Volcano
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, gases to escape from a magma chamber below the surface. Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle. Therefore, on Earth, volcanoes are found where tectonic plates are diverging or converging, most are found underwater. For example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates whereas the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates. Volcanoes can form where there is stretching and thinning of the crust's plates, e.g. in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "plate hypothesis" volcanism. Volcanism away from plate boundaries has been explained as mantle plumes; these so-called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth.
Volcanoes are not created where two tectonic plates slide past one another. Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. One such hazard is that volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere. Volcanic winters have caused catastrophic famines; the word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan, the god of fire in Roman mythology. The study of volcanoes is sometimes spelled vulcanology. At the mid-oceanic ridges, two tectonic plates diverge from one another as new oceanic crust is formed by the cooling and solidifying of hot molten rock; because the crust is thin at these ridges due to the pull of the tectonic plates, the release of pressure leads to adiabatic expansion and the partial melting of the mantle, causing volcanism and creating new oceanic crust.
Most divergent plate boundaries are at the bottom of the oceans. Black smokers are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed. Subduction zones are places where two plates an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges, under the continental plate, forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, thus creating magma; this magma tends to be viscous because of its high silica content, so it does not attain the surface but cools and solidifies at depth. When it does reach the surface, however, a volcano is formed. Typical examples are the volcanoes in the Pacific Ring of Fire. Hotspots are volcanic areas believed to be formed by mantle plumes, which are hypothesized to be columns of hot material rising from the core-mantle boundary in a fixed space that causes large-volume melting.
Because tectonic plates move across them, each volcano becomes dormant and is re-formed as the plate advances over the postulated plume. The Hawaiian Islands are said to have been formed in such a manner; this theory, has been doubted. The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; the features of volcanoes are much more complicated and their structure and behavior depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus. Vents that issue volcanic material and gases can develop anywhere on the landform and may give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Hawaii's Kīlauea. Other types of volcano include cryovolcanoes on some moons of Jupiter and Neptune. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when the mud volcano is a vent of an igneous volcano.
Volcanic fissure vents are linear fractures through which lava emerges. Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent, they do not explode catastrophically. Since low-viscosity magma is low in silica, shield volcanoes are more common in oceanic than continental settings; the Hawaiian volcanic chain is a series of shield cones, they are common in Iceland, as well. Lava domes are built by slow eruptions of viscous lava, they are sometimes formed within the crater of a previous volcanic eruption, as in the case of Mount Saint Helen
Yosemite Valley
Yosemite Valley is a glacial valley in Yosemite National Park in the western Sierra Nevada mountains of Central California. The valley is about 7.5 miles long and 3000–3500 feet deep, surrounded by high granite summits such as Half Dome and El Capitan, densely forested with pines. The valley is drained by the Merced River, a multitude of streams and waterfalls flow into it, including Tenaya, Illilouette and Bridalveil Creeks. Yosemite Falls is the highest waterfall in North America, is a big attraction in the spring when the water flow is at its peak; the valley is renowned for its natural environment, is regarded as the centerpiece of Yosemite National Park, attracting visitors from around the world. The Valley is the main attraction in the park for the majority of visitors, a bustling hub of activity during tourist season in the summer months. Most visitors pass through the Tunnel View entrance. Visitor facilities are located in the center of the valley. There are both hiking trail loops that stay within the valley and trailheads that lead to higher elevations, all of which afford glimpses of the park's many scenic wonders.
Yosemite Valley is located on the western slope of the Sierra Nevada mountains, 150 miles due east of San Francisco. It stretches for 7.5 miles in a east–west direction, with an average width of about 1 mile. Yosemite Valley represents only one percent of the park area, but this is where most visitors arrive and stay. More than half a dozen creeks tumble from hanging valleys at the top of granite cliffs that can rise 3000–3500 feet above the valley floor, which itself is 4000 ft above sea level; these streams combine into the Merced River, which flows out from the western end of the valley, down the rest of its canyon to the San Joaquin Valley. The flat floor of Yosemite Valley holds both forest and large open meadows, which have views of the surrounding crests and waterfalls. Below is a description of these features, looking first at the walls above, moving west to east as a visitor does when entering the valley visiting the waterfalls and other water features, returning east to west with the flow of water.
