The final exam of this class will be held on Friday,December 31,at 10AM.The exam will be held in our lecture room in Oriental Garden.

For the exam, I will select 10 of the following 25 questions. Most of the questions require short (1-2 paragraphs) essay type answers.

1,why do rocks of the Earth's crust have much higher 87Sr/86Sr ratios than rocks from the mantle?


87Sr is a radioactive isotope, and it can be created by the decay of 87Rb which behaves like K element(Rb是K的类质同像), so it can substitute K element in the common minerals, e.g. feldspar, mica. But 86Sr is a stable isotope which concentration is constant either in the earth crust or in the mantle.

And the rock of earth crust is intermediate-acidic rock(中酸性,富K), and it has much feldspar and mica, which enrich K element, so in these rocks there are also high concentration of Rb. But the rock from mantle is always ultramafic rock(超基性) which has low concentration of K element, so the concentration of Rb is also lower than the rock of crust.

More 87Rb can create more 87Sr, so rocks of the earth crust have much higher 87Sr/86Sr ratios than rocks from the mantle.


2,Mid-ocean ridge basalts and ocean island basalts both come from the oceanic mantle.Why are they so different in composition?


I think that their different tectonic environment cause the difference in their composition.

Firstly, the mid-ocean ridge basalt is formed generally in low pressure and high temperature conditions by highly partial melting formation. The mid-ocean ridge is ongoing extension at high speed and the mid-ocean ridge area is the earth magma frequent zone, so basalt source mantle often losses basaltic components,especially lherzolit(二辉橄榄岩) and 方辉橄榄岩(没查到单词). In addition, magma source area also losses incompatible components,and there is no enriched incompatible components granitic rocks.To sum up,the Mid-ocean ridge basalts losses K2O、TiO2 and incompatible components.

Secondly, the form process of ocean island basalts is not releated to any subduction structure.It’s formation is caused by the heat of mantle plume.So,the composition of ocean island basalts is samilar to the general oceanic mantle basalts. 

So,though mid-ocean ridge basalts and ocean island basalts both come from the oceanic mantle,they have much difference in their composition.


3,What is the difference between analytical accuracy and analytical precision  and how do we measure each of these?


 (Lecture2, slides9-12) Precision is the average error produced by repeated analyses, and it actually means the average deviation of the analyses to the average of the analyses done on a same sample. To put it simply, you conduct a few analyses of a same sample and get the average, then you calculate deviations of your analyses to the average by distracting it from each analysis and therefore the absolute values is obtained. The precision will be the average of all the deviations. Precision measures the stability of the machine. And it should be about 1%.True precision means the average deviation of a set of precisions you’ve gotten from different samples of a same rock. That’s to calculate precision out of precisions. For better understanding, we can refer to the Chinese version given below.

 precision又称平均偏差(average deviation)。


   Accuracy measures the degree to which the analysis represents the true composition of a rock. It is measured by repeated analyses of several standards prepared in the same way. And it should be no more than 1-2%.


4,what is the difference between tholeiitic and calc-alkaline basalts?How do the magma compositions of these two group change with crystal fractionation.



According to Alkali vs. Silica diagram, volcanic rocks can be divided into two distinct groups: alkaline and subalkaline.Then AFM diagram can further subdivide the subalkaline magma series into a tholeiitic and a calc-alkaline series like the picture above shows. Comparaed to Tholeiitic basalt, cal-alkaline basalt is richer in K2O Na2Oand Al2O3, and the content of CaO,FeOT and MgO is lower. Tholeiitic is poorer in alkali, especially for K2O, and is also lower in TiO2comparing with calc-alkaline basalt.

From the picture we can see that, we can see that the fractionation features of calc-alkaline series are :

①There is no dramatic Fe enrichment, probably related to the high water content of calc-alkaline magmas in arcs, dissolves → high fO2

②TiO2 decreases due to Fe-Ti oxide.

③The CaO/Al2O3 ratio decrease due to fractionation by cpx.

the fractionation features of tholeiitic series are :

There is a dramatic Fe enrichment, probably related to the low water content and low fO2, and then there is a crystallization of magnetite, so the content of Mg, Fe declines.


5,describe the difference between compatible and incompatible trace elements. What factors control whether an element is compatible or incompatible?


