Rock Cycle

                                        
Rocks are composed primarily of minerals and can be an amalgam of different minerals or can be composed of one mineral. Over 3500 minerals have been identified; most of these can be found in the earth's crust. Some of the earth's minerals are exceedingly popular - fewer than 20 minerals compose more than 95% of the earth's crust.
There are three different ways rock can be created on earth and thus there are three main classifications of rock, based on the three processes - igneous, sedimentary, and metamorphic.

Igneous Rock

Igneous rocks are formed from the molten liquid minerals that lie below the earth's crust. They're formed from magma that cools beneath the earth's surface or from lava that cools upon the earth's surface. These two methods of igneous rock formation are known as intrusive and extrusive, respectively.
Intrusive igneous formations can be forced to the surface of the earth where they can exist as masses of rock known as plutons. The largest types of exposed plutons are called batholiths. The Sierra Nevada mountains are a large batholith of igneous granite rock.
Slowly cooling igneous rock will usually contain larger mineral crystals than igneous rock that cools more quickly. The magma that forms igneous rock beneath the surface of the earth can take thousands of years to cool. Quickly cooling rock, often extrusive lava that comes from volcanoes or fissures in the earth's surface has small crystals and may be quite smooth, such as the volcanic obsidian rock.
All rocks on earth were originally igneous as that's the only method entirely new rock can be formed. Igneous rocks continue to form today under and above the earth's surface as magma and lava cool to form new rock. The word igneous comes from Latin and means "fire formed."
Most of the rocks of the earth's crust are igneous although sedimentary rocks usually cover them. Basalt is the most common type of igneous rock and it covers the ocean floor and thus, exists over two-thirds of the earth's surface.

Sedimentary Rock

Sedimentary rocks are formed by the lithification (cementing, compacting, and hardening) of existing rock or the bones, shells, and pieces of formerly living things. Rocks are weathered and eroded into tiny particles which are then transported and deposited along with other pieces of rock called sediments.

Sediments are cemented together and compacted and hardened over time by the weight and pressure of up to thousands of feet of additional sediments above them. Eventually, the sediments are lithified and become solid sedimentary rock. These sediments that come together are known as clastic sediments. Sediments usually sort themselves by the size of the particles during the deposition process so sedimentary rocks tend to contain similarly sized sedimentary particles.
An alternative to clastic sediments are chemical sediments which are minerals in solution that harden. The most common chemical sedimentary rock is limestone, which is a biochemical product of calcium carbonate created by the parts of dead creatures.
Approximately three-quarters of the earth's bedrock on the continents is sedimentary.

Metamorphic Rock

Metamorphic rock, which comes from the Greek to "change form," is formed by applying great pressure and temperature to existing rock converting it into a new distinct type of rock. Igneous rocks, sedimentary rocks, and even other metamorphic rocks and be modified into metamorphic rocks.
Metamorphic rocks are usually created when they come under extreme pressure such as under many thousands of feet of bedrock or through being crushed at the junction of tectonic plates. Sedimentary rocks can become metamorphic rocks if the thousands of feet of sediments above them apply enough heat and pressure to further change the structure of the sedimentary rock.
Metamorphic rocks are harder than other types of rock so they're more resistant to weathering and erosion. Rock always converts into the same type of metamorphic rock. For example, the sedimentary rocks limestone and shale become marble and slate, respectively, when metamorphosed.

The Rock Cycle

We know that all three rock types can be turned into metamorphic rocks but all three types can also be changed through the rock cycle. All rocks can be weathered and eroded into sediments, which can then form sedimentary rock. Rocks can also be completely melted into magma and become
 reincarnated as igneous rock.

Read more

Animation

Rocks

Rocks

Rocks are always on the move through the rock cycle!
Click on parts of this rock cycle diagram to find out more about

• sedimentary rocks
• igneous rocks
• metamorphic rocks

and how they change over geologic time
All rock (except for meteorites!) that is on Earth today is made of the same stuff as the rocks that dinosaurs and other ancient life forms walked, crawled or swam over. While the stuff that rocks are made from stays the same, the rocks do not. Over millions of years, rocks are recycled into other rocks. Moving tectonic plates help to destroy and form many types of rocks.

What Is a Sedimentary Rock?

Have you ever been to the beach and nestled your toes in the sand? Over thousands of years that sand might become part of a sedimentary rock!
Sedimentary rocks make up about three-quarters of the rocks at the Earth’s surface. They form at the surface in environments such as beaches, rivers, the ocean, and anywhere that sand, mud, and other types of sediment collect. Sedimentary rocks preserve a record of the environments that existed when they formed. By looking at sedimentary rocks of different ages, scientists can figure out how climate and environments have changed through Earth’s history. Fossils of ancient living things are preserved in sedimentary rocks too.
Many sedimentary rocks are made from the broken bits of other rocks. These are called clastic sedimentary rocks. The broken bits of rocks are called sediment. Sediment is the sand you find at the beach, the mud in a lake bottom, the pebbles in a river, and even the dust on furniture. The sediment may, in time, form a rock if the little pieces become cemented together.
There are other types of sedimentary rocks whose particles do not come from broken rock fragments. Chemical sedimentary rocks are made of mineral crystals such as halite and gypsum formed by chemical processes. The sediment particles of organic sedimentary rocks are the remains of living things such as clamshells, plankton skeletons, dinosaur bones, and plants.