The first view of Yosemite Valley many visitors see is the Tunnel View. So many paintings were made from a viewpoint nearby that the National Park Service named that viewpoint Artist Point; the view from the lower end of the Valley contains the great granite monolith El Capitan on the left, Cathedral Rocks on the right with Bridalveil Fall. Just past this spot the Valley widens with the Cathedral Spires the pointed obelisk of Sentinel Rock to the south. Across the Valley on the northern side are the Three Brothers, rising one above the other like gables built on the same angle – the highest crest is Eagle Peak, with the two below known as the Middle and Lower Brothers. To this point the Valley has been curving to the left. Now a grand curve back to the right begins, with Yosemite Falls on the north, followed by the Royal Arches, topped by North Dome. Opposite, to the south, is Glacier Point, 3,200 feet above the Valley floor. At this point the Valley splits in two, one section slanting northeast, with the other curving from south to southeast.
Between them, at the eastern end of the valley, is Half Dome, among the most prominent natural features in the Sierra Nevada. Above and to the northeast of Half Dome is Clouds Rest. Snow melting in the Sierra forms lakes. In the surrounding region, these creeks flow to the edge of the Valley to form cataracts and waterfalls. A fan of creeks and forks of the Merced River take drainage from the Sierra crest and combine at Merced Lake; the Merced flows down to the end of its canyon, where it begins what is called the Giant Staircase. The first drop is Nevada Fall. Below is one of the most picturesque waterfalls in the Valley; the Merced descends rapids to meet Illilouette Creek, which drops from the valley rim to form Illilouette Fall. They combine at the base of the gorges that contain each stream, flow around the Happy Isles to meet Tenaya Creek at the eastern end of Yosemite Valley proper. Tenaya Creek flows southwest from Tenaya Lake and down Tenaya Canyon flowing between Half Dome and North Dome before joining the Merced River.
The following falls tumble from the Valley rim to join it at various points: Yosemite Falls 2,425 feet Upper Yosemite Fall 1,430 feet, the middle cascades 670 feet, Lower Yosemite Fall 320 feet. Snow Creek Falls 2,140 feet Sentinel Falls 1,920 feet Ribbon Fall 1,612 feet Royal Arch Cascade 1,250 feet Lehamite Falls 1,180 feet Staircase Falls 1,020 feet Bridalveil Fall 620 feet. Nevada Fall 594 feet Silver Strand Falls 574 feet Vernal Fall 318 feet The features in Yosemite Valley are made of granitic rock emplaced as plutons miles deep during the late Cretaceous. Over time the Sierra Nevada was uplifted; the oldest of these granitic rocks, at 114 million years, occur along the Merced River Gorge west of the valley. The El Capitan pluton intruded the valley, forming most of the granitic rock that makes up much of the central part of the valley, including Cathedral Rocks, Three Brothers, El Capitan; the youngest Yosemite Valley pluton is the 87-million-year-old Half Dome granodiorite, which makes up most of the rock at
Buoyancy
Buoyancy or upthrust, is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid, thus the pressure at the bottom of a column of fluid is greater than at the top of the column. The pressure at the bottom of an object submerged in a fluid is greater than at the top of the object; the pressure difference results in a net upward force on the object. The magnitude of the force is proportional to the pressure difference, is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid. For this reason, an object whose average density is greater than that of the fluid in which it is submerged tends to sink. If the object is less dense than the liquid, the force can keep the object afloat; this can occur only in a non-inertial reference frame, which either has a gravitational field or is accelerating due to a force other than gravity defining a "downward" direction.
