Pangaea, Gondwana and Laurasia are all names given to ancient continents formed and destroyed according to the theory of plate tectonics. It is thought Pangaea broke up to form Gondwana and Laurasia.
Q Why are women's voices higher than men's?
The larynx, or voice box, in the throat has a mucous membrane that forms two pairs of folds: an upper pair (the "false" vocal cords) and a lower pair (the "true" vocal cords). The false vocal cords hold the breath against pressure from beneath - say, when you strain to lift a heavy object. They do not produce sound.
The true vocal cords do produce sound. Under the folds are bands of elastic ligaments stretched between pieces of rigid cartilage like the strings on a guitar, with muscles attached to both the cartilage and the true vocal cords. When the muscles contract, they pull the elastic ligaments tight, stretching the vocal cords out into the air passageway; this narrows the space between them. Air directed against the vocal cords makes them vibrate, creating sound waves in the air in the throat, nose and mouth. The greater the air pressure, the louder the sound.
Pitch is controlled by the tension of the true vocal cords. If they are pulled taut they vibrate more rapidly, creating a higher-pitched sound. Male sex hormones mean the vocal cords are usually thicker and longer in men than women; they therefore vibrate more slowly, giving men a generally lower range of pitch than women.
Q Do all woods float?
Any wood (in fact, any object) with a density greater than the density of water will sink. Tallow wood, Queensland Red Ironwood and certain types of ebony will all sink.
Q Why do you get lines of flint in chalk cliffs?
Flints are made of an insoluble sort of silica (silicon dioxide in chemical terms), which is also sometimes called chert. Flint and chert commonly grow as lumps and nodules in limestones, and chalk is just a rather fine- grained, pure sort of limestone. The ground-waters present in buried rocks have a small amount of silica dissolved in them, which gets precipitated out as very fine crystals that amass in the lumps. The same ground-waters tend to dissolve holes in the chalk. Together, these processes mean that as fast as a bit of chalk is dissolved out, flint is deposited to fill the gap.
Q Why are my cat's eyes so bright at night?
Nocturnal animals such as cats - and fishes that live in deep water - have a shiny backing to the retina, a layer of cells called the tapetum. Their eyes seem to glow if a light shines into them. The tapetum reflects light back onto the receptors, making vision more effective in low light conditions. In hoofed mammals like deer the tapetum contains glistening fibres of connective tissue. In cats, it contains shiny crystals of guanine.Reuse content