PPT: Lceture 3 7+8

incompatible elements are concentrated in the melt :(KD or D) « 1; incompatible elements are divided into two groups as highly charged high field strength (HFS) elements   and low field strength large ion lithophile (LIL) elements. Incompatible elements will be rich in melts.

compatible elements are concentrated in the solid:(KD or D) » 1. The concentrations of these elements will be low in melts and high in minerals.

The factors include the charge , radius and the field strength of the ion.(1)the less charges and the larger the radius ,the more incompatible ,like the LILEs,(2)The ones who have large ionic valences, like the HFSEs.


6,why are the rare earth elements so useful for classifying igneous rocks and for determining their chemical evolution?


The rare earth elements has the following two features:one is that rare earth elements are incompatible elements ,the other is that rare earth elements have a stable record of the igneous rocks’ the geochemical characteristics when forming.

First , magma forming process have an important effect on the composition of rare earth elements ,however the later geological events hardly affect the composition of rare earth elements .So rare earth elements have a stable record of the igneous rocks’ the geochemical characteristics when forming. Therefore they are useful for classifying igneous rocks.Secondly, the rare earth elements are incompatible elements, with magma chemical evoluting, the ratio of CLi/Csi(CLi  is the composition in magma, Csi is the composition in solid mineral)has a significant increase .So we can use the CLi/Csi to describe the process of magma chemical evolution.


7,what can igneous rock textures tell us about the rate of cooling of the magma and the sequence of mineral crystallization?



The textures of igneous rocks depend mainly on the rate of cooling of the magma from which the rocks are crystallized. Rapid cooling at the surface produces fine-grained laves whereas slower cooling at depth produces coarser-grained plutonic rocks. There are common not clear boundaries between the different textures and they grade into one another.

For example:

Quench texture—Rapid cooling produces a very fine-grained rock with tiny plagioclase lath and sheaves of poorly crystallized.

Cryptocrystalline texture—These rocks usually consist of devitrified glass in which one can not see any individual mineral. Fresh glass is isotropic under crossed polars but devitrified glass transmits some light.

Intersertal texture—Slightly coarser-grained texture in which plagioclase laths are clearly visible but the spaces between the laths are filled with glass or cryptocrystalline material.

Granular texture—Well crystallized mixture of minerals in coarse-grained rocks.

Cumulate texture—Large individual crystals with a regular or rounded shape, usually of olivine or pyroxene, are surrounded by other minerals, usually plagioclase.


8.Briefly describe the following terms: Eutectic, Liquidus, Solidus, Degree of Freedom in an igneous system.


此题答案可在岩石学书上P31 页找到具体答案,给大家参考参考。


Eutectic(共结点)三相共存,F=2-3+1=0; 为不变点,温度与组成都不能变,直到结晶结束,熔浆耗尽。[Three-phase coexist, F=2-3+1=0; Temperature and compositions must not be changed until crystallization end and magma exhausted.]


自由度 f=C+2-P     (C为独立组分数,P为相数,2-代表温度和压力) 

f=C+1-P   (当岩浆就位之后,压力固定,此时影响体系平衡的外因只有温度,则f=C+1-P)

Degree of Freedom(自由度)是指体系中的可变因素(如温度、压力、浓度)的数目。这些因素在一定范围内任意改变,不引起相的改变。【Degree of Freedom is the number of the system variables (such as temperature, pressure, concentration)】


Liquidus(液相线)液相线是指岩石熔融作用结束,即固体全部转换为液体那一瞬间的p T X条件,或者,反过来说,岩浆刚刚开始结晶作用的p T X条件。 见图

Solidus(固相线)岩石开始发生局部熔融的p T X(压力 温度 组分)条件,或者,指岩石结晶作用过程中残余岩浆最终消失之前一刹那的p T X条件。



  具体的  液相线  和  固相线  的定义,大家在考试时可以画类似左边的图或者画P32 页的图 指出共结点、液相线  和  固相线,在图中标出即可!





















9,what is exsolution in igneous minerals and which minerals are most likely to show exsolution? Are exsolution features common in vocanic rocks?if not, why not?


Solid solution can exist at high temperature but when the temperature goes down, the solid solution may separate into diffrerent components,we usually call this phenonenon exsolution.

Pyroxene and feldspar are most likely to show exsolution. Pyroxene shows cpx and opx perthitic texture while feldspar shows albite and K-feldspar perthitic texture.

