Homes and buildings have been destroyed.
(Lahars are similar to pyroclastic flows but contain more water )
The graphic shows the progress of the eruption and its impact on
the island.
Volcanic activity has calmed down in recent years and people
have begun to return to the island.
Eruption
Ash
Disaster
Evacuation
The island of
Montserrat in the
Caribbean.
How deep is the ash in this photo?
What effect might it have had on the people living here?
Few people died in this eruption but it is still a disaster. Why?
Why do people live close to volcanoes?
People live close to volcanoes because Geothermal energy can be harnessed by using the steam from
underground which has been heated by the Earth's magma. This steam is used to drive turbines in
geothermal power stations to produce electricity for domestic and industrial use. Countries such as Iceland
and New Zealand use this method of generating electricity.
Volcanoes attract millions of visitors around the world every year. Apart from the volcano itself, hot
springs and geysers can also bring in the tourists. This creates many jobs for people in the tourism
industry. This includes work in hotels, restaurants and gift shops. Often locals are also employed as tour
guides.
Lava from deep within the earth contains minerals which can be mined once the lava has cooled. These
include gold, silver, diamonds, copper and zinc, depending on their mineral composition. Often, mining
develop around volcanoes.
Volcanic areas often contain some of the most mineral rich soils in the world. This is ideal for farming.
Lava and material from pyroclastic flows are weathered to form nutrient rich soil which can be cultivated
to produce healthy crops and rich harvests.