Intrusive rock is formed when magma crystallizes and solidifies underground to form intrusions, for example plutons, dikes, sills and volcanic necks. Intrusive rock forms within Earth's crust from the crystallization of magma. Many mountain ranges, such as the Sierra Nevada in California, are formed from large granite intrusions. Intrusions are one of the two ways igneous rock. Technically an intrusion is any formation of intrusive igneous rock. In contrast, an extrusion consists of extrusive rock. Large bodies of magma that solidify underground before they reach the surface of the crust are called plutons. Plutonic rocks form 7% of the Earth's current land surface. Coarse-grained intrusive igneous rocks that form at depth within the earth are called abyssal while those that form near the surface are called subvolcanic or hypabyssal. Intrusive structures are classified according to whether or not they are parallel to the bedding planes or foliation of the country rock: if the intrusion is parallel the body is concordant, otherwise it is discordant.
An intrusive suite is a group of plutons related in time and space.. Intrusions vary from mountain-range-sized batholiths to thin veinlike fracture fillings of aplite or pegmatite. Intrusions can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes: Batholith: a large irregular discordant intrusion Chonolith: an irregularly-shaped intrusion with a demonstrable base Cupola: a dome-shaped projection from the top of a large subterranean intrusion Dike: a narrow tabular discordant body nearly vertical Laccolith: concordant body with flat base and convex top with a feeder pipe below Lopolith: concordant body with flat top and a shallow convex base, may have a feeder dike or pipe below Phacolith: a concordant lens-shaped pluton that occupies the crest of an anticline or trough of a syncline Volcanic pipe or volcanic neck: tubular vertical body that may have been a feeder vent for a volcano Sill: a thin tabular concordant body intruded along bedding planes Stock: a smaller irregular discordant intrusive Boss: a small stock A body of intrusive igneous rock which crystallizes from magma cooling underneath the surface of the Earth is called a pluton.
If the pluton is large, it may be called a stock. Intrusive rocks are characterized by large crystal sizes, as the individual crystals are visible, the rock is called phaneritic; this is as the magma cools underground, while cooling may be fast or slow, cooling is slower than on the surface, so larger crystals grow. If it runs parallel to rock layers, it is called a sill. If an intrusion makes rocks above rise to form a dome, it is called a laccolith. How deep-seated intrusions burst through the overlying strata causes intrusive rock to be recognized: Veins spread out into branches, or branchlike parts result from filled cracks, the high temperature is evident in how they alter country rock; as heat dissipation is slow, as the rock is under pressure, crystals form, no vitreous chilled matter is present. The intrusions did not flow. Contained gases could not escape through the thick strata, thus form cavities, which can be observed; because their crystals are of the rough equal size, these rocks are said to be equigranular.
There is no distinction between a first generation of large well-shaped crystals and a fine-grained ground-mass. The minerals of each have formed in a definite order, each has had a period of crystallization that may be distinct or may have coincided with or overlapped the period of formation of some of the other ingredients. Earlier crystals originated at a time when most of the rock was still liquid and are more or less perfect. Crystals are less regular in shape because they were compelled to occupy the spaces left between the already-formed crystals; the former case is said to be idiomorphic. There are many other characteristics that serve to distinguish the members of these two groups. For example, orthoclase is feldspar from granite, while its modifications occur in lavas of similar composition; the same distinction holds for nepheline varieties. Leucite is common in lavas but rare in plutonic rocks. Muscovite is confined to intrusions; these differences show the influence of the physical conditions under which consolidation takes place.
Intrusive rocks formed at greater depths are called abyssal. Some intrusive rocks solidified in fissures as dikes and intrusive sills at shallow depth and are called subvolcanic or hypabyssal, they show structures intermediate between those of plutonic rocks. They are commonly porphyritic and sometimes vesicular. In fact, many of them are petrologically indistinguishable from lavas of similar composition. Ellicott City Granodiorite Guilford Quartz Monzonite Methods of pluton emplacement Norbeck Intrusive Suite Volcanic rock Woodstock Quartz Monzonite
Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, the processes by which they change over time. Geology can include the study of the solid features of any terrestrial planet or natural satellite such as Mars or the Moon. Modern geology overlaps all other earth sciences, including hydrology and the atmospheric sciences, so is treated as one major aspect of integrated earth system science and planetary science. Geology describes the structure of the Earth on and beneath its surface, the processes that have shaped that structure, it provides tools to determine the relative and absolute ages of rocks found in a given location, to describe the histories of those rocks. By combining these tools, geologists are able to chronicle the geological history of the Earth as a whole, to demonstrate the age of the Earth. Geology provides the primary evidence for plate tectonics, the evolutionary history of life, the Earth's past climates. Geologists use a wide variety of methods to understand the Earth's structure and evolution, including field work, rock description, geophysical techniques, chemical analysis, physical experiments, numerical modelling.