This features are not commom in volcanic rocks.The reason is :whether exsolution can occur depends on the velocity of temperature decreasing.In volcanic rocks,temperature decreases quickly,and there is no enough time for the solid solution to divide completely, so exsolution features are not common in volcanic rocks.


10what are tectonic discrimination diagrams for volcanic rocks? Are they reliable for identifying the source of igneous rocks?



plotting  some data such as the trace element to make a discrimination diagram can distinguish different geological settings and  reflect the origin of magma. For example, Zr-Ti/100-Y, Ti/40-Si/100-Sr,Nb*25-Na/100-Sr,TiO2-MnO*10-P2O5*10, these triangular diagrams can be used to infer tectonic setting of ancient (meta)volcanic. Diagrams such as Rb/(Y+Nb),Yb/Y can be used distinguish the granitoid tectonic discrimination. Diagrams such as Ti-Zr, Zr/Y vs Zr, Y vs Cr are also can be used. they are not reliable for all igneous rocks, but it can be useful for most igneous rocks, and also it is not reliable by using the discrimination diagram alone ,as said above .


11what is the difference between congruent and incongruent melting? which is most common in igneous systems?


The difference between congruent melting and incongruent melting is congruent melting occurs as a compound melts,the composition of the liquid it forms is the same as the composition of the solid,while incongruent melting occurs when a substance does not melt uniformly and decomposes into another substance. Incongruent melting is more common in igneous systems.


12what can cause fractionation of stable isotopes,such as those of oxygen?why are oxygen isotopes so useful in geology.


L5,P5,(它幻灯片上说的是,chemical fractionation is impossible while mass fractionation is the only type possible,我不认同,我按照网上资料答的,大家参考吧)另可参考地球化学P246There are three types of isotope fractionation:1,Equilibrium isotope fractionations the partial separation of isotopes between two or more substances in chemical equilibrium. An example of equilibrium isotope fractionation is the        concentration of heavy isotopes of oxygenin liquid water, relative to water vapor, At 20oC, the equilibrium fractionation factor for this reaction is  2,Kinetic fractionation’s a process that separates stable isotopes from each other by their mass during unidirectional processes .One naturally occurring example of kinetic fractionation is the evaporation of seawater to form clouds. In this instance, isotopic ally lighter water molecules (i.e., those with 16O)will evaporate slightly more easily than will the isotopically heavier water molecules with 18O.3,Mass-independent fractionation: Most isotopic fractionations(includingtypicalkineticfractionationsandequilibriumfractionations) are caused by the effects of the mass of an isotope on atomic or molecular velocities, diffusivities or bond strengths. It can be observed in the he three-body ozone formation reaction. Oxygen is very common in nature, and it is important component for most rock-forming minerals. It is also an important tracing element in petrogenetic and mineralizing process. The mineralizing of oxygen isotopes in hydrosphere and atmosphere



13,How is zircon used for isotopic dating and why is it so valubale.


As zircon enrichs radioactive eliments, such as, U and Th, we use the U-Pb-Th System to doing the isotopic dating job. It is a very complex system which involved 3 radioactive isotopes of U: 234U, 235U, 238U and 3 radiogenic isotopes of Pb:206Pb,207Pb, and208Pb and 1 radioactive isotope of Th: 232Th. Only 204Pb is strictly non-radiogenic. U, Th, and Pb are incompatible elements, they are concentrate in early melts.Isotopic composition of Pb in rocks = function of    238U®234U®206Pb  (l = 1.5512 x 10-10a-1)    235U®207Pb       (l = 9.8485 x 10-10a-1)  232Th®208Pb            (l = 4.9475 x 10-11a-1)So we can the following equation:206Pb*/238U=eλ1t-1207Pb*/235U=eλ2t-1 We can use some testing methods such as,LA-ICP-MS ,SHRIMP  to get the value of206Pb*/238Uand207Pb*/235U.By using different “t” value, we can get a concordia which represents the true simultaneous co-evolution of 206Pb and 207Pb via:238U®234U®206Pb;235U®207Pb. However after the zircon forms, there must be some event which break the encapsulation of the system and make Pb lose. So the date we plot may be below the concordia, but they must be linear. This line and the Concordia can form two point of intersection.The upper one represents the formation time of the zircon, the rest one represent the time of s Pb loss.  Becaue the the U-Pb-Th System have two important requests:1the encapsulation is good 2 the initial value of isotopic ratio must be reasonable The zircon have high tempreture of encapsulation, and it is difficult to be affected by the lateral event. The initial value of206Pb/204Pb,207Pb/204Pb,and208Pb/204Pb can be regard as 0.And also the Instrument accuracy requirements have been met. So it is valuable.

