The rock cycle is a web of processes that outlines how each of the three major rock types—igneous, metamorphic, and sedimentary—form and break down based on the different applications of heat and pressure over time. For example, sedimentary rock shale becomes slate when heat and pressure are added. The more heat and pressure you add, the further the rock metamorphoses until it becomes gneiss.
If it is heated further, the rock will melt completely and reform as an igneous rock. Empower your students to learn about the rock cycle with this collection of resources. According to the United States Geologic Survey, there are approximately 1, potentially active volcanoes worldwide. Most are located around the Pacific Ocean in what is commonly called the Ring of Fire. A volcano is defined as an opening in the Earth's crust through which lava, ash, and gases erupt.
The term also includes the cone-shaped landform built by repeated eruptions over time. Teach your students about volcanoes with this collection of engaging material. The structure of the earth is divided into four major components: the crust, the mantle, the outer core, and the inner core. Each layer has a unique chemical composition, physical state, and can impact life on Earth's surface. Movement in the mantle caused by variations in heat from the core, cause the plates to shift, which can cause earthquakes and volcanic eruptions.
These natural hazards then change our landscape, and in some cases, threaten lives and property. Learn more about how the earth is constructed with these classroom resources. Igneous rocks are one of three main types of rocks along with sedimentary and metamorphic , and they include both intrusive and extrusive rocks.
Join our community of educators and receive the latest information on National Geographic's resources for you and your students.
Skip to content. Twitter Facebook Pinterest Google Classroom. Article Vocabulary. Friday, October 31, Magma is a molten and semi-molten rock mixture found under the surface of the Earth. This mixture is usually made up of four parts: a hot liquid base, called the melt ; mineral s crystal lized by the melt; solid rock s incorporate d into the melt from the surrounding confine s; and dissolve d gas es.
When magma is eject ed by a volcano or other vent , the material is called lava. Magma that has cooled into a solid is called igneous rock.
This heat makes magma a very fluid and dynamic substance, able to create new landform s and engage physical and chemical transform ations in a variety of different environment s. Earth is divided into three general layers. The core is the superheated center, the mantle is the thick, middle layer, and the crust is the top layer on which we live.
Most of the mantle and crust are solid, so the presence of magma is crucial to understanding the geology and morphology of the mantle. Differences in temperature , pressure , and structural formations in the mantle and crust cause magma to form in different ways. Decompression melting involves the upward movement of Earth's mostly-solid mantle. This hot material rises to an area of lower pressure through the process of convection.
Areas of lower pressure always have a lower melting point than areas of high pressure. This reduction in overlying pressure, or decompression, enables the mantle rock to melt and form magma. Decompression melting often occurs at divergent boundaries, where tectonic plate s separate.
The feeder-dyke thicknesses range from 0. Using these and the above values, Eq. On comparison with the depths of the earthquake foci associated with the pressure change in the assumed main magma chamber that supplied magma during the — El Hierro eruption, the feeder-dykes have generally a very similar source depth as the magma chamber Fig. Vertical cross-sections through the island of El Hierro oriented parallel A and perpendicular B to the seismic swarm, indicted by hypocentres see Fig.
Also shown are the six feeder-dykes D1—D6 , as red lines and their calculated depths of origin depths to the source magma chamber as red dots. We divide the earthquakes into three main groups according to depth of foci Figs. These earthquakes are mostly related to magma movement in connection with the eruption itself; many occur at the eruption site Fig.
The shallow chamber is most likely partially molten 4 , so that some of the earthquakes may occur inside the crystalline matrix of the chamber itself. This follows because at the high strain rates associating with earthquake-fracture propagation, part of the crystalline matrix may behave as brittle.
Most of the earthquakes, however, are likely to occur in the host rock in response to magma-pressure changes in the chamber. The pressure-induced stresses fall off with distance from the chamber - for example, with the cube and square of the distance from spherical and cylindrical chambers, respectively Thus, much of the earthquake activity is likely to be comparatively close to the magma-pressure source, that is, the fluid part of the chamber. Based on this and the density of the earthquake foci Fig.
On this assumption, the depth of origin of most of the feeder-dykes would coincide with the estimated depth of the roof of the presently active shallow magma chamber beneath El Hierro Fig. This is also roughly the depth of Moho beneath El Hierro 18 , 19 , a major discontinuity likely to encourage sill emplacement and magma-chamber formation 4.
The feeder-dykes have a radial arrangement with respect to the location of the main clouds of earthquake foci which we interpret as being approximately the locations of the presently active shallow and deep magma chambers Fig. Theoretically, all the feeders could therefore have been injected from the present magma chambers.
These estimates offer new constraints on the depths of the magma chambers and can be compared with other estimates, such as from the inversion of geodetic data, earthquake foci and geochemical and petrological studies. With modern GPS and InSAR techniques, the opening displacements and lengths of volcanic fissures can generally be determined quite accurately 1 , 2.
Where feeder-dykes can be measured and dated in an active volcano, it is also possible to detect those major long-term changes that may have occurred in the location of the source magma chamber.
This offers the possibility of using the present method to trace the spatial evolution of the source magma chamber of an active volcano. Geodetic and seismic methods for determining the locations of magma chambers in volcanoes are very recent. The depths to magma chambers obtained from the length-opening ratios of volcanic fissures using geodetic measurements can thus trace the spatial evolution of the magma chamber for, at most, several decades.