The center of buoyancy of an object is the centroid of the displaced volume of fluid. Archimedes' principle is named after Archimedes of Syracuse, who first discovered this law in 212 B. C. For objects and sunken, in gases as well as liquids, Archimedes' principle may be stated thus in terms of forces: Any object, wholly or immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object — with the clarifications that for a sunken object the volume of displaced fluid is the volume of the object, for a floating object on a liquid, the weight of the displaced liquid is the weight of the object. More tersely: buoyancy = weight of displaced fluid. Archimedes' principle does not consider the surface tension acting on the body, but this additional force modifies only the amount of fluid displaced and the spatial distribution of the displacement, so the principle that buoyancy = weight of displaced fluid remains valid; the weight of the displaced fluid is directly proportional to the volume of the displaced fluid.
In simple terms, the principle states that the buoyancy force on an object is equal to the weight of the fluid displaced by the object, or the density of the fluid multiplied by the submerged volume times the gravitational acceleration, g. Thus, among submerged objects with equal masses, objects with greater volume have greater buoyancy; this is known as upthrust. Suppose a rock's weight is measured as 10 newtons when suspended by a string in a vacuum with gravity acting upon it. Suppose that when the rock is lowered into water, it displaces water of weight 3 newtons; the force it exerts on the string from which it hangs would be 10 newtons minus the 3 newtons of buoyancy force: 10 − 3 = 7 newtons. Buoyancy reduces the apparent weight of objects that have sunk to the sea floor, it is easier to lift an object up through the water than it is to pull it out of the water. Assuming Archimedes' principle to be reformulated as follows, apparent immersed weight = weight − weight of displaced fluid inserted into the quotient of weights, expanded by the mutual volume density density of fluid = weight weight of displaced fluid, yields the formula below.
The density of the immersed object relative to the density of the fluid can be calculated without measuring any volumes.: density of object density of fluid = weight weight − apparent immersed weight Example: If you drop wood into water, buoyancy will keep it afloat. Example: A helium balloon in a moving car. During a period of increasing speed, the air mass inside the car moves in the direction opposite to the car's acceleration; the balloon is pulled this way. However, because the balloon is buoyant relative to the air, it ends up being pushed "out of the way", will drift in the same direction as the car's acceleration. If the car slows down, the same balloon will begin to drift backward. For the same reason, as the car goes round a curve, the balloon will drift towards the inside of the curve; the equation to calculate the pressure inside a fluid in equilibrium is: f + div σ = 0 where f is the force density exerted by some outer field on the fluid, σ is the Cauchy stress tensor. In this case the stress tensor is proportional to the identity tensor: σ i j = − p δ i j.
Here δij is the Kronecker delta. Using this the above equation becomes: f = ∇ p. Assuming the outer force field is conservative, it can be written as the negative gradient of some scalar valued function: f = − ∇
Weathering
Weathering is the breaking down of rocks and minerals as well as wood and artificial materials through contact with the Earth's atmosphere and biological organisms. Weathering occurs in situ, that is, in the same place, with little or no movement, thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, snow, wind and gravity and being transported and deposited in other locations. Two important classifications of weathering processes exist – physical and chemical weathering. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water and pressure; the second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals known as biological weathering in the breakdown of rocks and minerals. While physical weathering is accentuated in cold or dry environments, chemical reactions are most intense where the climate is wet and hot.
However, both types of weathering occur together, each tends to accelerate the other. For example, physical abrasion decreases the size of particles and therefore increases their surface area, making them more susceptible to chemical reactions; the various agents act in concert to convert primary minerals to secondary minerals and release plant nutrient elements in soluble forms. The materials left over after the rock breaks down combined with organic material creates soil; the mineral content of the soil is determined by the parent material. In addition, many of Earth's landforms and landscapes are the result of weathering processes combined with erosion and re-deposition. Physical weathering called mechanical weathering or disaggregation, is the class of processes that causes the disintegration of rocks without chemical change; the primary process in physical weathering is abrasion. However and physical weathering go hand in hand. Physical weathering can occur due to temperature, frost etc. For example, cracks exploited by physical weathering will increase the surface area exposed to chemical action, thus amplifying the rate of disintegration.
