Inside the Earth
The Earth is made of many different and distinct layers. The deeper layers are composed of heavier
materials; they are hotter, denser and under much greater pressure than the outer layers.
Core: The Earth has a iron-nickel core that is about 2,100 miles in radius. The inner core may have a
temperature up to about 13,000°F (7,200°C = 7,500 K), which is hotter than the surface of the Sun.
The inner core (which has a radius of about 750 miles (1,228 km) is solid. The outer core is in a liquid
state and is about 1,400 miles (2,260 km) thick.
Mantle: Under the crust is the rocky mantle, which is composed of silicon, oxygen, magnesium, iron,
aluminum, and calcium. The upper mantle is rigid and is part of the lithosphere (together with the
crust). The lower mantle flows slowly, at a rate of a few centimeters per year. The asthenosphere is
a part of the upper mantle that exhibits plastic properties. It is located below the lithosphere (the
crust and upper mantle), between about 100 and 250 kilometers deep.
Convection (heat) currents carry heat from the hot inner mantle to the cooler outer mantle. The
mantle is about 1,700 miles (2,750 km) thick. The mantle gets warmer with depth; the top of the
mantle is about 1,600° F (870° C); towards the bottom of the mantle, the temperature is about
4,000-6,700° F (2,200-3,700° C). The mantle contains most of the mass of the Earth. The Gutenberg
discontinuity separates the outer core and the mantle.
Surface and crust: The Earth's surface is composed mostly of water, basalt and granite. Oceans cover
about 70% of Earth's surface. These oceans are up to 3.7 km deep. The Earth's thin, rocky crust is
composed of silicon, aluminum, calcium, sodium and potassium.
The crust is divided
into continental plates
which drift slowly
(only a few
centimeters each year)
atop the less rigid
mantle. The crust is
thinner under the
oceans (6-11 km
thick); this is where new crust is formed. Continental crust is about 25-90 km thick. The lithosphere
is defined as the crust and the upper mantle, a rigid layer about 100-200 km thick. The Mohorovicic
discontinuity is the separation between the crust and the upper mantle.
THE EARTH'S ATMOSPHERE
The Earth's atmosphere is a thin layer of gases that surrounds the Earth. It composed of 78%
nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases. This
thin gaseous layer insulates the Earth from extreme temperatures; it keeps heat inside the
atmosphere and it also blocks the Earth from much of the Sun’s incoming ultraviolet radiation.
The Earth's atmosphere is about 300 miles (480 km) thick, but most of the atmosphere (about 80%)
is within 10 miles (16 km) of the surface of the Earth. There is no exact place where the atmosphere
ends; it just gets thinner and thinner, until it merges with outer space.
Air Pressure:
At sea level, the air pressure is about 14.7 pounds per square inch. As your altitude increases (for
example, if you climb a mountain), the air pressure decreases. At an altitude of 10,000 feet, the air
pressure is 10 pound per square inch (and there is less oxygen to breathe).
The Layers of the Atmosphere:
Thermosphere: The thermosphere is a thermal
classification of the atmosphere. In the
thermosphere, temperature increases with
altitude. The thermosphere includes the
exosphere and part of the ionosphere.
•Exosphere: The exosphere is the outermost
layer of the Earth's atmosphere. The exosphere
goes from about 400 miles (640 km) high to
about 800 miles (1,280 km). The lower boundary
of the exosphere is called the critical level of
escape, where atmospheric pressure is very low
(the gas atoms are very widely spaced) and the
temperature is very low.
•Ionosphere: The ionosphere starts at about 4350 miles (70-80 km) high and continues for
hundreds of miles (about 400 miles = 640 km). It
contains many ions and free electrons (plasma).
The ions are created when sunlight hits atoms
and tears off some electrons. Auroras occur in
the ionosphere.
•Mesosphere: The mesosphere is characterized by temperatures that quickly decrease as height
increases. The mesosphere extends from between 31 and 50 miles (17 to 80 kilometers) above the
earth's surface.
•Stratosphere: The stratosphere is characterized by a slight temperature increase with altitude and
the absence of clouds. The stratosphere extends between 11 and 31 miles (17 to 50 kilometers)
above the earth's surface. The earth's ozone layer is located in the stratosphere. Ozone, a form of
oxygen, is crucial to our survival; this layer absorbs a lot of ultraviolet solar energy. Only the highest
clouds (cirrus, cirrostratus, and cirrocumulus) are in the lower stratosphere.