14,explain how one can obtain the initial 87Sr/86Sr value for a rock using a Rb-Sr isochron.



we need 3 or more cogenetic samples with a range of Rb/Sr, these samples could be cogenetic rocks derived from the same magma or minerals with different K/Ca ratios in a single rock.

Then, we can get recast age by dividing through by stable 86Sr, that is

87Sr/86Sr = (87Sr/86Sr)o + (87Rb/86Sr)(eλt -1)   (1)

For values of λt less than 0.1, so eλt-1 @λt

Thus eq. 87Sr/86Sr = (87Sr/86Sr)o + (87Rb/86Sr)(eλt -1) for t<70Ga reduces to :

87Sr/86Sr = (87Sr/86Sr)0 + (87Rb/86Sr) λt       (2)

We suppose 87Sr/86Sr as y, (87Sr/86Sr)as b, 87Rb/86Sr as an arbitrary value, x, λt as m. So, the formula (2) can be replaced by equation y=b+xm, then we can get a line in 87Sr/86Sr vs. 87Rb/86Sr.

We should begin with 3 rocks or minerals plotting a b c at time t, after that, we can obtain a line like the one described in Fig.1




We know the samples have the ration begin with according to Fig.1


As the time gose on, 87Rb decays,then we get another line at t, which is described in Fig.2



And the sample has more 87Rb start with would have more 87Sr, that means the higher 87Sr/86Sr ratio, just like the Cpoint.

Then we can get the age by the slope of the line at t1.besides, if we prolongate the line down to the original point, we can get the initial 87Sr/86Sr value.


15, Briefly describe the plume theory for the formation of hotspots and large igneous provinces. Is there an alternate explanation for these features?


A mantle plume is an upwelling of abnormally hot rock within the Earth's mantle. As the heads of mantle plumes can partly melt when they reach shallow depths, they are often invoked as the cause of volcanic centers known as hotspots and their related continental flood basalts, which form the LIPs.

Other ways may explain the features, for examples the delamination and sinking of large portions of lower continental crust (assumed to have transformed into the dense rock eclogite), which allows the influx of asthenosphere from the low velocity zone and subsequently melts to form continental flood basalts.


16,Cumulate layering is very common in large mafic intrusions. Briefly describe how such layering may from and which minerals would most common in such layers. What is the difference between adcumulate and orthocumulate texture?



The cumulates rocks are formed of crystal concentrates,and the layering is formed when the mafic magma crystallizes under the form of gravity at low pressure.
Chromite, olivine, pyroxene and plagioclase are most common in such layers。 
In Orthocumulates texture the cumulus crystals are enclosed in material that has crystallised from the interstitial melt。
In Adcumulates texture the cumulus crystals continue to grow and displace the intercumulus liquid。


17 why are island arc volvanoes so dangerous?what are the most dangerous types of eruptions?


Island arc volcanism is characterized by activity along arcuate volcanic (island) chains that parallel subduction zones. Formed by explosive eruptions, island arc volcanoes are dominated by fluids with higher viscosity and high gas content. Since island arc volcanoes are composed of loose pyroclastic material and minor lava flows, all these products are dangerous to people. Pyroclastic falls (mainly ash fall) can cover large areas around the volcanoes. And pyroclastic flows are fast moving, high density flow of hot ash, crystals, blocks and pumice following topographic lows. They can be hot enough after deposition to weld fuse vitric fragments. So they can kill all the living things and destroy all the buildings within its coverage. What’s more, they release a lot of poisonous gases that are deadly to people. Sometimes, the volcanism can last for a long time thus resulting in permanent destruction.

Explosive eruptions are the most dangerous types of eruptions. Because of the productions of these types of volcanoes are dangerous and the processes of eruptions are fast, they can cause a tremendous loss of lives and properties of people in affected areas.