Abrasion by water and wind processes loaded with sediment can have tremendous cutting power, as is amply demonstrated by the gorges and valleys around the world. In glacial areas, huge moving ice masses embedded with soil and rock fragments grind down rocks in their path and carry away large volumes of material. Plant roots pry them apart, resulting in some disintegration. However, such biotic influences are of little importance in producing parent material when compared to the drastic physical effects of water, ice and temperature change. Thermal stress weathering, sometimes called insolation weathering, results from the expansion and contraction of rock, caused by temperature changes. For example, heating of rocks by sunlight or fires can cause expansion of their constituent minerals; as some minerals expand more than others, temperature changes set up differential stresses that cause the rock to crack apart. Because the outer surface of a rock is warmer or colder than the more protected inner portions, some rocks may weather by exfoliation – the peeling away of outer layers.
This process may be accelerated if ice forms in the surface cracks. When water freezes, it expands with a force of about 1465 Mg/m^2, disintegrating huge rock masses and dislodging mineral grains from smaller fragments. Thermal stress weathering comprises thermal shock and thermal fatigue. Thermal stress weathering is an important mechanism in deserts, where there is a large diurnal temperature range, hot in the day and cold at night; the repeated heating and cooling exerts stress on the outer layers of rocks, which can cause their outer layers to peel off in thin sheets. The process of peeling off is called exfoliation. Although temperature changes are the principal driver, moisture can enhance thermal expansion in rock. Forest fires and range fires are known to cause significant weathering of rocks and boulders exposed along the ground surface. Intense localized heat can expand a boulder; the thermal heat from wildfire can cause significant weathering of rocks and boulders, heat can expand a boulder and thermal shock can occur.
The differential expansion of a thermal gradient can be understood in terms of stress or of strain, equivalently. At some point, this stress can exceed the strength of the material. If nothing stops this crack from propagating through the material, it will result in the object's structure to fail. Frost weathering called ice wedging or cryofracturing, is the collective name for several processes where ice is present; these processes include frost frost-wedging and freeze -- thaw weathering. Severe frost shattering produces huge piles of rock fragments called scree which may be located at the foot of mountain areas or along slopes. Frost weathering is common in mountain areas where the temperature is around the freezing point of water. Certain frost-susceptible soils expand or heave upon freezing as a result of water migrating via capillary action to grow ice lenses nea
Erosion
In earth science, erosion is the action of surface processes that removes soil, rock, or dissolved material from one location on the Earth's crust, transports it to another location. This natural process is caused by the dynamic activity of erosive agents, that is, ice, air, plants and humans. In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion, snow erosion, wind erosion, zoogenic erosion, anthropogenic erosion; the particulate breakdown of rock or soil into clastic sediment is referred to as physical or mechanical erosion. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres. Natural rates of erosion are controlled by the action of geological weathering geomorphic drivers, such as rainfall; the rates at which such processes act control. Physical erosion proceeds fastest on steeply sloping surfaces, rates may be sensitive to some climatically-controlled properties including amounts of water supplied, wind speed, wave fetch, or atmospheric temperature.
Feedbacks are possible between rates of erosion and the amount of eroded material, carried by, for example, a river or glacier. Processes of erosion that produce sediment or solutes from a place contrast with those of deposition, which control the arrival and emplacement of material at a new location. While erosion is a natural process, human activities have increased by 10-40 times the rate at which erosion is occurring globally. At well-known agriculture sites such as the Appalachian Mountains, intensive farming practices have caused erosion up to 100x the speed of the natural rate of erosion in the region. Excessive erosion causes both "on-site" and "off-site" problems. On-site impacts include decreases in agricultural productivity and ecological collapse, both because of loss of the nutrient-rich upper soil layers. In some cases, the eventual end result is desertification. Off-site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment-related damage to roads and houses.
