Mountain Building | How are Mountains formed ?
Mountain Building | How are Mountains formed ?
The process involved in mountain building -- termed orogenesis -- occur as a result of the movement of Earth's crustal plates.
There are three main types of mountains : volcanic mountains, fold mountains, and block mountains.
Most volcanic mountains have been formed along plate boundaries where plates have come together or moved apart and lava and other debris have been ejected onto the Earth's surface.
The lava and debris may have built up to form a dome around the vent of a volcano.
Fold mountains are formed where plates push together and cause the rock to buckle upwards.
Where oceanic crust meets less dense continental crust, the oceanic crust is forced under the continental crust.
The continental crust is buckled up by the impact.
This is how folded mountain ranges, such as the Appalachian Mountains in North America, were formed.
Fold mountains are also formed where two areas of continental crust meet.
The Himalayas, for example, began to form when India collided with Asia, buckling the sediments and parts of the oceanic crust between them.
Block mountains are formed when a block of land is uplifted between two faults as a result of compression or tension in the Earth's crust.
Often, the movement along faults has taken place gradually over millions of years.
However, two plates may cause an earthquake by suddenly sliding past each other along a fault line.
The changing Earth | How has the Earth changed over time?
The Earth formed from a cloud of dust and gas drifting through space about 4,600 million years ago.
Dense minerals sank to the centre while lighter ones formed a thin rocky crust.
However, the first known life-forms -- bacteria and blue-green algae -- did not appear until about 3,400 million years ago, and it was only about 700 million years ago that more complex plants and animals began to develop.
Since then, thousands of animal and plant species have evolved; some, such as dinosaurs, survived for many millions of years, while others died out quickly.
The Earth itself is continually changing.
Although continents neared their present locations about 50 million years ago, they are still drifting slowly over the planet's surface, and mountain ranges such as Himalayas -- which began to form 40 million years ago -- are continually being built up and worn away.
Climate is also subject to change : the Earth has gone a series of ice ages intersperced with warmer periods (the most recent glacial period was at its height about 20,000 years ago).
Asteroids, Comets, and Meteoroids | What are Meteoroids ?
Asteroids, Comets, and Meteoroids are all debris remaining from the nebula from which the Solar System formed 4.6 billion years ago.
Asteroids are rocky bodies up to about 1,000 kilometres in diameter, although most are much smaller.
Most of them orbit the Sun in the asteroid belt, which lies between the orbits of Mars and Jupiter.
Cometary nuclei exist in a huge cloud (called the Oort Cloud) that surround the planetary part of the Solar System.
They are made of frozen water and dust, and are a few kilometres in diameter.
Occasionally, a comet is deflected from the Oort Cloud on to a long, elliptical path that brings it much closer to the Sun.
As the comet approaches the Sun, the cometary nucleus starts to vaporize in the heat, producing both a brightly shining coma (a huge sphere of gas and dust around the nucleus), and a gas tail, and a dust tail.
Meteoroids are small chunks of stone or stone and iron, which are fragments of asteroids and comets.
Meteoroids range in size from tiny dust particles to objects tens of metres across.
If a meteoroid enters the Earth's atmosphere, it is heated by friction and appears as a glowing streak of light called a meteor (also known as a shooting star).
Meteor showers occur when the Earth passes through the trail of dust particles left by a comet.
Most meteoroids burn up in the atmosphere.
The remnants of the few that are large enough to reach the Earth's surface are termed as meteorites.
Uranus | What is Uranus made of ?
Uranus is the seventh planet from the Sun and the third largest, with a diameter of about 51,000 kilometres.
It is thought to consist of a dense mixture of different types of ice and gas around a solid core.
Its atmosphere contains traces of methane, giving the planet a blue-green hue, and the temperature at the cloud tops is about -210 degree centigrade.
Uranus is the most featureless planet to have been closely observed : only a few icy clouds of methane have been seen so far.
Uranus is unique among the planets in that its axis of rotation lies close to its orbital plane.
As a result of its extremely tilted rotational axis, Uranus rolls on its side along its orbital path around the Sun, whereas other planets spin more or less upright.
Uranus is encircled by main rings that consist of rocks intersperced with dust lanes and too distant outer rings made of dust.
The rings contain some of the darkest matter in the Solar System and are extremely narrow, making them difficult to detect : most of them are less than 10 kilometres wide, whereas most of Saturn's rings are thousands of kilometres in width.
There are 27 known Uranian moons, all of which are icy and most of which are further out than the rings.
The 13 inner moons are small and dark, with diametres of less than 160 kilometres, and the five major moons are between 470 and 1,600 kilometres in diameter.
The major moons have a wide variety of surface features.
Mirinda has the most varied surface, with cratered areas broken up by huge ridges and cliffs 20 kilometres high.
Beyond these are nine much more distant moons with diametres less than 150 km.
Mars | Was there life on Mars ?
Mars, known as the red planed, is the fourth planet from the Sun and the outermost rocky planet.
In the 19th century, astronomers first observed what what were thought to be the signs of life on Mars.
The signs included apparent canal-like lines on the surface, and dark patches that were thought to be vegetation.
It is now known that the "canals" are an optical illusion, and the dark patches are areas where the red dust that covers most of the planet has been blown away.