Reference:PPT -Lecture 10 subduction related volcanism



18,Both island arc and mid-ocean ridge magmas apparently come from the mantle yet they have different compositions. Briefly discuss these differences and explain some of the factors that are responsible for the differences.


There’re three different MORBs: N-MORBs tap an upper, depleted mantle, E-MORBs tap a deeper enriched source, T-MORBs are mixing of N- and E- magmas during ascent and/or in shallow chambers. MORBs are relativly rich in Al2O3 and Cr and short of LIL elements and LREE, and they have a plat REE pattern. There’s no effort of oceanic crust matirials. Trace element and isotopic characteristics of melt reflect equilibrium distribution between melt and source reservoir . The major element (and hence mineralogical) character is controlled by equilibrium between melt and residual mantle during rise until melt separates as a system with its own distinct character (shallow). The chemical compostition of OIBs can vary from tholeiite to alkali basalt within the same island group, but is never calc-alkaline and tholeiite is the dominant type. The LIL trace elements are incompatible and are all enriched in OIB magmas with respect to MORBs. HFS elements (Th, U, Ce, Zr, Hf, Nb, Ta, and Ti) are also incompatible, and are enriched in OIBs > MORBs. But OIBs are too enriched for any known mantle process that they must correspond to crustal rocks and/or sediments. EMI is Slightly enriched,in which Deeper continental crust or oceanic crust may participate. EMII is More enriched,Specially in 87Sr (Rb parent) and Pb (U/Th parents) because of adding in of the upper continental crust or ocean-island crust .HIMU has very high 206Pb/204Pb ratio, indicating the existing of subducted and recycled oceanic crust (± seawater) probably. To remain isotopically distinct,it could not have rehomogenized or re-equilibrated with rest of mantle.


19 the lavas from island arcs and continental arcs have many similarities but also some differences. What are the main differences and why do they occur.





1 Dry peridotite solidus too high for melting of anhydrous mantle to occur anywhere in thermal regime – H2O must be involved in process.

2 LIL/HFS ratios of arc magmas imply water plays significant role in arc magmatism.

3 LIL/HFS trace element data underscore the importance of slab-derived water and a MORB-like mantle wedge source.

4 The flat HREE pattern argues against garnet-bearing (eclogite or garnet peridotite) source

5 Thus modern opinion has swung toward non-melted slab for most cases.



Petrogenesis of the Andes

1. Major and trace element data consistent with origin from fluid-fluxed and LIL-enriched mantle wedge above subducting and dehydrating Nazca plate.

2. Each of volcanic zones indicate magmas interact with local continental crust as they pass through crust. However, initial melts must come from mantle wedge.

Continental arc activity Potential differences with respect to Island Arcs:

1. Thick sialic crust contrasts greatly with mantle-derived partial melts - and is likely source of more pronounced effects of contamination.

2. Low density of crust may retard ascent, and produce stagnation of magmas and more potential for differentiation.

3. Low melting point of crust allows for partial melting and crustally derived melts.

4. Subcontinental lithospheric mantle is likely to be locally enriched, and this will be reflected in magmas that are derived from this mantle.



20How do the phenocryst assemblages in mid-ocean ridge lavas, ocean island lavas and island arc lavas differ and what accounts for these differences?


island arc basalt has largely Pl and Cpx/Opx as phenocryst, MORB are mainly Pl and Ol. OIB are Cpx、Ol. 三种岩浆不同的原因主要为:①不同成分的上地幔,在部分熔融时由于体系温度、压力和熔融程度的不同,形成不同成分的原生玄武岩浆;②原生玄武岩浆在向地表运移的过程中,成分发生改变(比如经过地壳的混染作用)产生一系列的进化岩浆。关于区别的原因老师似乎没讲,所以是自己查资料看的,只能做个参考了。底下的是自己翻译的可能不是很准确。The main reasons for the difference:1、different composition of upper mantle, when taking the part melting ,as the T、P and the degree of melting are different, so  form the different primary basalt magma.2、With the progress of primary basalt magma going up, the composition changed, such as experience assimilation and so on. it can form a series of magma.