Water and wind erosion are the two primary causes of land degradation. Intensive agriculture, roads, anthropogenic climate change and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion. However, there are many prevention and remediation practices that can curtail or limit erosion of vulnerable soils. Rainfall, the surface runoff which may result from rainfall, produces four main types of soil erosion: splash erosion, sheet erosion, rill erosion, gully erosion. Splash erosion is seen as the first and least severe stage in the soil erosion process, followed by sheet erosion rill erosion and gully erosion. In splash erosion, the impact of a falling raindrop creates a small crater in the soil, ejecting soil particles; the distance these soil particles travel can be as much as 0.6 m vertically and 1.5 m horizontally on level ground. If the soil is saturated, or if the rainfall rate is greater than the rate at which water can infiltrate into the soil, surface runoff occurs.
If the runoff has sufficient flow energy, it will transport loosened soil particles down the slope. Sheet erosion is the transport of loosened soil particles by overland flow. Rill erosion refers to the development of small, ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes. Where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are of the order of a few centimetres or less and along-channel slopes may be quite steep; this means that rills exhibit hydraulic physics different from water flowing through the deeper, wider channels of streams and rivers. Gully erosion occurs when runoff water accumulates and flows in narrow channels during or after heavy rains or melting snow, removing soil to a considerable depth. Valley or stream erosion occurs with continued water flow along a linear feature; the erosion is both downward, deepening the valley, headward, extending the valley into the hillside, creating head cuts and steep banks.
In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V cross-section and the stream gradient is steep. When some base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain; the stream gradient becomes nearly flat, lateral deposition of sediments becomes important as the stream meanders across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone
Half Dome
Half Dome is a granite dome at the eastern end of Yosemite Valley in Yosemite National Park, California. It is a well-known rock formation in the park, named for its distinct shape. One side is a sheer face while the other three sides are smooth and round, making it appear like a dome cut in half; the granite crest rises more than 4,737 ft above the valley floor. The impression from the valley floor that this is a round dome that has lost its northwest half is an illusion. From Washburn Point, Half Dome can be seen as a thin ridge of rock, an arête, oriented northeast-southwest, with its southeast side as steep as its northwest side except for the top. Although the trend of this ridge, as well as that of Tenaya Canyon, is controlled by master joints, 80 percent of the northwest "half" of the original dome may well still be there; as late as the 1870s, Half Dome was described as "perfectly inaccessible" by Josiah Whitney of the California Geological Survey. The summit was reached by George G. Anderson in October 1875, via a route constructed by drilling and placing iron eyebolts into the smooth granite.
Today, Half Dome may now be ascended in several different ways. Thousands of hikers reach the top each year by following an 8.5 mi trail from the valley floor. After a rigorous 2 mi approach, including several hundred feet of granite stairs, the final pitch up the peak's steep but somewhat rounded east face is ascended with the aid of a pair of post-mounted braided steel cables constructed close to the Anderson route in 1919. Alternatively, over a dozen rock climbing routes lead from the valley up Half Dome's vertical northwest face; the first technical ascent was in 1957 via a route pioneered by Royal Robbins, Mike Sherrick, Jerry Gallwas, today known as the Regular Northwest Face. Their five-day epic was the first Grade VI climb in the United States, their route has now been free. Other technical routes ascend the west shoulder; the Half Dome Cable Route hike runs from the valley floor to the top of the dome in 8.2 mi, with 4,800 ft of elevation gain. The length and difficulty of the trail used to keep it less crowded than other park trails, but in recent years the trail traffic has grown to as many as 800 people a day.
The hike can be done from the valley floor in a single long day, but many people break it up by camping overnight in Little Yosemite Valley. The trail climbs past Vernal and Nevada Falls continues into Little Yosemite Valley north to the base of the northeast ridge of Half Dome itself; the final 400 ft ascent is steeply up the rock between two steel cables used as handholds. The cables are raised onto a series of metal poles in late May; the cables are taken down from the poles for the winter in early October, but they are still fixed to the rock surface and can be used. The National Park Service recommends against climbing the route when the cables are down and when the surface of the rock is wet and slippery; the Cable Route is rated class 3, while the same face away from the cables is rated class 5. The Cable Route can be crowded. In past years, as many as 1,000 hikers per day have sometimes climbed the dome on a summer weekend, about 50,000 hikers climb it every year. Since 2011, all hikers who intend to ascend the Cable Route must now obtain permits before entering the park.