Igneous Rocks

Igneous rocks form when molten rock cools and becomes solid. Molten rock is called magma when it is below the Earth’s surface and lava when it is above.
Igneous rocks are divided into two groups, based on where the rock forms.
Igneous rocks that form below the Earth’s surface are called intrusive igneous rocks (or plutonic). They form when magma enters an underground chamber, cools very slowly, and forms rocks full of large crystals.
Igneous rocks that form above the Earth’s surface are called extrusive igneous rocks. These rocks, also called volcanic rocks, form when lava cools quickly at or above the Earth’s surface.

Metamorphic Rocks

Have you have heard that caterpillars metamorphose into butterflies? Well, rocks can metamorphose too! They don't grow wings like a butterfly. But they do change! Rocks metamorphose when they are in a place that is very hot and pressure is high. You can find such a place where Earth's tectonic plates are coming together. There, the colliding plates squish rocks, and hot pools of magma heat them deep underground.
Some rocks only change a little, while others change a lot. When a rock is metamorphosed, its mineral crystals change. Usually, the same chemical ingredients are used to form new crystals during metamorphism. Sometimes new types of minerals grow that weren't in the rock before.
Often, flat minerals like mica become lined up perpendicular (at a right angle) to the direction of pressure. When minerals within a metamorphic rock are organized this way, it is called foliation. Some metamorphic rocks are foliated and others are non-foliated.
Any type of rock, can be metamorphosed. The rocks are changed either in small areas of contact metamorphism or large areas of regional metamorphism

 


Read more
http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry13.html

Deformation of Rock

Deformation of Rock Stress and Strain Stress is a force applied over an area. One type of stress that we are all used to is a uniform stress, called pressure. A uniform stress is a stress wherein the forces act equally from all directions. In the Earth the pressure due to the weight of overlying rocks is a uniform stress, and is sometimes referred to as confining stress.
If stress is not equal from all directions then we say that the stress is a differential stress. Three kinds of differential stress occur. Tensional stress (or extensional stress), which stretches rock; Compressional stress, which squeezes rock; and Shear stress, which result in slippage and translation.

When rocks deform they are said to strain. A strain is a change in size, shape, or volume of a material. Stages of Deformation When a rock is subjected to increasing stress it passes through 3 successive stages of deformation.
Elastic Deformation -- wherein the strain is reversible.   Ductile Deformation -- wherein the strain is irreversible.   Fracture - irreversible strain wherein the material breaks.

We can divide materials into two classes that depend on their relative behavior under stress. Brittle materials have a small or large region of elastic behavior but only a small region of ductile behavior before they fracture.  Ductile materials have a small region of elastic behavior and a large region of ductile behavior before they fracture.

How a material behaves will depend on several factors. Among them are: Temperature - At high temperature molecules and their bonds can stretch and move, thus materials will behave in more ductile manner. At low Temperature, materials are brittle.   Confining Pressure - At high confining pressure materials are less likely to fracture because the pressure of the surroundings tends to hinder the formation of fractures. At low confining stress, material will be brittle and tend to fracture sooner.  Strain rate -- At high strain rates material tends to fracture. At low strain rates more time is available for individual atoms to move and therefore ductile behavior is favored.  Composition -- Some minerals, like quartz, olivine, and feldspars are very brittle. Others, like clay minerals, micas, and calcite are more ductile This is due to the chemical bond types that hold them together. Thus, the mineralogical composition of the rock will be a factor in determining the deformational behavior of the rock. Another aspect is presence or absence of water. Water appears to weaken the chemical bonds and forms films around mineral grains along which slippage can take place. Thus wet rock tends to behave in ductile manner, while dry rocks tend to behave in brittle manner.

Brittle-Ductile Properties of the Lithosphere
We all know that rocks near the surface of the Earth behave in a brittle manner. Crustal rocks are composed of minerals like quartz and feldspar which have high strength, particularly at low pressure and temperature. As we go deeper in the Earth the strength of these rocks initially increases. At a depth of about 15 km we reach a point called the brittle-ductile transition zone. Below this point rock strength decreases because fractures become closed and the temperature is higher, making the rocks behave in a ductile manner. At the base of the crust the rock type changes to peridotite which is rich in olivine. Olivine is stronger than the minerals that make up most crustal rocks, so the upper part of the mantle is again strong. But, just as in the crust, increasing temperature eventually predominates and at a depth of about 40 km the brittle-ductile transition zone in the mantle occurs. Below this point rocks behave in an increasingly ductile manner.