•Tropopause: The tropopause is the boundary zone (or transition layer) between the troposphere
and the stratosphere. The tropopause is characterized by little or no change in temperature altitude
increases.
•Troposphere: The troposphere is the lowest region in the Earth's (or any planet's) atmosphere. On
the Earth, it goes from ground (or water) level up to about 11 miles (17 kilometers) high. The
weather and clouds occur in the troposphere. In the troposphere, the temperature generally
decreases as altitude increases.
Formation of the Atmosphere:
The Earth's atmosphere was formed by planetary degassing, a process in which gases like carbon
dioxide, water vapour, sulphur dioxide and nitrogen were released from the interior of the Earth
from volcanoes and other processes. Life forms on Earth have modified the composition of the
atmosphere since their evolution.
Important Geophysical Phenomena
Earthquakes:
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Seismic wave: a pulse of energy released during an earthquake.
Earthquakes can cause a Tsunami. E.g. Tsunami in Japan in 2011 which caused Fukushima
disaster was caused by an earthquake.
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Technically, an earthquake is a vibration that travels through the Earth's crust. Quakes can be
caused by a variety of things, including meteor impacts and volcanic eruptions, and even
sometimes man-made events like mine collapse and underground nuclear tests. But most
natural earthquakes are caused by tectonic activities i.e. movement of pieces of earth’s
surface.
Over the last decade alone, earthquakes and the tsunamis, avalanches and
landslides caused by them -- have killed 688,000 people around the world.
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Plate Tectonics Theory: the Earth's crust, or lithosphere, comprises many plates that slide
over a lubricating asthenosphere layer. At the boundaries between these huge plates of rock
and soil, the plates sometimes move apart, and magma, or molten rock, comes to the
surface, where it's called lava. It cools and forms new parts of the crust. The line where this
happens is called a divergent plate boundary. The plates also can push against each other.
Sometimes, one of the plates will sink underneath the other into the hot layer of magma
beneath it and partially melt. Other times, the edges of the two plates will push against each
other and rise upward, forming mountains. This area is called a convergent plate boundary.
But in other instances, plates will slide by and brush against each other but very, very slowly.
At the region between the two plates, called a transform boundary, pent-up energy builds in
the rock. A fault line, a break in the Earth's crust where blocks of crust are moving in different
directions, will form.
Earthquake Faults: There are three types of faults; 1. Normal fault (divergent plate boundary)
2. Reverse fault (convergent plate boundary) 3.strike slip fault (transform boundary).
Causes of Earthquake: With all these faults, rocks push together tightly, creating friction. If
there's enough friction, they become locked, so that they won't slide anymore. Meanwhile, the
Earth's forces continue to push against them, increasing the pressure and pent-up energy. If
the pressure builds up enough, it will overcome the friction, the lock will give way suddenly,
and the rocks will snap forward. To put it another way, as the tectonic forces push on the
"locked" blocks, potential energy builds. When the plates are finally moved, this built-up
energy becomes kinetic. The sudden, intense shifts along already formed faults are the main
sources of earthquakes.
Why do earthquakes occur around plate boundaries? : Most earthquakes occur around
plate boundaries because this is where strain from plate movements is felt most intensely,
creating fault zones, groups of interconnected faults.
Why sometimes several earthquakes occur at the same time? : In a fault zone, the
release of kinetic energy at one fault may increase the stress -- the potential energy -- in a
nearby fault, leading to other earthquakes. That's one reason why several earthquakes may
occur in an area in a short period of time. These additional quakes are
called foreshocks and aftershocks. The quake with the largest magnitude is called
the mainshock.
Seismic Waves:
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Body Waves (Primary waves and Secondary waves.) and Surface Waves or Long
waves or L waves.