21 what are the four main types of granitic rocks.Briefly discuss how they differ in composition and isotopic character




Chappell and White  (SIAM Classification)

– (M)antle type; (I)gneous type; (S)edimentary type; (A)norogenic type


A-type granites are a diverse group of felsic rocks typically created in anorogenic areas and having diverse sources and origins。

Whole-rock silica is generally from 60 to 77 wt.%. They have moderately high total alkalies; high K/Na,(K+Na)/Al, Fe/Mg, Ga/Al; halogens (F,Cl); high-field-strength (HFS) cations (Zr,Nb,Ta,Zn); and REEs but low Cr, Ni, and Sr. High concentrations of incompatible elements form abundant accessory minerals, including zircon(in wholly crystalline rocks) and, in biotite granites, allanite. Cassiterite is found in some rocks. The δ18O and Sr87/Sr86 is various.

I-type granites are believed to have been derived from magmas generated by partial melting of mafic and intermediate igneous rocks。It typically created in subduction zone Infracrustal Mafic to intermed igneous source.

Whole-rock silica is generally from 53 to 76 wt.%. These mostly metaluminous granites contain more Na and Ca than S-type granites. They have high Ca and Sr in mafic rocks. They have low K2O/Na2O ,low Cr, Ni; They have moderately high Fe3+/Fe2+, when it comes to istope,theδ18O<9‰and Sr87/Sr86<0.705.

S-type granites were derived from magmas generated by par-tial melting of sedimentary source rock composed in part of metamorphosed clay minerals.

Whole-rock silica is generally from 65 to 74 wt.%.  Typically created in subduction zone and having suracrustal sedimentary source. They have high K2O/Na2O,high Cr,Ni,Rb,Th,U. The δ18O >9‰ and Sr87/Sr86 >0.707‰.

M-type granites are generated by magma from mantle and crust, typically created in subduction zone or ocean-intraplate Mantle-derived .

Whole-rock silica is generally from 46 to 70 wt.%. They have low K2O/Na2O, Fe3+/Fe2+,Cr,Ni, Rb, Th,U,HFS; but they have high Ca and Sr. Theδ18O<9‰and Sr87/Sr86<0.705.


22Highly alkaline rocks occur in two tectonic environments on Earth. What are these environments and what is the likely source of the alkaline magmas?


Highly alkaline rocks occur in almost all of the tectonic environments and mainly in continental rifts and the edge of the plates such as the arc environment. The alkaline magma may come from the melting fractionation of the ordinary basaltic magma after most of the Si rich compositions have crystallized. And also by partial melting of ultramafic rocks, it will form alkaline magma. Another sauce of alkaline magma is from the mixing or assimilation of the ordinary magma with the highly Na&K rich rocks in earth crust.


23 what is a carbonatite and how can such a rock from?


Carbonatite is a kind of igneous rock that mainly contains the carbonate minerals and has a close relationship with the alkaline ultrabasic complex with its placement and formation.It may be crystallized 1)from the ultrabasic magma or 2)from the high CO2 rich hydrotherm which is from the alkaline ultrabasic magma and then metasomatized the alkaline rock or ultrabasic rock or 3)after the liquid immiscibility of the original homogeneous silicate melt that has the carbonate composition already.

24 what are mafic enclaves in granitic rocks and how might you determine their origin?


The mafic enclaves in the granitic rocks, usually shaped rounded, ellipse, and takes on dark color for the abundant mafic minerals ,which are different from the host rocks. Sometimes the granite minerals may appear in the enclaves. mafic enclaves have been recognized as evidence of mixing between two at least partially molten magmas.

(1)   we can analyze the chemical composition and compare it with other several reservoirs, like the deep crustal, the upper mantle to determine the potential origins.

(2)   Experiment is also a good way ,via the experiment, we can imitate the pressure and the temperature and other parameters to constrain the source conditions, like the depth of the origin.

(3)   We can distinguish the origin through the texture of the enclave,and from the minerals mineral paragenesis


25 Briefly describe the lines of evidence we use to determine the structure and composition of the Earth`s mantle.


As we know the average density and seismic velocity of each layer, but experimental studies tell us that only a few rocks can have these densities and velocities. And also, the mineralogies and chemical compositions of these few rocks are known. Experimental studies show that mantle magmas must be formed by melting of such rocks. So these natural phenomenon and some experimental studies can help us determine the composition of the Earth’s mantle. Exposures of upper mantle rocks provide direct evidence of composition. Xenoliths in volcanic rocks provide evidence of deeper mantle rocks.

The structure of the mantle can be known through the geothermal method and geophysical method such as the seismic method