The fine dust particles are often whipped up by winds into dust storms that occasionaly obscure almost all the surface.
Residual fine dust in the atmosphere gives the Martian sky a pinkish hue.
The northern hemisphere of Mars has many has many large plains formed of solidified volcanic lava, whereas the southern hemisphere has many craters and large impact basins.
There are also several huge, extinct volcanoes, including Olympus Mons, which, at 600 kilometers across and 23 kilometres high, is the largest known volcano in the Solar system.
The surface also has many canyons and branching channels.
The canyons were formed by movements of the surface crust, but the channels are thought to have been formed by flowing water that has now dried up.
The Martian atmosphere is much thinner than Earth's, with only a few clouds and morning mists.
Mars has two tiny, irregularly shaped moons called Phobos and Deimos.
Their small size indicates that they may be asteroids that have been captured by the gravity of Mars.
The Moon | Dark side of the Moon
The Moon is the Earth' s only natural satellite.
It is relatively large for a moon, with a diameter of about 3,470 kilometres - just over a quarter that of the Earth.
The Moon takes the same time to rotate on its axis as it takes to orbit the Earth (27.3 days), and so the same side (the outer side) always faces us.
However, the amount of the surface we can see - the phase of the Moon - depends on how much of the near side is in sunlight.
The Moon is dry and barren, with negligible atmosphere and water.
It consists mainly of solid rock, although its core may contain molten rock or iron.
The surface is dusty, with highlands covered in craters caused by meteorite impacts, and lowlands in which large craters have been filled by solidified lava to form dark areas called maria or "seas".
Maria occur mainly on the near side, which has a thinner crust than the far side.
Many of the craters are rimmed by mountain ranges that form the crater walls and can be thousands of metres high.
The Earth | What if the Moon crashed into Earth ?
The Earth is the third of the eight planets that orbit the sun.
It is the largest and densest rocky planet, and the only known to support life.
About 70% of the earth' s surface is covered by water, which is not found in liquid form on the surface of any other planet.
There are four main layers: the inner core, the outer core, the mantle, and the crust.
At the heart of the planet the solid inner core has a temperature of about 6,600 degree centigrade.
The heat from the inner core causes material in the molten outer core and mantle to circulate in convection currents.
It is thought that these convection current generate the earth' s magnetic field, which extends into space as the magnetosphere.
The Earth' s atmosphere helps screen out some of the harmful radiation from the Sun, stops most meteoroids from reaching the planet' s surface, and traps enough heat to prevent extremes of cold.
The Earth has one natural satellite, the Moon, which is thought to have formed when a huge asteroid impacted Earth in the distant past
Mercury | Why is the orbit of Mercury different ?
Mercury is the smallest planet to the Sun, orbiting at an average distance of about 58 million kilometres.
Because Mercury is the closest planet to the Sun, it moves faster than any other planet, travelling at an average speed of nearly 48 kilometres per second and completing an orbit in just under 88 days.
Mercury is very small ( only 40% bigger than the Moon ) and rocky.
Most of the surface has been heavily cratered by the impact of meteorites, although there are also smooth, sparcely cratered lava-covered plains.
The Caloris basin is the largest crater, measuring about 1,500 kilometres across.
It is thought to have been formed when a 60 kilometre-diametre asteroid hit the planet, and is surrounded by concentric rings of mountains thrown up by the impact.
The surface also has many cliff-like ridges (called ropes) that are thought to have been formed when the hot core of the young planet cooled and shrank about four billion years ago, buckling the planet' s surface in the process.
The planet rotates about in its axis slowly, taking about 59 earth days to complete one rotation.
As a result, a solar day (sunrise to sunrise ) on mercury is about 176 earth days - twice as long as the 88-day Mercurian year.
Mercury has extreme surface temperature, ranging from a maximum of 430 degree celcius on the sunlit side to -170 degree celcius on the dark side.
At nightfall, temperature drops very quickly because the planet' s atmosphere is almost non-existent.
It consists only of minute amounts of helium and hydrogen captured from the solar wind, plus traces of other gases.
The Sun | Why do we need the Sun ?
The Sun is the star at the centre of the Solar System.
It is about 5 billion years old and will continue to shine as it does now for about another five billion years.
The Sun is a yellow main sequence star about 1.4 million kilometres in diameter.
It consists almost entirely of hydrogen and helium.
In the Sun's core, hydrogen is converted into helium by nuclear fusion, releasing energy in the process.
The energy travels from the core, through the radiative and convective zones, to the photosphere (visible surface), where it leaves the Sun in the form of heat and light.
On the photosphere there are often dark, relatively cool areas called sunspots, which usually appears in pairs or groups and are caused by the cooling effect of the magnetic field.
Other types of Solar activity are flares, which are usually associated with sunspots, and prominences.
Flares are sudden discharges of high energy-radiation and atomic particles.
Prominences are huge loops or filaments of gas extending inti the solar atmosphere; some last for hours, others for months.
Beyond the photosphere is the chromosphere (inner atmosphere) and the extremely rarified corona (outer atmosphere), which extends millions of kilometres into space.
Tiny particles that escape from the corona give rise to the solar wind, which streams through space at hundreds of kilometres per second.
The chromosphere and the corona can be seen from Earth when the Sun is totally eclipsed by the moon.