Primary Waves: Or P waves: are the fastest moving waves, traveling at 1 to 5 miles per
second (1.6 to 8 kilometres per second). They can pass through solids, liquids and gases
easily. As they travel through rock, the waves move tiny rock particles back and forth -pushing them apart and then back together -- in line with the direction the wave is traveling. P
waves generally travel 1.7 times faster than S waves. Using this ratio, scientists can calculate
the distance between any point on the Earth's surface and the earthquake's focus the
breaking point where the vibrations originated. (How focus of an earthquake is
calculated?) If you gather this information from three or more points, you can determine the
location of the focus through a process called trilateration. (In the same way as positioning is
done by using satellite data from 3 satellites)
Secondary Waves or S waves: They move a little more slowly than P waves, and can only
pass through solids. As S waves move, they displace rock particles outward, pushing them
perpendicular to the path of the waves. This results in the first period of rolling associated with
earthquakes. Unlike P waves, S waves don't move straight through the Earth. They only
travel through solid material, and so are stopped at the liquid layer in the Earth's core.
Surface waves or L waves: Unlike body waves, surface waves (also known as long waves,
or simply L waves) move along the surface of the Earth. Surface waves are to blame for most
of an earthquake's carnage. They move up and down the surface of the Earth, rocking the
foundations of man-made structures. Surface waves are the slowest moving of all waves,
which means they arrive the last. So the most intense shaking usually comes at the end of an
earthquake.
Measurement of intensity of an earthquake:
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Measurement of release of energy is done by Richter scale (1-10). The instrument used in
measuring the intensity is known as Seismograph. The Richter scale is logarithmic, meaning
that whole-number jumps indicate a tenfold increase. A majority of quakes register less than 3
on the Richter scale; these tremors, called microquakes, aren't even felt by humans.
Measurement of Damage: By Mercalli Scale (I - XII). Richter ratings only give you a rough
idea of the actual impact of an earthquake, though. An earthquake's destructive power varies
depending on the composition of the ground in an area and the design and placement of
man-made structures. The extent of damage is rated on the Mercalli scale. Mercalli ratings,
which are given as Roman numerals, are based on largely subjective interpretations. A low
intensity earthquake, one in which only some people feel the vibration and there is no
significant property damage, is rated as a II. The highest rating, a XII, is applied to
earthquakes in which structures are destroyed, the ground is cracked and other natural
disasters, such as landslides or tsunamis, are initiated.
Predicting earthquakes:
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Today's scientists understand earthquakes a lot better than we did even 50 years ago, but
they still can't match the quake-predicting prowess of the common toad, which can detect
seismic activity days in advance of a quake. Researchers aren't quite sure how the toads do
this, but it's believed that they can detect subtle signs, such as the release of gases and
charged particles, that may occur before a quake. Scientists can predict where major
earthquakes are likely to occur, however, based on the movement of the plates in the Earth
and the location of fault zones. They also can make general guesses about when
earthquakes might occur in a certain area, by looking at the history of earthquakes in the
region and detecting where pressure is building along fault lines. For example, if a region has
experienced four magnitude 7 or larger quakes during the past 200 years, scientists would
calculate the probability of another magnitude 7 quake occurring in the next 50 years at 50
per cent. But these predictions may not turn out to be reliable because, when strain is
released along one part of a fault system, it may actually increase strain on another part. As a
result, most earthquake predictions are vague at best. Scientists have had more success
predicting aftershocks, additional quakes following an initial earthquake. These predictions
are based on extensive research of aftershock patterns.
Earthquake Preparedness:
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Over the past 50 years, major advances have been made in earthquake preparedness -particularly in the field of construction engineering.
Uniform Building Code (1973): an international set of standards for building construction,
added specifications to fortify buildings against the force of seismic waves.
Structural "fuses": a structure equipped with structural "fuses" that, instead of toppling,
deliberately collapse upon themselves and then reform after the quake subsides.
"smart" building material: are capable of coping with the tremendous forces generated by a
quake. One idea is to include fiber-optic sensors that can sense when a structure is about to
fail; the sensors would then send signals to tiny ceramic strips built into the walls and frame,
which would change shape to absorb the energy.
Educating the public:
TSUNAMI
A tsunami (pronounced sue-nahm-ee) is a series of huge waves that can cause great
devastation and loss of life when they strike a coast.