Deformation in Progress Only in a few cases does deformation of rocks occur at a rate that is observable on human time scales. Abrupt deformation along faults, usually associated with earthquakes caused by the fracture of rocks occurs on a time scale of minutes or seconds. Gradual deformation along faults or in areas of uplift or subsidence can be measured over periods of months to years with sensitive measuring instruments. Evidence of Former Deformation Evidence of deformation that has occurred in the past is very evident in crustal rocks. For example, sedimentary strata and lava flows generally follow the law of original horizontality. Thus, when we see such strata inclined instead of horizontal, evidence of an episode of deformation. In order to uniquely define the orientation of a planar feature we first need to define two terms - strike and dip.

For an inclined plane the strike is the compass direction of any horizontal line on the plane. The dip is the angle between a horizontal plane and the inclined plane, measured perpendicular to the direction of strike.  In recording strike and dip measurements on a geologic map, a symbol is used that has a long line oriented parallel to the compass direction of the strike. A short tick mark is placed in the center of the line on the side to which the inclined plane dips, and the angle of dip is recorded next to the strike and dip symbol as shown above.  For beds with a 900 dip (vertical) the short line crosses the strike line, and for beds with no dip (horizontal) a circle with a cross inside is used as shown below..

Fracture of Brittle Rocks  Faults - Faults occur when brittle rocks fracture and there is an offset along the fracture. When the offset is small, the displacement can be easily measured, but sometimes the displacement is so large that it is difficult to measure. Types of Faults Faults can be divided into several different types depending on the direction of relative displacement. Since faults are planar features, the concept of strike and dip also applies, and thus the strike and dip of a fault plane can be measured. One division of faults is between dip-slip faults, where the displacement is measured along the dip direction of the fault, and strike-slip faults where the displacement is horizontal, parallel to the strike of the fault. Dip Slip Faults - Dip slip faults are faults that have an inclined fault plane and along which the relative displacement or offset has occurred along the dip direction. Note that in looking at the displacement on any fault we don't know which side actually moved or if both sides moved, all we can determine is the relative sense of motion. For any inclined fault plane we define the block above the fault as the hanging wall block and the block below the fault as the footwall block. Normal Faults - are faults that result from horizontal tensional stresses in brittle rocks and where the hanging-wall block has moved down relative to the footwall block.

Horsts & Gabens - Due to the tensional stress responsible for normal faults, they often occur in a series, with adjacent faults dipping in opposite directions. In such a case the down-dropped blocks form grabens and the uplifted blocks form horsts. In areas where tensional stress has recently affected the crust, the grabens may form rift valleys and the uplifted horst blocks may form linear mountain ranges. The East African Rift Valley is an example of an area where continental extension has created such a rift. The basin and range province of the western U.S. (Nevada, Utah, and Idaho) is also an area that has recently undergone crustal extension. In the basin and range, the basins are elongated grabens that now form valleys, and the ranges are uplifted horst blocks.

Half-Grabens - A normal fault that has a curved fault plane with the dip decreasing with depth can cause the down-dropped block to rotate. In such a case a half-graben is produced, called such because it is bounded by only one fault instead of the two that form a normal graben.

Reverse Faults - are faults that result from horizontal compressional stresses in brittle rocks, where the hanging-wall block has moved up relative the footwall block.

A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 15o. Thrust faults can have considerable displacement, measuring hundreds of kilometers, and can result in older strata overlying younger strata.

Strike Slip Faults - are faults where the relative motion on the fault has taken place along a horizontal direction. Such faults result from shear stresses acting in the crust. Strike slip faults can be of two varieties, depending on the sense of displacement. To an observer standing on one side of the fault and looking across the fault, if the block on the other side has moved to the left, we say that the fault is a left-lateral strike-slip fault. If the block on the other side has moved to the right, we say that the fault is a right-lateral strike-slip fault. The famous San Andreas Fault in California is an example of a right-lateral strike-slip fault. Displacements on the San Andreas fault are estimated at over 600 km.

Transform-Faults are a special class of strike-slip faults. These are plate boundaries along which two plates slide past one another in a horizontal manner. The most common type of transform faults occur where oceanic ridges are offset. Note that the transform fault only occurs between the two segments of the ridge. Outside of this area there is no relative movement because blocks are moving in the same direction. These areas are called fracture zones. The San Andreas fault in California is also a transform fault.

Evidence of Movement on Faults Slikensides are scratch marks that are left on the fault plane as one block moves relative to the other. Slickensides can be used to determine the direction and sense of motion on a fault. Fault Breccias are crumbled up rocks consisting of angular fragments that were formed as a result of grinding and crushing movement along a fault. Folding of Ductile Rocks When rocks deform in a ductile manner, instead of fracturing to form faults, they may bend or fold, and the resulting structures are called folds. Folds result from compressional stresses acting over considerable time. Because the strain rate is low, rocks that we normally consider brittle can behave in a ductile manner resulting in such folds. We recognize several different kinds of folds.  Monoclines are the simplest types of folds. Monoclines occur when horizontal strata are bent upward so that the two limbs of the fold are still horizontal. .