Permits are checked by a ranger on the trail, no hikers without permits are allowed to hike beyond the base of the sub-dome or to the bottom of the cables. Hikers caught bypassing the rangers to visit either the sub-dome or main dome without a permit face fines of up to $5,000 and/or 6 months in jail. Backpackers with an appropriate wilderness permit can receive a Half Dome permit when they pick up their wilderness permit with no additional reservation required. Rock climbers who reach the top of Half Dome without entering the subdome area can descend on the Half Dome Trail without a permit; the top of Half Dome is a flat area where climbers can relax and enjoy their accomplishment. The summit offers views of the surrounding areas, including Little Yosemite Valley and the Valley Floor. A notable location to one side of Half Dome is the "Diving Board", where Ansel Adams took his photograph "Monolith, The Face of Half Dome" on April 10, 1927. Confused with "the Visor," a small overhanging ledge at the summit, the Diving Board is on the shoulder of Half Dome.
From 1919 when the cables were erected through 2011, there have been six fatal falls from the cables. The latest fatality occurred on May 21, 2018. Lightning strikes can be a risk while near the summit. On July 27, 1985, five hikers were struck by lightning; the Cable Route was added to the National Register of Historic Places in 2012. 1875 George G. Anderson via drilled spikes on the east slope. 1946 Salathe Route on southwest face, FA by John Salathe and Anton Nelson 1957 Northwest Face, FA by Royal Robbins, Jerry Gallwas and Mike Sherrick. First Grade VI in North America. 1963 Direct Northwest Face, FA by Royal Robbins and Dick McCracken 1969 Tis-sa-ack, FA by Royal Robbins and Don Peterson. 1973 First "clean ascent" of NW face by Dennis Hennek, Doug Robinson, Galen Rowell, Hennek is on the cover of June 1974 National Geographic leading a nut protected traverse see Super Topo too 1987 The Big Chill, FA by Jim Bridwell, Peter Mayfield, Sean Plunkett and Steve Bosque 1989 Shadows, FA by Jim Bridwell, Charles Row, Cito Kirkpatrick, William Westbay 1997 Blue Shift FA by Jay Smith and Karl McConachie.
1964 Salathe Route, FFA by Fran
Country rock (geology)
Country rock is a geological term meaning the rock native to an area, in which there is an intrusion of viscous geologic material magma, or rock salt or unconsolidated sediments. Magma is less dense than the rock it intrudes and filling existing cracks, sometimes melting the already-existing country rock; the term "country rock" is similar to, in many cases interchangeable with, the terms basement and wall rocks. Country rock can denote the widespread lithology of a region in relation to the rock, being discussed or observed. Settings in geology when the term country rock is used include: When describing a pluton or dike, the igneous rock can be described as intruding the surrounding country rock, the rock into which the pluton has intruded; when country rock is intruded by dyke, perpendicular to the bedding plane, it is called discordant intrusion, while a parallel intrusion by a sill indicates a sub-parallel or concordant intrusion. Most intrusions into country rock are via magma. Country rock is intruded by an igneous body of rock which formed when magma forced upward through fractures, or melted through overlying rock.
Magma cooled into solid rock, different from the surrounding country rock. Sometimes, a fragment of country rock will break off and become incorporated into the intrusion, is called a xenolith, from Greek, ξένος, xenos, "strange,", λίθος, the ancient Greek word for "stone." The heat of the intrusions changes the country rock to contact metamorphic rock. Hornfels is produced, or skarn; when describing recent alluvium, the material that has arrived through volcanic, glacial or fluvial action can be described as a veneer on the country rock