Tsunamis are caused by an underwater earthquake, a volcanic eruption, an sub-marine
rockslide, or, more rarely, by an asteroid or meteoroid crashing into in the water from
space. Most tsunamis are caused by underwater earthquakes, but not all underwater
earthquakes cause tsunamis - an earthquake has to be over about magnitude 6.75 on
the Richter scale for it to cause a tsunami. About 90 percent of all tsunamis occur in the
Pacific Ocean.
Many tsunamis could be detected before they hit land, and the
loss of life could be minimized, with the use of modern
technology, including seismographs (which detect earthquakes),
computerized offshore buoys that can measure changes in wave
height, and a system of sirens on the beach to alert people of
potential tsunami danger.
NOTE: If you see the water recede quickly and unexpectedly
from a beach (this is called drawback), run toward higher ground
or inland -- there may be a tsunami coming. Also, if you are on the coast and there is an
earthquake, it may have caused a tsunami, so run toward higher ground or inland. Some
beaches have tsunami warning sirens -- do not ignore them. The first wave in a tsunami is
often not the largest; if you experience one abnormally-huge wave, go inland quickly -- even
bigger waves could be coming soon.
The Word Tsunami:
The word tsunami comes from the Japanese word
meaning "harbor wave." Tsunamis are sometimes
incorrectly called "tidal waves" -- tsunamis are not
caused by the tides (tides are caused by the
gravitational force of the moon on the sea). Regular
waves are caused by the wind.
The Development of a Tsunami:
A tsunami starts when a huge volume of water is quickly shifted. This rapid movement can
happen as the result of an underwater earthquake (when the sea floor quickly moves up or
down), a rock slide, a volcanic eruption, or another high-energy event.
After the huge volume of water has moved, the resulting wave is very long (the distance
from crest to crest can be hundreds of miles long) but not very tall (roughly 3 feet tall). The
wave propagates (spreads) across the sea in all directions; it can travel great distances from
the source at tremendous speeds.
The Size of a Tsunami:
Tsunamis have an extremely long wavelength (wavelength is the distance between the crest
(top) of one wave and the crest of the next wave) -- up to several hundred miles long. The
period (the time between two successive waves) is also very long -- about an hour in deep
water.
In the deep sea, a tsunami's height can be only about 1 m (3 feet) tall. Tsunamis are often
barely visible when they are in the deep sea. This makes tsunami detection in the deep sea
very difficult.
The Speed of a Tsunami:
A tsunami can travel at well over 970 kph (600 mph) in the open ocean - as fast as a jet flies.
It can take only a few hours for a tsunami to travel across an entire ocean. A regular wave
(generated by the wind) travels at up to about 90 km/hr.
A Tsunami Hits the Coast:
As a tsunami wave approaches the coast (where the sea becomes shallow), the trough
(bottom) of a wave hits the beach floor, causing the wave to slow down, to increase in
height (the amplitude is magnified many times) and to decrease in wavelength (the distance
from crest to crest).
At landfall, a tsunami wave can be hundreds of meters tall. Steeper shorelines produce
higher tsunami waves.
In addition to large tsunami waves that crash onto shore, the waves push a large amount of
water onto the shore above the regular sea level (this is called runup). The runup can cause
tremendous damage inland and is much more common than huge, thundering tsunami
waves.
Tsunami Warning Systems:
Tsunami warning systems exist in many places around the world. As scientists continuously
monitor seismic activity (earthquakes), a series of buoys float off the coast and monitor
changes in sea level. Unfortunately, since tsunamis are not very tall in height when they are
out at sea, detection is not easy and there are many false alarms. Sirens at affected beaches
may be activated -- do not ignore them!
Wind-Generated Waves vs. Tsunami Waves:
Regular waves (caused by the wind) are very different from tsunami waves. Tsunami waves
are much faster than wind-generated waves and they have a much longer wavelength (the
distance from crest to crest). In the deep sea, tsunami waves are very small, but by the
coast, they dwarf regular waves.
How often do Tsunamis Occur?
Tsunamis are very rare. There are roughly six major tsunamis each century.
Tiny Model of a Tsunami:
You can make a tiny model of a tsunami by dropping a rock into a bowl of water, causing
ripples to propagate (travel) outwards from the site of impact. Another way is to slightly jolt
the bowl of water and watch it slosh over the rim on one side.
