Specific gravity 2.9 – 3.4
Oceanic crust is dark coloured lava, containing large quantities of iron and magnesium. The iron and magnesium content makes the lava weather easily and gives oceanic crust a greater specific gravity than continental crust. The lavas are fluid and the volcanoes have low angles (sheild volcanoes) – examples in Hawaii and Iceland.
Composition
Most of the basaltic, dark-coloured lava is poured out along the mid-ocean ridges where the ocean floor is tearing apart. This action causes earthquakes, volcanoes and the moving apart of the continents which float along as passengers. Much of the floor of the Atlantic Ocean is no older than 60 million years. Vents of hot volcanic gasses (black smokers) occur along mid-ocean ridges. They deposit metals such as gold, nickel and copper. The environment also provides an oasis of life – from bacteria to 3 metre long tube worms and large clams.
As basalt cools, magnetite crystals within it cool past the Curie Point and take on the direction of magnetic north at that time. These compass directions may be "read" like the universal product codes at the supermarket checkout! Over 50 reversals of the north and south poles have taken place since the end of the Cretaceous period 65 million years ago.
The Hawaiian Islands are a good example of volcanoes associated with a mantle plume. Rocks of the Pacific Plate move westwards over a plume of magma rising up from the mantle. The further north westward you go in the Hawaiian Island chain the older the volcanoes are. Look at a bathymetric map of the Pacific Ocean and follow the line of these volcanoes further west and then northward toward the Aleutians.
Volcanoes – continental crust
Specific gravity 2.7
Continental crust is composed mostly of oxides of silicon and aluminum. The rocks are light in colour and resist weathering. The continental crust "floats" on top of the oceanic crust. Volcanoes of this type have steep angles to their slopes, and violent, explosive eruptions. Examples: Mount Saint Helens and Mount Pelee, Martinique.
Composition
As the North American Plate moves westwards it pushes against the Pacific Plate which plunges down. As it moves down it melts, and tear-drop-shaped blobs of molten magma rise up through the overlying rocks giving the possibility of a violent eruption, or gradual cooling of the mass to form a pluton of granitic rock.
Volcanoes formed of continental crust have the most violent eruptions. Glowing clouds of ash killed 29,000 people in Martinique in 1902. When Krakatoa, Indonesia erupted in 1883 a tidal wave killed 96,400 people. Fine dust from this eruption coloured the sunsets for a year after the eruption. Famine and epidemics have also added to the toll of destruction caused by volcanoes of this type.
The temperature of lava is measured by various devices. The optical pyrometer is the most useful. The colour of a lamp filament is matched with the colour of the lava.
Approximate colour scale of temperature in degrees celsius:
1,100 C = yellowish red heat
1,260 C = incipient white heat
1,480 C = white heat