The outer layer of the earth is called the lithosphere. This layer is (relatively) cold and rigid and comprises the crust and top part of the mantle.
The lithosphere is made of tectonic plates which are less dense than the mantle below.
The crust is too thick to drill through, so most of our information about the earth is collected from seismic waves produced by earthquakes and man made explosions.
More on the earths structure
The mantle is the zone between the core and the crust. It is cold and rigid just below the crust, but at greater depths it is hot and non-rigid and able to move.
The earths core transfers energy, so the temperature of the mantle increases with depth.
Convection currents slowly move plates.
Oceanic crust is denser than continental crust. When these plates collide, the oceanic plate (which is cooler at the margins) sinks, pulling more of the plate down, and partly melting as it reaches the hotter part of the mantle. This is called subduction.
Plate tectonic theory
Most scientists now accept the theory of plate tectonics, which suggests that Afric and South America could once have been one land mass, because;
-it explains a wide range of evidence
-it has been discussed and tested by many scientists.
Wegener's continental drift theory (1914) was not accepted by scientists at the time.
In the 1960s, new sea floor spreading evidence was found. Subsequent research led to wegener's theory slowly becoming accepted.
magma and rocks
Magma rises up through the earth's crust because it is less dense than the crust. This can cause volcanoes.
Magma can have different types of composition which cause different types of eruption.
Geologists study volcanoes to try to forecast eruptions and reveal more about the structure of the earth.
Different types of igneous rock are formed from lava.
Iron-rich basalt rock comes from runny lava in slower volcanic eruptions. Silica-rich rhylite rock comes from thick lava in explosive eruptions.
Some raw materials used to make construction materials are found in the earths crust.
Hardness can be compared by rubbing two materials together. Granite is harder than marble, and marble is harder than limestone.
Many buildings look like they are made from one material, but they are often made with different materials and only lined with an expensive material.
Limestone is a sedimentary rock. It is made from soft sediments compressed and cemented together. This makes limestone quite soft.
Marble is a metamorphic rock formed when limestone is changed by heat and pressure, and is typically composed of an interlocking mosaic of carbonate crystals. Marble is harder than the limestone because it has been baked.
Granite is formed when magma cools and solidifies. It is an igneous rock with interlocking crystals. This makes it very hard.