The fall of volcanic material from Mount Lawu, Central Java: a case study of magnetic susceptibility

Mount Lawu was recorded to have erupted last on November 28, 1885, releasing volcanic material including magnetic minerals. The material settles in various places carried by the wind and river flow. The movement of wind and river flow is very influential in the process of transporting volcanic material. Large material directly falls around the mountain, usually in the form of large rocks, and undergoes physical and chemical weathering. Small-sized materials such as volcanic ash and small-mass rocks will be carried by the wind and river flows in all directions. However, the displacement does not remove the magnetic minerals contained in it. Magnetic minerals can be evaluated by the method of rock magnetism using MS. Bartington MS2 device with a dependent MS2B sensor by mass. From this tool, the low field susceptibility values in the range of 1467.5 x 10−8 m3/Kg – 5262.1 x 10−8 m3/Kg and high field susceptibility in the range of 1467,1 x 10−8 m3/Kg. The volcanic material of Mount Lawu belongs to the Ilmenite (FeTiO3) group and belongs to the group of weak magnetism. Then further review the frequency-dependent susceptibility (%) to see the magnetic domains contained in the sample. From the magnetic susceptibility data, the percentage of frequency-dependent susceptibility (%) is less than 2%. From these data, the sample is not included in the superparamagnetic (SP) and in general the sample is classified as a multidomain item (MD).


Introduction
Mount Lawu (ML) is one of the mountains located in Central and East Java.ML erupted on November 28, 1885, [1].The mountain emitted various materials.The material can be volcanic ash, lava, lava and many more [2].The material comes out of the bowels of the earth and then settles in various places.The place can be in the form of plantations, rivers, or lakes around the mountain.Usually, the material around the center of the eruption is found in very large rock sizes [3][4][5].Due to the very large mass, the rocks immediately fell and could not be carried by the wind and river flow.Likewise with the conditions found in locations far from the point of eruption.In theory, the farther from the eruption distance, the smaller the sample size, and vice versa [6].
The material released by the mountain contains minerals stored in the core.These minerals are called magnetic minerals.Because the material contains metals.The minerals contained in it can be minerals with weak to strong magnetism [7][8][9].This is the initial indication for researchers to study the magnetic minerals of Mount Lawu.Magnetic minerals can be Magnetite (Fe 3 O 4 ), Hematite (αFe2O3), Maghemite (γFe 2 O 3 ), Ilmenite (FeTiO 3 ), Pyrite (FeS 2 ), Pyrrhotite (Fe 7 S 8 ), and Goethite (αFeOOH) [2], [10].While the elements contained in volcanic material are mostly Fe and Si [4], [11], [12].Each mountain has a different character from one mountain to another.This is an indication for the two researchers to study Mount Lawu.
At this time the researchers wanted to examine Mount Lawu in the context of the magnetic minerals contained in the Mount Lawu material using MS data and refined using X-Ray Fluorescence analysis.Researchers used several kinds of samples with different categories and sizes from one another.The sample taken is in the form of rocks that cannot be moved by people.Because the easiest way to find the remaining material from the eruption of Mount Lawu is in the form of rocks along the path of the eruption of Mount Lawu.Researchers are more focused on the river that flows from Mount Lawu to all places.Because the eruption of Mount Lawu is not so big as the ancient Mount Maninjau [2], [13], Mount Toba [14][15][16][17] and Mount Krakatau [18][19][20].So, the eruption is classified into the type of pelee and Hawaiian.This type is shown from the geological map of Mount Lawu.The results of the eruption are not spread far from the center of the eruption.In contrast to the results of the eruption of Mount Maninjau, Mount Toba and Mount Krakatau.The results of these eruptions can be found Far from eruption.The geological map of Mount Lawu is the basis for sampling around ML.
The method of rock magnetism can be used to determine magnetic minerals (paleomagnetic).MS as a tool called Bartington MS2 with MS2B sensor.The MS value is used as initial data to see the mineral content contained in the sample.The data cannot be a pure benchmark for the minerals contained.Researchers must use X-Ray Fluorescence (XRF) data.X-Ray Fluorescence is used to focus on the elements contained in the sample.These elements are presented on the table and graph.The data generated by the table is in the form of the percentage of elements contained in the sample.