Anticlines are folds where the originally horizontal strata has been folded upward, and the two limbs of the fold dip away from the hinge of the fold.

Synclines are folds where the originally horizontal strata have been folded downward, and the two limbs of the fold dip inward toward the hinge of the fold. Synclines and anticlines usually occur together such that the limb of a syncline is also the limb of an anticline.

Geometry of Folds - Folds are described by their form and orientation. The sides of a fold are called limbs. The limbs intersect at the tightest part of the fold, called the hinge. A line connecting all points on the hinge is called the fold axis. In the diagrams above, the fold axes are horizontal, but if the fold axis is not horizontal the fold is called a plunging fold and the angle that the fold axis makes with a horizontal line is called the plunge of the fold. An imaginary plane that includes the fold axis and divides the fold as symmetrically as possible is called the axial plane of the fold.

Note that if a plunging fold intersects a horizontal surface, we will see the pattern of the fold on the surface.

Classification of Folds Folds can be classified based on their appearance. If the two limbs of the fold dip away from the axis with the same angle, the fold is said to be a symmetrical fold. If the limbs dip at different angles, the folds are said to be asymmetrical folds.

If the compressional stresses that cause the folding are intense, the fold can close up and have limbs that are parallel to each other. Such a fold is called an isoclinal fold (iso means same, and cline means angle, so isoclinal means the limbs have the same angle). Note the isoclinal fold depicted in the diagram below is also a symmetrical fold.

If the folding is so intense that the strata on one limb of the fold becomes nearly upside down, the fold is called an overturned fold.

An overturned fold with an axial plane that is nearly horizontal is called a recumbant fold.

A fold that has no curvature in its hinge and straight-sided limbs that form a zigzag pattern is called a chevron fold.

The Relationship Between Folding and Faulting Because different rocks behave differently under stress, we expect that some rocks when subjected to the same stress will fracture or fault, while others will fold. When such contrasting rocks occur in the same area, such as ductile rocks overlying brittle rocks, the brittle rocks may fault and the ductile rocks may bend or fold over the fault.

Also since even ductile rocks can eventually fracture under high stress, rocks may fold up to a certain point then fracture to form a fault.

Folds and TopographySince different rocks have different resistance to erosion and weathering, erosion of folded areas can lead to a topography that reflects the folding. Resistant strata would form ridges that have the same form as the folds, while less resistant strata will form valleys (see figure 9.25 in you text). Mountain Ranges - The Result of Deformation of the Crust One of the most spectacular results of deformation acting within the crust of the Earth is the formation of mountain ranges. Mountains originate by three processes, two of which are directly related to deformation. Thus, there are three types of mountains:

Fault Block Mountains - As the name implies, fault block mountains originate by faulting. As discussed previously, both normal and reverse faults can cause the uplift of blocks of crustal rocks. The Sierra Nevada mountains of California, and the mountains in the Basin and Range province of the western U.S., as discussed previously, were formed by faulting processes and are thus fault block mountains. Fold & Thrust Mountains - Large compressional stresses can be generated in the crust by tectonic forces that cause continental crustal areas to collide. When this occurs the rocks between the two continental blocks become folded and faulted under compressional stresses and are pushed upward to form fold and thrust mountains. The Himalayan Mountains (currently the highest on Earth) are mountains of this type and were formed as a result of the Indian Plate colliding with the Eurasian plate. Similarly the Appalachian Mountains of North America and the Alps of Europe were formed by such processes. Volcanic Mountains - The third type of mountains, volcanic mountains, are not formed by deformational processes, but instead by the outpouring of magma onto the surface of the Earth. The Cascade Mountains of the western U.S., and of course the mountains of the Hawaiian Islands and Iceland are volcanic mountains  Read more   http://www.geologyclass.org/Structures%20Concepts.htm http://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/Structural.html

Time and Geology

Time and Geology

Important concepts

• Principle of Uniformitarianism (Hutton, 1790) “Present is key to the past”

• Principle of superposition – Order in which the strata were deposited is from bottom to top

• Law of horizontality – Strata deposited in horizontal/nearly horizontal and parallel/nearly parallel to the earth surface.                                                

• Unconformities – A substantial break or gap in sedimentary records – Three types

Geologic Age

• Relative age – Key beds – Index fossils (eg. Trilobites, 530 Ma ago)

• Highly abundant

• Widespread geographic occurrence

• Absolute age – C14, K40 radioactive dating

Stratigraphic Correlations

• Rock stratigraphic units – A body of rock forming a discrete and recognizable unit of reasonable homogeneity – Basic unit is “formation” – Divisions are group, formation, member and bed

• Time stratigraphic units – All rocks deposited in a given time – “System” is the primary time-stratigraphic unit – Divisions are erathem, system, series and stage