In practical terms, geology is important for mineral and hydrocarbon exploration and exploitation, evaluating water resources, understanding of natural hazards, the remediation of environmental problems, providing insights into past climate change. Geology is a major academic discipline, it plays an important role in geotechnical engineering; the majority of geological data comes from research on solid Earth materials. These fall into one of two categories: rock and unlithified material; the majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three major types of rock: igneous and metamorphic; the rock cycle illustrates the relationships among them. When a rock solidifies or crystallizes from melt, it is an igneous rock; this rock can be weathered and eroded redeposited and lithified into a sedimentary rock. It can be turned into a metamorphic rock by heat and pressure that change its mineral content, resulting in a characteristic fabric.
All three types may melt again, when this happens, new magma is formed, from which an igneous rock may once more solidify. To study all three types of rock, geologists evaluate the minerals; each mineral has distinct physical properties, there are many tests to determine each of them. The specimens can be tested for: Luster: Measurement of the amount of light reflected from the surface. Luster is broken into nonmetallic. Color: Minerals are grouped by their color. Diagnostic but impurities can change a mineral’s color. Streak: Performed by scratching the sample on a porcelain plate; the color of the streak can help name the mineral. Hardness: The resistance of a mineral to scratch. Breakage pattern: A mineral can either show fracture or cleavage, the former being breakage of uneven surfaces and the latter a breakage along spaced parallel planes. Specific gravity: the weight of a specific volume of a mineral. Effervescence: Involves dripping hydrochloric acid on the mineral to test for fizzing. Magnetism: Involves using a magnet to test for magnetism.
Taste: Minerals can have a distinctive taste, like halite. Smell: Minerals can have a distinctive odor. For example, sulfur smells like rotten eggs. Geologists study unlithified materials, which come from more recent deposits; these materials are superficial deposits. This study is known as Quaternary geology, after the Quaternary period of geologic history. However, unlithified material does not only include sediments. Magmas and lavas are the original unlithified source of all igneous rocks; the active flow of molten rock is studied in volcanology, igneous petrology aims to determine the history of igneous rocks from their final crystallization to their original molten source. In the 1960s, it was discovered that the Earth's lithosphere, which includes the crust and rigid uppermost portion of the upper mantle, is separated into tectonic plates that move across the plastically deforming, upper mantle, called the asthenosphere; this theory is supported by several types of observations, including seafloor spreading and the global distribution of mountain terrain and seismicity.
There is an intimate coupling between the movement of the plates on the surface and the convection of the mantle. Thus, oceanic plates and the adjoining mantle convection currents always move in the same direction – because the oceanic lithosphere is the rigid upper thermal boundary layer of the convecting mantle; this coupling between rigid plates moving on the surface of the Earth and the convecting mantle is called plate tectonics. The development of plate tectonics has provided a physical basis for many observations of the solid Earth. Long linear regions of geologic features are explained as plate boundaries. For example: Mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, are seen as divergent boundaries, where two plates move apart. Arcs of volcanoes and earthquakes are theorized as convergent boundaries, where one plate subducts, or moves, under another. Transform boundaries, such as the San Andreas Fault system, resulted in widespread powerful earthquakes.
Plate tectonics has provided a mechan
A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks; the method is called "polarized light microscopy". Depending on the grade of observation required, petrological microscopes are derived from conventional brightfield microscopes of similar basic capabilities by: Adding a Nicol prism polarizer filter to the light path beneath the sample slide Replacing the normal stage with a circular rotating stage Adding a second rotatable and removable Nicol prism filter, called the analyzer, to the light path between objective and eyepiece Adding a Phase telescope known as a Bertrand Lens, which allows the viewer to see conoscopic interference patterns Adding a slot for insertion of wave platesPetrographic microscopes are constructed with optical parts that do not add unwanted polarizing effects due to strained glass, or polarization by reflection in prisms and mirrors.
These special parts add to the complexity of the microscope. However, a "simple polarizing" microscope is made by adding inexpensive polarizing filters to a standard biological microscope with one in a filter holder beneath the condenser, a second inserted beneath the head or eyepiece; these can be sufficient for many non-quantitative purposes. The two Nicol prisms of the petrographic microscope have their polarizing planes oriented perpendicular to one another; when only an isotropic material such as air, water, or glass exists between the filters, all light is blocked, but most crystalline materials and minerals change the polarizing light directions, allowing some of the altered light to pass through the analyzer to the viewer. Using one polarizer makes it possible to view the slide in plane polarized light. A particular light pattern on the upper lens surface of the objectives is created as a conoscopic interference pattern characteristic of uniaxial and biaxial minerals, produced with convergent polarized light.
