How Old is the Earth
Thus, isochrons do indeed seem to contain somewhat of an internal indicator or control for contamination that indicates the general suitability or unsuitability of a specimen for dating. In other experiments muscovite was synthesized from a colloidal gel under similar temperatures and Ar pressures, the resultant muscovite retaining up to 0. The weighted mean of these two measurements is Retrieved 17 August
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Keep Exploring Britannica Earthquake. Still, this image approximates the appearance of the full Moon as seen from the Earth adequately for the purpose of this image. And yet, with a relatively large amount of argon in the air, argon filtering up from rocks below, excess argon in lava, the fact that argon and potassium are water soluble, and the fact that argon is mobile in rock and is a gas, we are still expecting this wisp of argon gas to tell us how old the rock is? Gamma rays very small bundles of energy are the device by which an atom rids itself of excess energy. The size frequency distribution SFD of crater diameters on a given surface that is, the number of craters as a function of diameter approximately follows a power law with increasing number of craters with decreasing crater size. This example shows an age of 3. Thus the assumption of immense ages has not been proven.
Three Luna spacecraft returned with grams Rocks from the Moon have been measured by radiometric dating techniques. They range in age from about 3. In contrast, the oldest ages of rocks from the Earth are between 3. Moon rocks fall into two main categories: The terrae consist dominantly of mafic plutonic rocks. Regolith breccias with similar protoliths are also common. Mare basalts come in three distinct series in direct relation to their titanium content: Almost all lunar rocks are depleted in volatiles and are completely lacking in hydrated minerals common in Earth rocks.
In some regards, lunar rocks are closely related to Earth's rocks in their isotopic composition of the element oxygen. The Apollo moon rocks were collected using a variety of tools, including hammers , rakes , scoops , tongs , and core tubes. Most were photographed prior to collection to record the condition in which they were found.
They were placed inside sample bags and then a Special Environmental Sample Container for return to the Earth to protect them from contamination. In contrast to the Earth, large portions of the lunar crust appear to be composed of rocks with high concentrations of the mineral anorthite.
The mare basalts have relatively high iron values. Furthermore, some of the mare basalts have very high levels of titanium in the form of ilmenite. The Soviet Union attempted, but failed to make manned lunar landings in the s, due to failure to develop their N1 rocket , but they succeeded in landing three robotic Luna spacecraft with the capability to collect and return small samples to Earth. A combined total of less than one kilogram of material was returned.
Primary igneous rocks in the lunar highlands compose three distinct groups: Lunar breccias, formed largely by the immense basin-forming impacts, are dominantly composed of highland lithologies because most mare basalts post-date basin formation and largely fill these impact basins.
The ferroan anorthosite suite is the most common group in the highlands, and is inferred to represent plagioclase flotation cumulates of the lunar magma ocean, with interstitial mafic phases formed from trapped interstitial melt or rafted upwards with the more abundant plagioclase framework.
This reflects the extreme depletion of the bulk moon in alkalis Na, K as well as water and other volatile elements. Ferroan anorthosites have been dated using the internal isochron method at "circa" 4. These rocks represent later intrusions into the highlands crust ferroan anorthosite at round 4.
An interesting aspect of this suite is that analysis of the trace element content of plagioclase and pyroxene require equilibrium with a KREEP -rich magma, despite the refractory major element contents.
The alkali suite is so-called because of its high alkali content—for moon rocks. The alkali suite consists of alkali anorthosites with relatively sodic plagioclase An , norites plagioclase-orthopyroxene , and gabbronorites plagioclase-clinopyroxene-orthopyroxene with similar plagioclase compositions and mafic minerals more iron-rich than the magnesian suite. The alkali suite spans an age range similar to the magnesian suite.
Lunar granites are relatively rare rocks that include diorites , monzodiorites, and granophyres. They consist of quartz, plagioclase, orthoclase or alkali feldspar, rare mafics pyroxene , and rare zircon. The alkali feldspar may have unusual compositions unlike any terrestrial feldspar, and they are often Ba-rich.
These rocks apparently form by the extreme fractional crystallization of magnesian suite or alkali suite magmas, although liquid immiscibility may also play a role. U-Pb date of zircons from these rocks and from lunar soils have ages of 4.