That time window is normally far too short to show any significant changes in the location of the chamber. However, for many well-monitored volcanoes that have had frequent fissure eruptions in this time window, such as Etna in Italy 21 , 22 and Piton de la Fournaise in Reunion 25 , the present method makes it possible to estimate the depths to the source chambers. In particular, many volcanoes such as in El Hierro are apparently supplied with magma from a double magma chamber, that is, a small shallow chamber that, in turn, is fed by a larger, deeper reservoir Careful analysis of the location of the volcanic fissures and the composition of the eruptive materials issued should then, in combination with the depth estimates from the aspect ratios of the fissures, allow us to distinguish between the two magma sources.
The present method is currently the only one that makes it possible to trace the long-term spatial evolution of magma chambers in volcanoes. This follows because the method can not only be applied to volcanic fissures, but also to feeder-dykes. Feeder-dykes record the opening and the length of the associated volcanic fissure. Measurements of the thickness and length of a feeder-dyke make it possible, using the present method, to calculate the depth to its source chamber. The present estimates of the depths to the source chambers of the — eruption in El Hierro, based on the feeder-dyke data and the seismic data, are in agreement with new petrological results 27 , 28 , These results indicate that two interconnected mafic magma chambers were active during the eruption.
As indicated above, feeder-dykes have not been widely reported from active volcanoes. The present results, where six clear feeder-dykes have been identified in a comparatively small island, indicate that systematic search for feeder-dykes could result in many more being observed. This conclusion is supported by recent studies of dykes in well-exposed caldera walls where many feeder-dykes have been observed 7 , 8 , These dykes can be connected to volcanic fissures at the surface, making it possible to calculate their aspect ratios and infer the depth of origin of the magma.
Similarly, systematic studies of feeder-dykes in other volcanoes should make it possible to trace the evolution of the associated magma chambers over similar or longer periods, that is, over significant parts of, or the entire history of, the associated volcanoes. The theoretical methods used here derive from, first, elastic fracture mechanics and, second, overpressure estimates for magma-filled fractures.
The equation applies to mode I through cracks, such as feeder-dykes must be, that is, extension fractures that extend from one free surface here a fluid-filled magma chamber to another free surface here the earth's surface. The overpressure estimates Eq. The density of basaltic magma - the main focus here - is roughly constant in a feeder-dyke until at a shallow depth. Gas expansion in the uppermost few hundred metres of the feeder-dyke may lower the magma density considerably 34 , but the overall average density for many basaltic magmas is similar to that used in the present calculations The crustal density is also taken as a constant, that is, an average crustal density is used.
Crustal density varies with depth - it increases on average. In the present case, there is insufficient data on the crustal layers to take the variation in crustal density into account. This is possible for volcanic areas where more detailed mechanical data on the crustal layers is available.
In most case and in the absence of drilling, the densities of the crustal layers are generalised values for thick units rather than individual layers. In the present calculations, as is common, the average density of all these units combined has been estimated and used to calculate the buoyancy effect on the magma overpressure at the point where the feeder-dyke reaches the surface.
The excess pressure p e in the source magma chamber must be similar to the in-situ tensile strength of the host rock at the time of its rupture and dyke formation. General theoretical stress analyses sometimes suggest that the chamber could hold magma under excess pressure exceeding the tensile strength - such as in the case of an ideal spherical chamber However, all chambers are, in detail, somewhat irregular in geometry 4 and the rupture and dyke injection occur at those locations at the chamber boundary where notches and other irregularities raise concentrate the tensile stress until it reaches the local in situ tensile strength.
The in situ tensile strength is generally low: most measurements range between 0. Healing and sealing of fractures, partly through geothermal activity, is more common close to the magma chamber than at the surface. Thus, we use a somewhat higher tensile strength for the host rock of the magma chamber 2. The data on the six feeder-dykes in this paper are, as regards thickness, based on direct field measurements.
These are well-exposed dykes so that the thickness can be measured accurately. The lengths of the feeder-dykes, or rather the associated volcanic fissures, are obtained from direct measurements on aerial images. Dzurisin, D. Volcano deformation: new geodetic monitoring techniques Springer Verlag, Berlin, Segall, P. Earthquake and volcano deformation Princeton University Press, Princeton, Sturkell, E.
Volcano geodesy and magma dynamics in Iceland. Fractional crystallization is thought to be one way of producing rocks of different compositions from the same magma. Partial melting and magma contamination are also important. If a rock is not exposed to a high enough temperature to melt all of its minerals, only some minerals will melt.
This is known as partial melting. If a rock melts only partially, the magma produced will have a different chemical composition than the rock from which the magma originated.
As magma rises toward the earth's surface it may also cause rocks in the overlying crust to partially melt, contaminating the magma with molten rock of a different composition. The composition of magma therefore depends on many factors, including original magma composition resulting from partial melting, fractional crystallization, and magma contamination. It ranges in temperature from about o C to o C o F to o F.
Andesitic magma has moderate amounts of these minerals, with a temperature range from about o C to o C o F to o F. Rhyolitic magma is high in potassium and sodium but low in iron, magnesium, and calcium.
It occurs in the temperature range of about o C to o C o F to o F. Both the temperature and mineral content of magma affect how easily it flows. The viscosity thickness of the magma that erupts from a volcano affects the shape of the volcano.
Volcanoes with steep slopes tend to form from very viscous magma, while flatter volcanoes form from magma that flows easily. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
The Rights Holder for media is the person or group credited. Tyson Brown, National Geographic Society. National Geographic Society. For information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher.
0コメント