Hurricane/Cyclone/Typhoon
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Hurricanes, typhoons, cyclones are all the same, just different names for tropical storms in
different parts of the world; Hurricane in the Atlantic, Typhoon in the Pacific and Cyclone in
the Indian Ocean.
A cyclone is a powerful, rotating storm that forms over warm oceans near the Equator.
Hurricanes have strong, rotating winds (at least 74 miles per hour or 119 kilometres per
hour), a huge amount of rain, low air pressure, thunder and lightning. The cyclonic winds of
a hurricane rotate in a counter-clockwise direction around a central, calm eye.
Cyclones often travel from the ocean to the coast and on to land, where the wind, rain, and
huge waves can cause extensive destruction.
Generally, when a cyclone moves over land (or over cold ocean waters) the storm begins to
weaken and quickly dies down because the storm is fuelled by warm water.
How Cyclones are formed?
 Hurricanes need four conditions to form:
1. low air pressure
2. warm temperatures
3. moist ocean air
4. Tropical winds (near the equator).
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Cyclones form in the tropics, over warm ocean water (over 80ºF or 27ºC) and at latitudes
between 8° and 20°. These powerful storms are fuelled by the heat energy that is released
when water vapour condenses (turns into liquid water -- rain).
The End of a Storm:
When a cyclone travels over land or cold water, its energy source (warm water) is gone and the
storm weakens, quickly dying.
Structure of a cyclone:
Volcano
A volcano is a place on the Earth's surface (or any other planet's or moon's surface) where molten
rock, gases and pyroclastic debris erupt through the earth's crust. Volcanoes vary quite a bit in their
structure - some are cracks in the earth's crust where lava erupts, and some are domes, shields, or
mountain-like structures with a crater at the summit.
Magma is molten rock within the Earth's crust. When magma erupts through the earth's surface it is
called lava. Lava can be thick and slow-moving or thin and fast-moving. Rock also comes from
volcanoes in other forms, including ash (finely powdered rock that looks like dark smoke coming
from the volcano), cinders (bits of fragmented lava), and pumice (light-weight rock that is full of air
bubbles and is formed in explosive volcanic eruptions - this type of rock can float on water).
The Word Volcano:
The word volcano comes from the Roman god of fire, Vulcan. Vulcan was said to have had a forge (a
place to melt and shape iron) on Vulcano, an active volcano on the Lipari Islands in Italy.
Extreme Volcanoes:
The largest volcano on Earth is Hawaii’s Mauna Loa. Mauna Loa is about 6 miles (10 km) tall from
the sea floor to its summit (it rises about 4 km above sea level). It also has the greatest volume of
any volcano, 10,200 cubic miles (42,500 cubic kilometres). The largest volcano in our solar system is
perhaps Olympus Mons on the planet Mars. This enormous volcano is 17 miles (27 km) tall and over
320 miles (520 km) across.
Types of Volcanos:
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Shield Volcano - A gently-sloping volcano that emits mostly basaltic lava (very fluid lava) that
flows in long-lasting, relatively gentle eruptions - explosions are minimal. Shield volcanoes
can be very big. An example is Mt. Kilauea (in Hawaii, USA).
Composite or Strato Volcano - A steep-coned volcano that explosively emits gases, ash,
pumice, and a small amount of stiff, silica lava (called rhyolite). This type of volcano can have
eruptions accompanied by lahars -- deadly mudflows. Most volcanoes on Earth are of this
type. Strato volcanoes kill more people than any other type of volcanoes - this is because of
their abundance on Earth and their powerful mudflows. Examples are Krakatoa in Indonesia,
Mt. Pinatubo in the Philippines, and Mt. St. Helens in Washington state, USA.
Lava Dome - A bulbous (rounded) volcano that forms when very viscous lava barely flows.
An example is Mont Pelée in Martinique.
Cinder Cone - A cone-shaped volcano whose steep sides are formed by loose, fragmented
cinders that fall to the Earth close to the vent. The lava flows through a single vent that is
usually only up to about 1,000 feet tall. There is usually a bowl-shaped crater at the top. As
the gas-filled lava erupts into the air, the lava fragments into pieces and forms cinders.