O'Keefe and others linked lunar granites with tektites found on Earth although many researchers refuted these claims. According to one study, a portion of lunar sample has a chemistry that closely resembles javanite tektites found on Earth. Lunar breccias range from glassy vitrophyre melt rocks, to glass-rich breccia, to regolith breccias. The vitrophyres are dominantly glassy rocks that represent impact melt sheets that fill large impact structures.
They contain few clasts of the target lithology, which is largely melted by the impact. Glassy breccias form from impact melt that exit the crater and entrain large volumes of crushed but not melted ejecta. It may contain abundant clasts that reflect the range of lithologies in the target region, sitting in a matrix of mineral fragments plus glass that welds it all together.
Some of the clasts in these breccias are pieces of older breccias, documenting a repeated history of impact brecciation, cooling, and impact. Regolith breccias resemble the glassy breccias but have little or no glass melt to weld them together.
As noted above, the basin-forming impacts responsible for these breccias pre-date almost all mare basalt volcanism, so clasts of mare basalt are very rare. When found, these clasts represent the earliest phase of mare basalt volcanism preserved. Mare basalts are named as such because they frequently constitute large portions of the lunar maria.
They are similar to terrestrial basalts, but have many important differences; for example, mare basalts show a large negative europium anomaly.
At the bottom of the lunar stratigraphical sequence the pre-Nectarian unit of old crater plains can be found. The stratigraphy of Mercury is very similar to the lunar case. The lunar landscape is characterized by impact craters , their ejecta, a few volcanoes , hills, lava flows and depressions filled by magma.
The most distinctive aspect of the Moon is the contrast between its bright and dark zones. Lighter surfaces are the lunar highlands, which receive the name of terrae singular terra , from the Latin for Earth , and the darker plains are called maria singular mare , from the Latin for sea , after Johannes Kepler who introduced the name in the 17th century.
The highlands are anorthositic in composition, whereas the maria are basaltic. The maria often coincide with the "lowlands," but it is important to note that the lowlands such as within the South Pole-Aitken basin are not always covered by maria. The highlands are older than the visible maria, and hence are more heavily cratered. The major products of volcanic processes on the Moon are evident to Earth-bound observers in the form of the lunar maria.
These are large flows of basaltic lava that correspond to low- albedo surfaces covering nearly a third of the near side. Only a few percent of the farside has been affected by mare volcanism. Even before the Apollo missions confirmed it, most scientists already thought that the maria are lava-filled plains, because they have lava flow patterns and collapses attributed to lava tubes.
The ages of the mare basalts have been determined both by direct radiometric dating and by the technique of crater counting. The oldest radiometric ages are about 4.
Volumetrically, most of the mare formed between about 3 and 3. The youngest lavas erupted within Oceanus Procellarum , whereas some of the oldest appear to be located on the farside. The maria are clearly younger than the surrounding highlands given their lower density of impact craters. A large portion of maria erupted within, or flowed into, the low-lying impact basins on the lunar nearside. However, it is unlikely that a causal relationship exists between the impact event and mare volcanism because the impact basins are much older by about million years than the mare fill.
Furthermore, Oceanus Procellarum , which is the largest expanse of mare volcanism on the Moon, does not correspond to any known impact basin. It is commonly suggested that the reason the mare only erupted on the nearside is that the nearside crust is thinner than the farside. Although variations in the crustal thickness might act to modulate the amount of magma that ultimately reaches the surface, this hypothesis does not explain why the farside South Pole-Aitken basin , whose crust is thinner than Oceanus Procellarum, was only modestly filled by volcanic products.
Another type of deposit associated with the maria, although it also covers the highland areas, are the "dark mantle" deposits. These deposits cannot be seen with the naked eye, but they can be seen in images taken from telescopes or orbiting spacecraft. Before the Apollo missions, scientists believed that they were deposits produced by pyroclastic eruptions. Some deposits appear to be associated with dark elongated ash cones , reinforcing the idea of pyroclasts. The existence of pyroclastic eruptions was later confirmed by the discovery of glass spherules similar to those found in pyroclastic eruptions here on Earth.
Many of the lunar basalts contain small holes called vesicles , which were formed by gas bubbles exsolving from the magma at the vacuum conditions encountered at the surface. It is not known with certainty which gases escaped these rocks, but carbon monoxide is one candidate. The samples of pyroclastic glasses are of green, yellow, and red tints. Rilles on the Moon sometimes resulted from the formation of localized lava channels.