Geologic Time Scale

• Extends from the beginning of the earth to the present

• Consists of Eons, Eras, Periods, epochs, ages and chrons

– Eon

 • Largest time interval in the earth history

• Composed of 4 intervals

 – Hadean - no records – Archean

– oldest rocks of the earth

 – Proterozoic – multicelled organisms (not preserved)

 – Phanerozoic– clues of the life

 – Era

 • Subdivisions of eons

– Palaeozoic (Old life)

 – Mesozoic (Middle life)

– Cenezoic (Recent life)

Palaeozoic Era

• Lasted for 300 Ma

• By the end, most of species disappeared or evolved

• Lot of invertebrate sea species appeared from the Cambrian, especially trilobites

• Plants grew from algae to trees through swamps

• Vertebrates evolved from the Ordovician (Age of Fish), amphibians

• Climate changed at the end of the Carboniferous turning forests into coal (Oil seams)

Mesozoic Era (Triassic, Jurassic, Cretaceous)

• The supercontinent Pangaea came to the existence

• Nonfloweringseed plants dominated (conifers)

• Giant reptiles evolved from the Permian dominated the land

• Flying and marine reptiles appeared from the Jurassic

• Mammals evolved from the Cretaceous

• By the end of the ear, all dinosaurs became a victim of mass extinction

• Flowering plants first appeared

• Pangaea split to Laurasia and Gondwanaland during the Cretaceous

Cenozoic Era Tertiary and Quaternary

• Also called Palaeogene and Neocene

• From the Palaeocene, continentals started spreading

• Woolly mammals dominated (eg. Mammoth)

• Modern fish appeared (Whales, dolphins)

• Flowering plants dominated the lands

• Primates evolved through the era

• during the Pleistocene, modern human evolved.

• Pleistocene and Holocene are known are Ice Ages

Ice Ages

• At least 11 ice ages

 • Earliest in the Proterozoic (2.7 to 2.3 Ba ago)

• Followed by another between 850 and 630 Ma ago and minor ice ages in Ordovician to Permian

 • In Pleistocene Major Ice age exited forming ice sheets in northern hemisphere (20 Ma ago) and it ended 10,000 years ago.

• Usually a glaciations lasts for 18,000 years, thus during ice ages, glaciation and non glaciation periods exists. Hence, retreated and redeveloped.

The Solar System and the Layered Earth

• The Solar System and the Layered Earth
  
  
  

   Origin of the Solar System

• age of universe: 15 billion years (a recent, more precise value is 13.73 +/- 0.12 billion years)
(NB for the interested: we know this from estimates of the lifetime of protons which are considered "stable" particles.)
• age of solar system⁽¹⁾ (and Earth): 4.5 billion years (Moon about 100,000 years younger)
⁽¹⁾estimated from the study of meteorites Currently, no hypothesis can explain all observations adequately but the modified nebular hypothesis, initially by Immanuel Kant (1755), then updated, is the most widely used/accepted. The nebular hypothesis supports the idea that sun and planets formed at the same time.

• initially huge rotating spherical cloud (nebula) of ice, gas, debris
• spinning cloud contracts
• to conserve angular momentum, spinning speeds up (like figure skater pulling arms in)
• flattening to disk
  (NB for the interested: the exact reason for this is poorly understood but one hypothesis is that particles from above and below the disk collided near the disk and propelled each other further out - this is needed to explain why the planets carry most of the angular momentum in the solar system)
• immense density at center leads to gravitational collapse and formation of sun
(NB for the interested: immense pressure and temperature during gravitational collapse started thermo-nuclear processes in the sun)
• in rotating disk, masses collide and stick together (accretion)
(NB: exactly how this was done is still under debate; two possible models are called gravitational instability theory and core-accretion theory)
• accretion first leads to planetesimals (a few km radius), then planets (> 1000km radius)
A publication in "American Scientist" likens the core-accretion process to what happens to dust particles under your bed when you don't vacuum for a while. If you are logged in from a UCSD campus machine, you can access the article.

• Members of the Solar System

   

  

• sun has 99.9% of all the mass in the solar system
• planets have 99% of all the angular momentum in the solar system


• terrestrial planets, inner planets: Mercury, Venus, Earth, Mars
(rocky planets)


• gas giants, outer planets: Jupiter, Saturn, Uranus, Neptune
(Jupiter and Saturn are gas planets with relatively small cores; Uranus and Neptune are icy planets; all four consist of mainly H₂ and He)
• Jupiter has 60% of all the mass of the planets and 70% of all the angular momentum
• all planets orbit the sun in the same direction (counterclockwise when viewed from above)
• all planets have nearly circular orbits
NB: a revolution about the sun can take from 80 days (Mercury) to 165 years (Neptune)
• all planets also revolve about a spin axis
a spin can take from 9h (Jupiter) to 243 days (Venus)
• all but Mercury and Venus have Moons/satellites