To observe the interference figure, true petrographic microscopes include an accessory called a Bertrand lens, which focuses and enlarges the figure. It is possible to remove an eyepiece lens to make a direct observation of the objective lens surface. In addition to modifications of the microscope's optical system, petrographic microscopes allow for the insertion of specially-cut oriented filters of biaxial minerals, into the optical train between the polarizers to identify positive and negative birefringence, in extreme cases, the mineral order when needed
Aphanite, or aphanitic as an adjective, is a name given to certain igneous rocks that are so fine-grained that their component mineral crystals are not detectable by the unaided eye. This geological texture results from rapid cooling in hypabyssal environments; as a rule, the texture of these rocks is not the same as that of volcanic glass, with volcanic glass being non-crystalline, having a glass-like appearance. Aphanites are porphyritic, having large crystals embedded in the fine groundmass or matrix; the large inclusions are called phenocrysts. They consist of fine-grained minerals, such as plagioclase feldspar, with hornblende or augite, may contain biotite and orthoclase. Andesite Basalt Basanite Dacite Felsite Phonolite Rhyolite Trachyte
A phanerite is an igneous rock whose microstructure is made up of crystals large enough to be distinguished with the unaided eye. Phaneritic texture forms when magma deep underground in the plutonic environment cools giving the crystals time to grow. Phanerites are described as coarse grained or macroscopically crystalline
Granite is a common type of felsic intrusive igneous rock, granular and phaneritic in texture. Granites can be predominantly white, pink, or gray depending on their mineralogy; the word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Speaking, granite is an igneous rock with between 20% and 60% quartz by volume, at least 35% of the total feldspar consisting of alkali feldspar, although the term "granite" is used to refer to a wider range of coarse-grained igneous rocks containing quartz and feldspar; the term "granitic" means granite-like and is applied to granite and a group of intrusive igneous rocks with similar textures and slight variations in composition and origin. These rocks consist of feldspar, quartz and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole peppering the lighter color minerals; some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic.
A granitic rock with a porphyritic texture is known as a granite porphyry. Granitoid is a descriptive field term for lighter-colored, coarse-grained igneous rocks. Petrographic examination is required for identification of specific types of granitoids; the extrusive igneous rock equivalent of granite is rhyolite. Granite is nearly always massive and tough; these properties have made granite a widespread construction stone throughout human history. The average density of granite is between 2.65 and 2.75 g/cm3, its compressive strength lies above 200 MPa, its viscosity near STP is 3–6·1019 Pa·s. The melting temperature of dry granite at ambient pressure is 1215–1260 °C. Granite has poor primary permeability overall, but strong secondary permeability through cracks and fractures if they are present. Granite is classified according to the QAPF diagram for coarse grained plutonic rocks and is named according to the percentage of quartz, alkali feldspar and plagioclase feldspar on the A-Q-P half of the diagram.
True granite contains both alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase, the rock is referred to as alkali feldspar granite; when a granitoid contains less than 10% orthoclase, it is called tonalite. A granite containing both muscovite and biotite micas is called two-mica granite. Two-mica granites are high in potassium and low in plagioclase, are S-type granites or A-type granites. A worldwide average of the chemical composition of granite, by weight percent, based on 2485 analyses: Granite containing rock is distributed throughout the continental crust. Much of it was intruded during the Precambrian age. Outcrops of granite tend to form rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels. Granite occurs as small, less than 100 km2 stock masses and in batholiths that are associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are associated with the margins of granitic intrusions.
In some locations coarse-grained pegmatite masses occur with granite. Granite is more common in continental crust than in oceanic crust, they are crystallized from felsic melts which are less dense than mafic rocks and thus tend to ascend toward the surface. In contrast, mafic rocks, either basalts or gabbros, once metamorphosed at eclogite facies, tend to sink into the mantle beneath the Moho. Granitoids have crystallized from felsic magmas that have compositions near a eutectic point. Magmas are composed of minerals in variable abundances. Traditionally, magmatic minerals are crystallized from the melts that have separated from their parental rocks and thus are evolved because of igneous differentiation. If a granite has a cooling process, it has the potential to form larger crystals. There are peritectic and residual minerals in granitic magmas. Peritectic minerals are generated through peritectic reactions, whereas residual minerals are inherited from parental rocks. In either case, magmas will evolve to the eutectic for crystallization upon cooling.