These generally fall into three categories, consisting of sinuous, arcuate, or linear shapes. By following these meandering rilles back to their source, they often lead to an old volcanic vent. An example of a sinuous rille exists at the Apollo 15 landing site, Rima Hadley , located on the rim of the Imbrium Basin. Based on observations from the mission, it is generally believed that this rille was formed by volcanic processes, a topic long debated before the mission took place.
These are believed to be formed by relatively viscous, possibly silica-rich lava, erupting from localized vents. The resulting lunar domes are wide, rounded, circular features with a gentle slope rising in elevation a few hundred meters to the midpoint. Some of the domes contain a small pit at their peak. Wrinkle ridges are features created by compressive tectonic forces within the maria.
These features represent buckling of the surface and form long ridges across parts of the maria. Some of these ridges may outline buried craters or other features beneath the maria. A prime example of such an outlined feature is the crater Letronne. Grabens are tectonic features that form under extensional stresses. Structurally, they are composed of two normal faults , with a down-dropped block between them.
Most grabens are found within the lunar maria near the edges of large impact basins. The origin of the Moon's craters as impact features became widely accepted only in the s. This realization allowed the impact history of the Moon to be gradually worked out by means of the geologic principle of superposition.
That is, if a crater or its ejecta overlaid another, it must be the younger. The amount of erosion experienced by a crater was another clue to its age, though this is more subjective. Adopting this approach in the late s, Gene Shoemaker took the systematic study of the Moon away from the astronomers and placed it firmly in the hands of the lunar geologists.
Impact cratering is the most notable geological process on the Moon. The kinetic energy of the impact creates a compression shock wave that radiates away from the point of entry.
This is succeeded by a rarefaction wave, which is responsible for propelling most of the ejecta out of the crater. Finally there is a hydrodynamic rebound of the floor that can create a central peak. In a very general sense, the lunar history of impact cratering follows a trend of decreasing crater size with time.
In particular, the largest impact basins were formed during the early periods, and these were successively overlaid by smaller craters. The size frequency distribution SFD of crater diameters on a given surface that is, the number of craters as a function of diameter approximately follows a power law with increasing number of craters with decreasing crater size. The vertical position of this curve can be used to estimate the age of the surface. The most recent impacts are distinguished by well-defined features, including a sharp-edged rim.
Small craters tend to form a bowl shape, whereas larger impacts can have a central peak with flat floors. Larger craters generally display slumping features along the inner walls that can form terraces and ledges. The largest impact basins, the multiring basins, can even have secondary concentric rings of raised material. The impact process excavates high albedo materials that initially gives the crater, ejecta, and ray system a bright appearance.
The process of space weathering gradually decreases the albedo of this material such that the rays fade with time. Gradually the crater and its ejecta undergo impact erosion from micrometeorites and smaller impacts. This erosional process softens and rounds the features of the crater. The crater can also be covered in ejecta from other impacts, which can submerge features and even bury the central peak. The ejecta from large impacts can include larges blocks of material that reimpact the surface to form secondary impact craters.
These craters are sometimes formed in clearly discernible radial patterns, and generally have shallower depths than primary craters of the same size. In some cases an entire line of these blocks can impact to form a valley.
These are distinguished from catena , or crater chains, which are linear strings of craters that are formed when the impact body breaks up prior to impact.
Generally speaking, a lunar crater is roughly circular in form. Laboratory experiments at NASA's Ames Research Center have demonstrated that even very low-angle impacts tend to produce circular craters, and that elliptical craters start forming at impact angles below five degrees.
However, a low angle impact can produce a central peak that is offset from the midpoint of the crater. Additionally, the ejecta from oblique impacts show distinctive patterns at different impact angles: Dark-halo craters are formed when an impact excavates lower albedo material from beneath the surface, then deposits this darker ejecta around the main crater. This can occur when an area of darker basaltic material, such as that found on the maria , is later covered by lighter ejecta derived from more distant impacts in the highlands.
This covering conceals the darker material below, which is later excavated by subsequent craters. The largest impacts produced melt sheets of molten rock that covered portions of the surface that could be as thick as a kilometer. Examples of such impact melt can be seen in the northeastern part of the Mare Orientale impact basin.