• Moon
• Earth's satellite, not a planet
• formed during early formation when Mars size impactor hit Earth (~4.4 billion years ago)
• Moon made mostly of Earth mantle rock
• Pluto
• discovered relatively late: on 13 February 1930 by Clyde W. Tombaugh
• Pluto is relatively small (less than half the diameter of Mercury; only 2/3 diameter of Moon); smaller than other celestial objects found in the solar system
• Pluto's orbit is highly elliptical and inclined with respect to all the other planets' orbits
• Pluto has a very large moon, Charon, whose diameter is about half that of Pluto
• Pluto is one of the trans-Neptunian objects (TNOs), meaning they lie beyond Neptune
• Pluto was found to be too unusual to fit the typical 'planet of the solar system'
• used to be one of the 9 planets but was downgraded to a dwarf planet in August 2006 during a conference of the IAU (International Astronomical Union) in Prague, Czech Republic


• dwarf planets:
Pluto is now a member of the dwarf planets. Other examples are Ceres in the asteroid belt and Eris (formerly known as Xena), a TNO further out than Pluto but larger and whose discovery was ultimately responsible to downgrade Pluto to a dwarf planet. Read about Pluto at wikipedia
• definition of a dwarf planet
• has own orbit around Sun
• has sufficient mass to assume nearly round shape
• is not a satellite of a planet or other nonstellar body
• has not "cleared the neighborhood" around its orbit
The only difference between the definition of a dwarf planet and a real planet is that real planets have "cleared the neighborhood" around its orbit, meaning that they have "scooped" up all the debris along its orbit as a result of its large mass and gravitational force.


• Current list of dwarf planets as of fall 2011, the IAU has recognized five dwarf planets in the Solar System: Ceres, Pluto, Haumea, Makemake and Eris. Only Ceres and Pluto have been observed in enough detail to demonstrate that they meet the definition of a dwarf planet. Eris has been accepted because it is more massive (by 27%) than Pluto. The two trans-Neptunian objects Haumea (rather egg-shaped, having 2 moons) and Makemake currently qualify because their brightness of +1 or larger suggests that their diameter is 838 km or larger). However, hundreds more objects are proposed for consideration. Using the brightness argument, four additional trans-Neptunian objects are candidates, as of August 2011: 2007 RQ₁₀, Sedna, Quaoar and Orcus.



• Pluto's Fall Continues: Pluto's struggle for status isn't over. Charon is currently considered Pluto's satellite but it does not really orbit it. Due to the size of Charon relative to Pluto, the center of mass for the Pluto-Charon system (which orbits the Sun) lies outside of Pluto. One could argue that Pluto does not have its own orbit around the Sun (one of the criteria to be a dwarf planet). There is therefore the suggestion that Pluto and Charon should be re-classified together to be a dual, or binary dwarf planet system.

NB: ER, Earth radius = 6371 km; EM, Earth mass = 5.974x10²⁴ kg; AU, astronomical unit = 1.4960x10⁸ km
* distance to Earth: 3.844x10⁵ km
** this is 2.5 times the mass of all the other planets taken together
*** Ganymede, Callisto, Io and Europa are similar to terrestrial planets; Ganymede is larger than Mercury
**** Titan (largely made of ice) is the only satellite with an atmosphere and is larger than Mercury
^oCeres was discovered on 1 January, 1801, 45 years before Netpune. It was considered a planet for half a century before reclassification as an asteroid.
^oo Charon is currently considered Pluto's satellite but Pluto/Charon are really a binary dwarf planet system
-- Eris is larger and more massive than Pluto

• asteroid belt between Mars and Jupiter:
• belt where there should be another planet, according to Bode's Law
• scientists believe that either an impactor destroyed a planet initially formed there or that, due to the tug of Jupiter, a planet could never form
• most objects too small to be rounded
• 3 objects larger than 500 km in diameter (Ceres, Pallas, Vesta)
• some of the larger objects are planetesimals or dwarf planets, with Ceres being the largest at about 1000 km in diameter
• some have Earth-crossing orbit (Apollos) -> past and future impactors on Earth
• some have Mars-crossing orbit (Amors)
• asteroids are low-density, rocky objects
NB: Ceres was actually considered a planet from 1801 until the 1860s.
NB: Neptune was discovered in 1846 and both Neptune and Pluto are not located where Bode's Law would place them. check out wikipedia.