Anatectic melts are produced by peritectic reactions, but they are much less evolved than magmatic melts because they have not separated from their parental rocks. The composition of anatectic melts may change toward the magmatic melts through high-degree fractional crystallization. Fractional crystallisation serves to reduce a melt in iron, titanium and sodium, enrich the melt in potassium and silicon – alkali feldspar and quartz, are two of the defining constituents of granite; this process operates regardless of the origin of parental magmas to granites, regardless of their chemistry. The composition and origin of any magma that differentiates into granite leave certain petrological evidence as to what the granite's parental rock was; the final texture and composition of a granite are distinctive as to its parental rock. For instance, a granite, derived from partial melting of meta
Porphyry is a textural term for an igneous rock consisting of large-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate rich aphanitic matrix or groundmass. The larger crystals are called phenocrysts. In its non-geologic, traditional use, the term porphyry refers to the purple-red form of this stone, valued for its appearance; the term porphyry is from Ancient Greek and means "purple". Purple was the color of royalty, the "imperial porphyry" was a deep purple igneous rock with large crystals of plagioclase; some authors claimed. "Imperial" grade porphyry was thus prized for monuments and building projects in Imperial Rome and later. Porphyry has hardness 7 on the Mohs scale of mineral hardness, corresponding to steel and quartz. Subsequently, the name was given to any igneous rocks with large crystals; the adjective porphyritic now refers to a certain texture of igneous rock regardless of its chemical and mineralogical composition. Its chief characteristic is a large difference in size between the tiny matrix crystals and the much larger phenocrysts.
Porphyries may be aphanites or phanerites, that is, the groundmass may have invisibly small crystals as in basalt, or crystals distinguishable with the eye, as in granite. Most types of igneous rocks display some degree of porphyritic texture. Porphyry deposits are formed. In the first stage, the magma is cooled deep in the crust, creating the large crystal grains with a diameter of 2 mm or more. In the second and final stage, the magma is cooled at shallow depth or as it erupts from a volcano, creating small grains that are invisible to the unaided eye; the term porphyry is used for a mineral deposit called a "copper porphyry". The different stages of cooling that create porphyritic textures in intrusive and hypabyssal porphyritic rocks lead to a separation of dissolved metals into distinct zones; this process, which occurs when fluids are driven off the cooling magma, is one of the main reasons for the existence in the world of rich, localized metal ore deposits such as those of gold, molybdenum, tin, zinc and tungsten.
This enrichment occurs in the porphyry itself, or in other related igneous rocks or surrounding country rocks carbonate rock. Collectively, these type of deposits are known as "porphyry copper deposits". Rhomb porphyry is a volcanic rock with gray-white large porphyritic rhomb- shaped phenocrysts embedded in a fine-grained red-brown matrix; the composition of rhomb porphyry places it in the trachyte–latite classification of the QAPF diagram. Rhomb porphyry lavas are only known from three rift areas: the East African Rift, Mount Erebus near the Ross Sea in Antarctica, the Oslo graben in Norway, it is intrusive. Pliny's Natural History affirmed that the "Imperial Porphyry" had been discovered at an isolated site in Egypt in AD 18, by a Roman legionary named Caius Cominius Leugas. Ancient Egyptians used other decorative porphyritic stones of a close composition and appearance, but remained unaware of the presence of the Roman grade although it was located in their own country; this particular Imperial grade of porphyry came from a single quarry in the Eastern Desert of Egypt, from 600 million-year-old andesite of the Arabian-Nubian Shield.
The road from the quarry westward to Qena on the Nile, which Ptolemy put on his second-century map, was first described by Strabo, it is to this day known as the Via Porphyrites, the Porphyry Road, its track marked by the hydreumata, or watering wells that made it viable in this utterly dry landscape. Porphyry was extensively used in Byzantine imperial monuments, for example in Hagia Sophia and in the "Porphyra", the official delivery room for use of pregnant Empresses in the Great Palace of Constantinople. After the fourth century the quarry was lost to sight for many centuries; the scientific members of the French Expedition under Napoleon sought it in vain, it was only when the Eastern Desert was reopened for study under Muhammad Ali that the site was rediscovered by James Burton and John Gardiner Wilkinson in 1823. As early as 1850 BC on Crete in Minoan Knossos there were large column bases made of porphyry. All the porphyry columns in Rome, the red porphyry togas on busts of emperors, the porphyry panels in the revetment of the Pantheon, as well as the altars and vases and fountain basins reused in the Renaissance and dispersed as far as Kiev, all came from the one quarry at Mons Porpyritis, which seems to have been worked intermittently between 29 and 335 AD.
Porphyry was used for the blocks of the Column of Constantine in Istanbul. In countries where many cars have studded winter tires such as Sweden and Norway, it is common that highways are paved with asphalt made of porphyry aggregate to make the wearing course withstand the extreme wear from the spiked tires. List of rock textures – A list of rock textural and morphological terms Quartz-porphyry – A type of volcanic rock containing large porphyritic crystals of quartz Sarcophagi of Helena and Constantina Tyrian purple – Natural dye extracted from Murex sea snails Pictures of the Mons Porphyrites, Red Sea, Egypt. Rhomb porphyry lavas at the Wayback Machine Flash showing rhomb porphyry formation at the Wayback Machine