The surface of the Moon has been subject to billions of years of collisions with both small and large asteroidal and cometary materials. Over time, these impact processes have pulverized and "gardened" the surface materials, forming a fine-grained layer termed regolith.
The thickness of the lunar regolith varies between 2 meters beneath the younger maria, to up to 20 meters beneath the oldest surfaces of the lunar highlands. The regolith is predominantly composed of materials found in the region, but also contains traces of materials ejected by distant impact craters. The term mega-regolith is often used to describe the heavily fractured bedrock directly beneath the near-surface regolith layer. The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during the impacts.
In most of the lunar regolith, half of the particles are made of mineral fragments fused by the glassy particles; these objects are called agglutinates. The chemical composition of the regolith varies according to its location; the regolith in the highlands is rich in aluminium and silica , just as the rocks in those regions. The lunar regolith is very important because it also stores information about the history of the Sun.
The atoms that compose the solar wind — mostly helium , neon , carbon and nitrogen — hit the lunar surface and insert themselves into the mineral grains. Upon analyzing the composition of the regolith, particularly its isotopic composition, it is possible to determine if the activity of the Sun has changed with time. The gases of the solar wind could be useful for future lunar bases, because oxygen, hydrogen water , carbon and nitrogen are not only essential to sustain life, but are also potentially very useful in the production of fuel.
The composition of the lunar regolith can also be used to infer its source origin. Lunar lava tubes form a potentially important location for constructing a future lunar base, which may be used for local exploration and development, or as a human outpost to serve exploration beyond the Moon.
A lunar lava cave potential has long been suggested and discussed in literature and thesis. The first rocks brought back by Apollo 11 were basalts. Although the mission landed on Mare Tranquillitatis , a few millimetric fragments of rocks coming from the highlands were picked up. These are composed mainly of plagioclase feldspar ; some fragments were composed exclusively of anorthositic plagioclase.
The identification of these mineral fragments led to the bold hypothesis that a large portion of the Moon was once molten, and that the crust formed by fractional crystallization of this magma ocean. A natural outcome of the hypothetical giant-impact event is that the materials that re-accreted to form the Moon must have been hot. Crystallization of this magma ocean would have given rise to a differentiated body with a compositionally distinct crust and mantle and accounts for the major suites of lunar rocks.
As crystallization of the lunar magma ocean proceeded, minerals such as olivine and pyroxene would have precipitated and sank to form the lunar mantle. After crystallization was about three-quarters complete, anorthositic plagioclase would have begun to crystallize, and because of its low density, float, forming an anorthositic crust.
Importantly, elements that are incompatible i. Evidence for this scenario comes from the highly anorthositic composition of the lunar highland crust, as well as the existence of KREEP-rich materials. The Apollo program brought back These rocks have proved to be invaluable in deciphering the geologic evolution of the Moon. Lunar rocks are in large part made of the same common rock forming minerals as found on Earth, such as olivine , pyroxene , and plagioclase feldspar anorthosite.
Plagioclase feldspar is mostly found in the lunar crust, whereas pyroxene and olivine are typically seen in the lunar mantle. The maria are composed predominantly of basalt , whereas the highland regions are iron-poor and composed primarily of anorthosite , a rock composed primarily of calcium -rich plagioclase feldspar.
Images: dating lunar rocks
Other factors and basic assumptions must also be considered.
K-Ar ages on stone meteorites range from about million years to nearly 5 billion years, with a large concentration at 4. Olivine , any member of a group of common magnesium, iron silicate minerals.
Cut fragment of Apollo 17 samplean impact melt breccia. Volcanic rocks produced by lava flows which occurred in Hawaii in the years were dated by the potassium-argon method. Radiometric dating has not been applied to just a few new free dating site in america dating lunar rocks from the geologic record. For example, many isochrons used to date meteorites are most probably the result of mixing since they are based on whole-rock analysis, not on crystalline analysis. Since Potassium-Argon and Argon-Argon dating techniques are the most common dating lunar rocks are considered, even by geologists, to be among the most accurate of all the radioisotope dating methods, lets consider these in particular detail. Many of these samples have not had so intense nor so complex histories as the oldest Earth rocks, and they commonly record events nearer or equal to the time of formation of the planets. However, close examination of his examples, a few of which are listed in Table 2shows that he misrepresents both the data and their dating lunar rocks.
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