Rhyolite Caldera Complex - these are the most explosive volcanoes. They do not look like
common volcanoes -- after an eruption, the result is a caldera (crater) caused when the area
around the vent collapses. Examples are Yellowstone in Wyoming, USA and Lake Taupo in
New Zealand (which erupted around A.D. 80).
The Ring of Fire:
The area encircling the Pacific Ocean is called the "Ring of Fire" because its edges mark a circle of
high volcanic and seismic activity (earthquakes). Most of the active volcanoes on Earth are located
on this circumference. On the periphery of the Pacific Ocean, the edge of the Pacific Continental
Plate is expanding in the seabed, and is hitting the North American Plate, the Nazca Plate, the
Eurasian Plate, and other plates, causing the margins of the plates to collide, buckle, and compress,
causing earthquakes and volcanoes.
Glaciers
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A glacier is a large, long-lasting mass of ice and snow that moves very slowly over the land
like a slow-moving frozen river. A glacier is formed as layers upon layers of snow are
compacted. As new snow falls, older layers are compressed into dense ice.
Glacial ice is made of frozen fresh water (not salt water); glaciers contain the largest
reservoir of fresh water on Earth. Glaciers contain about 75% of the fresh water on earth
and cover about 10% of the land. During the last Ice Age (which ended about 10,000 years
ago), glaciers covered almost a third of the land.
Where Glaciers are located?
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Glaciers are located in cold areas that get snow in the winter and have relatively cool
summers. Most glaciers are located near the North and South Poles or are high in the
mountains. Glaciers can be found on every continent on Earth except Australia (but
there are glaciers on other islands of Oceania). Glaciers are also found on some other
planets, including dry-ice glaciers on Mars.
How Glaciers Form and Shrink?
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Glaciers form when snow falls and accumulates in cold areas in which the snow does not
all melt in the summer. A glacier will grow in mass when winter snowfall is greater than
summer snowmelt. A glacier will shrink in mass when winter snowfall is less than
summer snowmelt.
As snow accumulates in a glacier it becomes heavier, and the snow is compressed under
the pressure, turning into ice. The top of a glacier is snow, but deeper layers are made of
snow that has been compressed into ice. The bottom of a glacier is a thin layer of water
(from glacial melting or from water that seeps through cracks in the glacier).
The mass of glacial ice combined with the slope of the Earth under it cause the ice to
inch its way downwards or outwards, sliding on the thin layer of water. Mountain
glaciers flow downhill and continental glaciers flow outwards. As a moving mass of ice
and snow, the ice mass is classified as a glacier.
When one parts of a glacier moves more rapidly than other parts, that area becomes
relatively unstable and deep cracks or chasms (called crevasses) form in the ice.
Compressed glacial ice looks bluish in colour (glacial ice that looks white is full of
bubbles).
Types of Glaciers?
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Two of the main types of glaciers are ice sheets (also called continental glaciers) and Alpine
glaciers (also called mountain glaciers).
Glaciers and Erosion:
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Erosion is the wearing down of an object by friction or abrasion. Glaciers cause erosion by
crushing and scraping along the soil and rock they sit on. The mass of snow and ice often
pluck up rocks and boulders and deposit them downstream. Glacial till is assorted sediment,
including soil, rock, and boulders that are moved around by glaciers. Also, grooves and
striations are often made in underlying rock by the massive moving ice.
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Many geological formations are created by glacial movement, including fjords (long, deep
inlets with steep sides), U-shaped glacial valleys (V-shaped valleys are rounded by a
glacier),cirques (bowl-shaped depressions formed at the beginning or end of a glacier -- a
cirque has high walls except for the downhill portion where the glacier flowed), and many
others.
Icebergs
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Calving is the process in which large chunks of ice break off a glacier. Icebergs are huge
chunks of floating fresh-water ice that break off a glacier or ice shelf and float out to sea.
Almost 90% of an iceberg is hidden beneath the water.
Glaciers and the Future
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As the Earth's temperature warms, most of the Earth's mountain glaciers have been
decreasing in size; ice sheets in Greenland are also decreasing in size. The ice sheets in
Antarctica are changing less dramatically.
Glaciers on Mars
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Glaciers cover much of the Polar Regions (the ice caps) of Mars. The glaciers on Mars are
made of frozen carbon dioxide, CO2 -- dry ice (and not water, H2O).
LANDSLIDES