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History of the Earth


If it was possible to dig throughto the centre of the Earth, what would you find?

What is the Earth made from?


At the very centre of the Earth is the core. This layer is made up of nickel and iron and is divided into two sections. The liquid outer core and the solid inner core.

Firstly, beneath our feet is a rocky layer called the crust. Despite all life existing on this layer, it makes up only a tiny part of the Earth’s mass (about 0.4%).It mostly consists of oxygen, magnesium, aluminium, silicon, calcium, sodium, potassium and iron.

Beneath the crust is the mantle, made of silicon, oxygen, aluminium and iron. The high temperature allows this layer to move slowly. The cooler section near to the crust is less mobile than the hotter section next to the core ( known as the asthenosphere).

Crust

Mantle

Outercore

Innercore

What are the layers of the Earth?


Cross-section of the Earth



In 1912, a German scientist called Alfred Wegener proposed that South America and Africa were once joined together and had subsequently moved apart.

Africa

SouthAmerica

Do the continents move?


What is continental drift?

PANGAEA

Wegener believed that all of the continents were once joined together as one big land mass called Pangaea and this was intact until about 200 million years ago.

The idea that continents are slowly shifting their positions is called continental drift


Continental drift


Wegener knew the continents had drifted but he couldn't explain how they drifted.

It wasn't until the 1960's that geologists used ocean surveys to explain continental drift with the theory of Plate Tectonics.

What is plate tectonics?


Continental drift and plate tectonics


What is continental drift?


What is plate tectonics?


The lithosphere is made up of the crust of the Earth and the largely solid layer of mantle beneath it. The lithosphereforms the plates that float and move on the liquid layer ofmantle.

Lithosphere(between 100km

- 300km thick)

Oceanic crustapprox. 6 -10 km thick

Continental crustapprox. 35-70 km thick

Layer of mantlebehaving in rigid manner

Layer of mantle behaving in liquid manner (asthenosphere)

Temperatureboundary approx. 1300oC

What is the lithosphere?


A destructive plate boundary causes earthquakes and volcanoes. Japan sits on this kind of plate boundary.

A collision plate boundary causes mountains, like the Himalayas, when two plates of relatively equal densities collide.

A conservative plate boundary causes earthquakes as plates rub past each other. California sits on this type of boundary.

Tectonic plates move, very slowly, in different ways:

A constructive plate boundary creates newcrust. This is the plate boundary at which seafloor spreading occurs.

How do tectonic plates move?


Tectonic plate movements


The energy transfer of these convection currents is powerful enough drive the movement of the lithosphere.This is how the continental and oceanic plates move.

The mantle nearest the core rises upwards towards the cooler crust. As this mantle begins to cool and solidify, it is pushed along by further hot mantle rising from beneath. This process is called convection.

Why do the plates move?


Cause of plate movement


The Earth's surface is made up of a number of large plates (like pieces of a jigsaw puzzle) that are in constant, slow motion.

The ocean floors are continually moving, spreading from the centre and sinking at the edges.

At the edges of these plates (plate boundaries) earthquakes and volcanoes occur.

Convection currents in the mantle move the plates. The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth.

Key points of plate movements


Review of key terms



Although volcanoes can be extremely destructive influence they are also very creative forces of nature.

Some volcanoes are more violent than others. This is due to the thickness of the lava. Runny lava produces more ‘gentle’ volcanoes, thick lavaproduces more explosive volcanoes.

How do volcanoes affect an area?

Volcanoes create new land, which is often very fertile. They also play a major part in the rock cycle. All igneous rocks rely on volcanic activity for their formation and transportation to the surface.


This causes the more buoyant magma to rise and collect in pockets called magma chambers.

Volcanoes and volcanic rock

Eventually, with enough pressure, some of this magma pushes itself through cracks and weaknesses in the Earth’s surface, which is where volcanoes can be found.

