Plate Tectonics
German scientist Alfred Wegener (1880-1930) proposed that the continenets were once one big landmass, which he called Pangaea from the Greek for 'entire Earth'. The theory of plate tectonics says that the Earth's surface is covered by a serioes of plates, which make up the Earth's crust.
German scientist Alfred Wegener (1880-1930) proposed that the continenets were once one big landmass, which he called Pangaea from the Greek for 'entire Earth'. The theory of plate tectonics says that the Earth's surface is covered by a serioes of plates, which make up the Earth's crust.
Evidence for Plate Tectonics
Density and convection
As discussed earlier, convection results from the movement of heat energy through the mantle. As the fluid rock heats, the particles move further apart and the density decreases. As it becomes less dense, it rises. Near the crust, the rock is pushed sideways and some of its heat energy is transferred to its surroundings. As the temperature of the rock falls, the particles more closer together, and so the density increases. The rock starts to sink and the cycle starts over again.
Density and convection
As discussed earlier, convection results from the movement of heat energy through the mantle. As the fluid rock heats, the particles move further apart and the density decreases. As it becomes less dense, it rises. Near the crust, the rock is pushed sideways and some of its heat energy is transferred to its surroundings. As the temperature of the rock falls, the particles more closer together, and so the density increases. The rock starts to sink and the cycle starts over again.
Crustal densities
Objects float because of density. A substance will float in a liquid of higher density. The density of the continents is on average less than that of the mantle, so the continents float.
Magnetic stripes
The magnetic elements that make up the rock align themselves with the local magnetic field. They point towards north. This is similar to watching the needle of a compass move, and when it points to magnetic north the needle is frozen so it can't move anymore.
Molten rock cools and solidfies. The rock's magnetic particles will point north and stay that way after they have cooled and solidified.
As more and more magma is pushed up through the crack in the crust, the seafloor is pushed sideways. As the magma cools, more tiny magnetic partilces are fixed so that they point towards magnetic north.
If the Earth's magnetic fields remained constant, then all the magnetic particles in rocks would point north. But, over tens of thousands of years, the Earth's magnetic north and south poles have swapped. These reversals mean that in rocks created at different times the magnetic particles point in opposite directions. THis is seen as striping on the seafloor either side of mid-ocean ridges.
The magnetic striping indicates new seabed rock is being formed along the mid-ocean ridges. New seafloor is spreading out from this ridge.
Objects float because of density. A substance will float in a liquid of higher density. The density of the continents is on average less than that of the mantle, so the continents float.
Magnetic stripes
The magnetic elements that make up the rock align themselves with the local magnetic field. They point towards north. This is similar to watching the needle of a compass move, and when it points to magnetic north the needle is frozen so it can't move anymore.
Molten rock cools and solidfies. The rock's magnetic particles will point north and stay that way after they have cooled and solidified.
As more and more magma is pushed up through the crack in the crust, the seafloor is pushed sideways. As the magma cools, more tiny magnetic partilces are fixed so that they point towards magnetic north.
If the Earth's magnetic fields remained constant, then all the magnetic particles in rocks would point north. But, over tens of thousands of years, the Earth's magnetic north and south poles have swapped. These reversals mean that in rocks created at different times the magnetic particles point in opposite directions. THis is seen as striping on the seafloor either side of mid-ocean ridges.
The magnetic striping indicates new seabed rock is being formed along the mid-ocean ridges. New seafloor is spreading out from this ridge.
Subduction zones
HMS Challenger's expedition in 1872 surveyed the seafloor and found the Mariana Trench. Scientists now understand why these trenches exist. They are boundaries where one heavier crustal plate is pushed under a lighter crustal plate. This is called subduction. As the subducted plate is pushed further below the crust of another plate, a huge amount of heat is generated. This turns the rock into molten magma.
HMS Challenger's expedition in 1872 surveyed the seafloor and found the Mariana Trench. Scientists now understand why these trenches exist. They are boundaries where one heavier crustal plate is pushed under a lighter crustal plate. This is called subduction. As the subducted plate is pushed further below the crust of another plate, a huge amount of heat is generated. This turns the rock into molten magma.
Volcanic activity usually occurs where there are subduction zones, such as in Indonesia. The volcanoes are particularly dangerous as they have an abundant supply of magma from the subducting crust below them. The areas of volcanic activity, or hot spots, are caused by plumes of hot magma making its way to the surface throug hthe Earth's crust.
Fossils
Scientists have found fossils of the freshwater reptile Mesosaurus in both sothern South America and the southern tip of Africa.
Fossils
Scientists have found fossils of the freshwater reptile Mesosaurus in both sothern South America and the southern tip of Africa.
These points of the globe are thousands of kilometers apart. The Mesosausus was not a good swimmer and could not possibly have swum such a large distance. Fossils of the Glossopteris have been found spread across South America, Africa, India, Antarctica and Australia, which shows that these continents were once joined.
Climate Change
A glacier is a river of ice and snow. Evidence of past glacial activity exists in the form of U-shaped valleys and the till or desposits that are left behind when the glacier has retreated or melted.
A glacier is a river of ice and snow. Evidence of past glacial activity exists in the form of U-shaped valleys and the till or desposits that are left behind when the glacier has retreated or melted.
The pictures below show the distribution of glaciers on the continent of Pangaea and currently. It explains why evidence of glaciers can occur in desert regions. If the continents were arranged as one large continent located near the South Pole, then it provides a more logical picture of why glacial evidence exists where it does today.
QUESTION
1. What explanation would you come to if you found:
a) almost identical fossils in very disparate continents.
b) evidence of glacial activity in countries near the equator.
TASK 1
Write a picture story book that explains plate tectonics to young children. It may include an animal, human and/or plant.
1. What explanation would you come to if you found:
a) almost identical fossils in very disparate continents.
b) evidence of glacial activity in countries near the equator.
TASK 1
Write a picture story book that explains plate tectonics to young children. It may include an animal, human and/or plant.