• comets:
  • have rocky core (a few km across) embedded in icy shell ("dirty snowball")
  • upon approach to sun, solar energy causes glow and ejections of icy material
  • left as debris to make for bright meteor showers when Earth approaches debris on its orbit (e.g. Perseid Meteor Showers in August)
  • the core of some comets are large enough to pose a serious impactor risk (e.g. Shoemaker-Levy 9 comet that hit Jupiter in 1994) view wikipedia page
  • In July 2009, another comet impacted on Jupiter see newsclip
  
  
• Kuiper belt:
  • on the plane of the solar system (ecliptic) beyond Neptune
  • extends out to 55 AU (1AU=Earth-Sun distance)
  • source of short-period comets (recurrence < 200 yrs)
  An example:
  • e.g. Halley's comet comes about every 75 yrs though Halley's comet originally may have come from the Oort cloud, last appearance/apparition: 1986)


• Oort cloud:
  • vast, diffuse spherical cloud around the solar system
  • extends out to 50,000 AU (1 light-year)
  • source of long-period comets (recurrence > 200 yrs)
  • period of comets change after encounter with gravitational pull of gas giants
  Some examples:
  • Kohoutek last came 150,000 yrs ago, will come again in 75,000 yrs, last appearance: 1973; its rocky composition suggests that it was originally from the Kuiper belt
  • Hale-Bopp last came 4,200 yrs ago, will come again in 2,533 yrs, last appearance: 1997; orbit almost perpendicular to the ecliptic, so collision unlikely
  • Hyakutake used to come every 17,000 yrs but now comes every 70,000 yrs, last appearance: 1996


• Meteoroids, Meteorites and Meteors:
  • meteoroid: fragment of asteroid or comet entering Earth's atmosphere
  • meteor: visual phenomenon when meteoroid burns up in Earth's atmosphere
  • meteorite: remaining piece(s) of meteoroid after it impacts on Earth's surface;
    there are two types
    iron/metallic meteorites are thought to resemble Earth's core
    stony/rocky meteorites are thought to resemble Earth's mantle; rarer than iron meteorites

• Earth's History

• ~ 4.56 Ga ago: initially aggregating mass of particles and gas
(NB: Ga comes from Giga for billion and "annum", latin for year)
• random collision of pieces -> planetesimal -> planet
• collision and compaction led to heating
• differentiation: heavier components (e.g. iron-rich) pulled toward center; lighter components toward surface
• formation of Earth's iron core while Earth's outer shells were still soft
• Mars-size impactor hit earth, ejecting parts of mantle into space to form the Moon
• Earth cools and forms lithosphere
• ~3.9 billion years ago: (small) continents, oceans, dense atmosphere (CO2 dominant)
• ~3.5 billion years ago: first life (bacteria)
• ~2.5 billion years ago: large continents
• plate tectonics for at least last 1.5 billion years

• The Layered Earth

• radius: 6371 km; diameter (without atmosphere): 12742 km
• circumference: ~40000 km
• dense, iron-rich core (radius: 3480 km; diameter: 6960 km); composition similar to metallic meteorites
• solid inner core (radius: 1222 km; diameter: 2444 km)
• liquid outer core (mostly iron), movement of electrically charged particles cause magnetic field
• core-mantle boundary: at 2891 km depth; transition from liquid iron to solid rock
• mantle around core; composition similar to chondritic, stony meteorites (peridotite)
• transition zone between upper and lower mantle: 410-660~km depth
• weak, ductile asthenosphere above 350~km
• strong, brittle lithosphere above about 125~km
• crust above 70~km; very brittle; rich in silicates (SiO2-rich rocks)
• the 5 layer model: inner core, outer core, mantle, asthenosphere, lithosphere
• the 3 layer model: core, mantle, crust

• Elastic, Ductile and Brittle Material

Elastic, ductile and brittle describes a material's reaction when a force (or stress) is applied to it.
elastic material
• can be bent without breaking
• returns to original state after bending force is removed
• example: a metal spring
• stores potential energy (e.g. a loaded spring)
ductile material
• also bends without breaking but deformation is plastic, not elastic
• does not return to original state after bending force is removed
• example: putty, play dough
• no storage of potential energy
brittle material
• may be elastic when force is applied slowly
• breaks if force is applied rapidly or if force is greater than the elasticity of the material allows
• may store potential energy (when force is acting slowly)
• potential energy released when material breaks

• The Lithosphere and Asthenosphere; Earth's Layers relevant to Plate Tectonics

Lithosphere
• from Greek "lithos" for rock
• crust and uppermost mantle
• cool and strong but brittle
• zone of earthquakes (EQs)
• broken into 12 major plates and a few others
• oceanic: thin but dense
• continental: thick and less dense

Asthenosphere
• from Greek "asthenes" for weak
• uppermost mantle below lithosphere
• warm, soft and weak
• ductile

• Isostasy

principle of buoyancy
• 2 counteracting forces: gravity and buoyancy
• isostatic equilibrium: forces are balanced and body floats
Relevance to Earth


• lithosphere less dense than asthenosphere
• rigid lithosphere floats on soft asthenosphere like icebergs in water
• floating continents have roots/keels beneath
• asthenosphere reacts to imbalance by flowing (e.g. like viscous honey)
Example 1:
• oceanic plate cools and thickens with age
• gets denser, eventually losing buoyancy
• sinks into Earth's mantle
• drives mantle convection and plate tectonics
• use of potential energy to drive plate tectonics
Example 2 (Load through Ice Sheets during Glaciation):
• ice sheet adds mass added on top of lithosphere
-> asthenosphere beneath flows away
-> lithosphere sinks
• elastic rebound after ice melts (postglacial rebound)

Read more
http://geol.queensu.ca/museum/index.php?option=com_content&view=article&id=50&Itemid=57

Earth – Overview

Earth is the third planet from the Sun and the fifth largest:

        orbit:    149,600,000 km (1.00 AU) from Sun

        diameter: 12,756.3 km

        mass:     5.972e24

Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, hundreds of other names for the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus - the fertile soil (Greek: Gaia, terra mater - Mother Earth).