Volcanoes occur because the magma or molten rock beneath the crust is less dense than the solid rock of the crust.

crater

central vent

strata (layers)

magna chamber


Types of volcanoes


Around which plate do we find most volcanoes?

Where are volcanoes found?


Some rock on the Earth's surface is formed by volcanic activity. As the magma cools, it solidifies and crystallizes to form igneous rocks. Some are formed above the ground and others form beneath the ground.

Granite, obsidian and basalt are all types of igneous rock.

USGS

What happens to molten rock?


The size of the crystals in an igneous rock is related to the rate of cooling in the molten rock.

Quick Cooling Slow Cooling

Magma is full of minerals which will turn into crystals in the right conditions. If the magma cools quickly, the crystals do not have much time to form and are therefore small in size. Likewise, if the magma cools slowly, then the crystals have time to grow and can become large.

Magma and rocks


Which of these rock types are quick cooling and slow cooling?

Rocks from lavawhich is iron rich with a low silicacontent.

Rocks from lavawith a high silica content

Rhyolite Granite

Basalt Gabbro

Quick Cooling Slow cooling

Quick Cooling Slow cooling

More on magma and rocks


The rock cycle



Is the Earth dangerous?

Although plate movements occurvery slowly, they can have dramatic effects. When the large plates rub together they can result in devastating earthquakes. The earthquake of 2005 in Kashmir, killed over 87,500 people.

Plate movements in the oceancan also cause powerful tsunamis, asseen in the Indian Ocean Tsunami of 2004.

Those living near volcanoes, face the possibility of eruptions that bring poisonous gases, falling debris, landslides and lava flows.


Scientists are fully aware of the places where earthquakesare likely to occur. However, they have only manageda couple of accurate earthquake predictions in human history.

In 1975, in Haicheng, China, the government evacuated thecity the day before a major earthquake. Scientists had noted changes in land elevation and there had been a number of minor foreshocks alongside unusual animal behaviour. Fourteen year later they predicted another earthquake by similar methods.

However, these are isolated cases, normally earthquakesstrike at any time and scientists have not yet been able tofind an accurate way of predicting exactly when and where.

Can earthquakes be predicted?


Predicting eruptions


Volcanologists (people who study volcanoes) are skilled at predicting the likelihood of an eruption.

However, it's very difficult to pinpoint exactly when an eruption will happen. Often, moving magma doesn't result in an eruption, but instead cools below the surface.

Monitoring potential eruptions is expensive. With many volcanoes erupting only once every few hundred years, it is not possible to monitor every site.

The problem of prediction


Tourists are attracted to areas of volcanic activity.

Geothermal energy can be produced in many volcanic areas.

Can you think of any other reasons?

This lava is weathered (broken down) to form a fertile soil.

Why do people live in volcanic areas?


Iceland sits on the boundaries of the mid-Atlantic tectonic plates. A plate boundary is not a peaceful place to be, but its situation does provide an endless supply of energy.

Icelanders use hot water out of the Earth to heat their homes and businesses. It's stored in enormous tanks on top of one Reykjavik's few hills. This allows gravity to distribute it around the town.

How can geothermal energy be used?


Geothermal energy



asthenosphere – The layer of hotter, more mobile mantle beneath the lithosphere.

core – The centre of the Earth, composed mainly of nickel and iron.

continental drift – Theory stating that the continents were once all joined together and have subsequently moved apart.

convection currents – Circular energy currents created by the heating and cooling of material in the mantle.

crust – The outer rocky layer of the Earth upon which we live.

Glossary (1/2)


Glossary (2/2)

igneous – A type of rock formed by volcanic activity.

lithosphere – The layer that makes up the moving plates, composed of the crust and upper level of the mantle.

mantle – The hot and slightly mobile rocky layer beneath the crust.

Pangaea – A super-continent that existed about 200 million years ago.

plate tectonics – The theory of moving plates that explains continental drift.

volcanologists – Scientists who study volcanoes.


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