It was not until the time of Copernicus (the sixteenth century) that it was understood that the Earth is just another planet.

Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful.

The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):

                 0-  40  Crust

                40- 400  Upper mantle

               400- 650  Transition region

               650-2700  Lower mantle

              2700-2890  D'' layer

              2890-5150  Outer core

              5150-6378  Inner core      

The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle.

Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole (values below x10^24 kilograms):

                 atmosphere     = 0.0000051

                 oceans         = 0.0014

                 crust          = 0.026

                 mantle         = 4.043

                 outer core     = 1.835

                 inner core     = 0.09675

The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by mass) is:

34.6%  Iron

    29.5%  Oxygen

    15.2%  Silicon

    12.7%  Magnesium

     2.4%  Nickel

     1.9%  Sulfur

     0.05% Titanium

The Earth is the densest major body in the solar system.

The other terrestrial planets probably have similar structures and compositions with some differences: the Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth.

Unlike the other terrestrial planets, Earth's crust is divided into several separate solid plates which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse motion at some plate boundaries (i.e. the San Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia). There are (at present) eight major plates:

• North American Plate - North America, western North Atlantic and Greenland
• South American Plate - South America and western South Atlantic
• Antarctic Plate - Antarctica and the "Southern Ocean"
• Eurasian Plate - eastern North Atlantic, Europe and Asia except for India
• African Plate - Africa, eastern South Atlantic and western Indian Ocean
• Indian-Australian Plate - India, Australia, New Zealand and most of Indian Ocean
• Nazca Plate - eastern Pacific Ocean adjacent to South America
• Pacific Plate - most of the Pacific Ocean (and the southern coast of California!)

There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate boundaries.

The Earth's surface is very young. In the relatively short (by astronomical standards) period of 500,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters). Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare. The oldest fossils of living organisms are less than 3.9 billion years old. There is no record of the critical period when life was first getting started.

Strait of Gibraltar Space Shuttle view of the Strait of Gibraltar

71 Percent of the Earth's surface is covered with water. Earth is the only planet on which water can exist in liquid form on the surface (though there may be liquid ethane or methane on Titan's surface and liquid water beneath the surface of Europa). Liquid water is, of course, essential for life as we know it. The heat capacity of the oceans is also very important in keeping the Earth's temperature relatively stable. Liquid water is also responsible for most of the erosion and weathering of the Earth's continents, a process unique in the solar system today (though it may have occurred on Mars in the past).

Earth's atmosphere Earth's atmosphere seen at the limb

The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. There was probably a very much larger amount of carbon dioxide in the Earth's atmosphere when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living plants. Plate tectonics and biological processes now maintain a continual flow of carbon dioxide from the atmosphere to these various "sinks" and back again. The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth's surface temperature via the greenhouse effect. The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from a frigid -21 C to a comfortable +14 C); without it the oceans would freeze and life as we know it would be impossible. (Water vapor is also an important greenhouse gas.)

Earth from Apollo 11 View from Apollo 11

The presence of free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive gas and under "normal" circumstances would quickly combine with other elements. The oxygen in Earth's atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen.

The interaction of the Earth and the Moon slows the Earth's rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 481 18-hour days in a year.

Earth has a modest magnetic field produced by electric currents in the outer core. The interaction of the solar wind, the Earth's magnetic field and the Earth's upper atmosphere causes the auroras (see the Interplanetary Medium). Irregularities in these factors cause the magnetic poles to move and even reverse relative to the surface; the geomagnetic north pole is currently located in northern Canada. (The "geomagnetic north pole" is the position on the Earth's surface directly above the south pole of the Earth's field.)

The Earth's magnetic field and its interaction with the solar wind also produce the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth. The outer belt stretches from 19,000 km in altitude to 41,000 km; the inner belt lies between 13,000 km and 7,600 km in altitude.

Earth's Satellite

Earth has only one natural satellite, the Moon. But

    thousands of small artificial satellites have also been placed in orbit around the Earth.

    Asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships with the Earth; they're not really moons, the term "companion" is being used. It is somewhat similar to the situation with Saturn's moons Janus and Epimetheus.

    Lilith doesn't exist but it's an interesting story.

           Distance  Radius    Mass

Satellite  (000 km)   (km)     (kg)

---------  --------  ------  -------

Moon       384         1738  7.35e22

Read more about Earth  l  Earth facts, pictures and information. by nineplanets.org