Time and Location
One of the mountains on the island of Java, especially in Central Java is Mount Lawu, precisely on Central and East Java Province.ML is included in three districts, namely Karanganyar, on Central Java, Ngawi, Magetan, on East Java.ML is currently still in the resting category, meaning that one day an eruption will occur.The last eruption was recorded on November 28, 1885 [1].From its peak, erosion has occurred.A 2019 study on geothermal flows shows that Mount Lawu is still active today [21].Evidence of the eruption of Mount Lawu can be seen in Figure 1. Figure 1 shows volcanic material coming out of the volcanic chamber and settling in various places.The deposition process occurs due to the influence of wind and river movement.
Figure 1 showed that the sample are several categories of deposits, namely lava flows, pyroclastic falls, lahar deposits, lytic pyroclastic flows, and pyroclastic flows [22].Lava flows are formed due to cooling during the displacement process.The lava flow is evenly distributed over the entire side of the mountain.Pyroclastic fall is volcanic material that shoots into the air during an eruption.Consists of loose pyroclastic material, has a clear layer with a thickness of 50 cm.The units are brownish yellow, well separated, and have pumice fragments measuring 1-2 cm.Lava deposits is a pile of lava that has cooled to the extreme.Usually in the form of large rocks (Bombs).In the Figure 1 it clearly indicated of sediment around that volcano.Geological maps are a strong basis for researchers to take samples consisting of eight samples (figure 2).In Figure 2

Sampling Process and measurement
The sample is in the form of rocks which are the result of the Lawu volcanic eruption.It is proven by the large size of the rock which is suspected to be impossible for anyone to change the position of the rock.Material that fell from Mount Lawu was spread in different places with different sample categories based on Figure 1.The sample is broken using a geological hammer to get small grain as shown in Figure 3. Then the grain is taken to the laboratory for the next step.The chunks were crushed using a hammer into small pieces.The grain is then put into a ball milling tool to get powder.Then the powder is filtered to separate the coarse grains from the fine ones.Then it was put into a beaker and then washed with distilled water at a ratio of 1:2 using a hot plate [23].The sample was washed at a speed of 350 rpm for 4 hours [24], [25].Every hour the distilled water is replaced with a new one, aiming to make the sample completely clean of impurities.
Samples that have gone through the washing process then enter the drying stage.Sample drying using a memmert oven that has been set at 100 o C runs for 6 hours [26][27][28][29].After the dry sample enters the magnetic separation stage using a permanent magnet.Samples that have been separated between magnetic and non-magnetic, especially magnetic materials, are inserted into plastic to be tested for MS and XRF.
The Bartington MS2 with MS2B sensor operates at a magnetic field of 80 A/m rms, and at two frequencies, namely low field (  ) and high field ( ℎ ) (470-4700 Hz) [30].The difference caused by the two frequencies is called the dependent frequency (  ) in units of (m 3 /Kg) or in a percentage state called   % (Frequency Dependent Susceptibility) according to the equation [31]: The result of the comparison between the low field and the percentage of the dependent frequency is called a scatter diagram.The results issued by the diagram can be interpreted using table 2.

Magnetic Susceptibility (MS) Result
Samples that have been prepared starting with taking based on Figure 2 to the process of withdrawing magnetic material using permanent magnets.The researcher's first step was to test the MS value.The result obtained using a MS2B sensor or a magnetic value based on mass.The results that have been obtained by researchers showed on table 3.

Sample filtered. Samples crushed using a hammer. ball milling process
The result of the filter process Samples were washed using 1:2 distilled water and a hot plate tool.

Drying process The process of separating magnetic materials and nonmagnetic materials
The result of the separation of magnetic materials Susceptibility Meter X-Ray Fluorescence Mineralogy Using equation ( 1) and ( 2) we can get (Table 4) From the fd% data obtained and juxtaposed with Figure 3.We can see the content of magnetic grains in the Lawu rock sample figure 5. From Figure 3 and juxtaposed with Figure 5, broadly the sample is included in the type of igneous rock.From the data that has been obtained in tables 3 and 4 and matched with table 2, it is found that (  %) is less than 2% and is strengthened by the presence of Figures 5 and 6, so it can be categorized in the eight samples that have been measured, no Superparamagnetic grains are found and if there is it is only around less than 10%.The result from table 3 indicated of the sample Single Domain grains ranged between ~30 x 10 -6 m 3 /Kg and for Superparamagnetic grains ranged from 75-160 x 10 -6 m 3 /Kg [30].Table 3 shows the value of low field MS (  ) which is not much different from high field ( ℎ ).  4 shows that the percentage of frequency dependent susceptibility (fds%) ranges from 0.032 -1.262%.According to [30] the values that are not too different between low-frequency and high-frequency MS identify the grains contained in the sample of Mount Lawu volcanic material including multi-domain (MD) and according to [32] the multi-domain (MD) material has a grain size of 10μm.Based on [33] Multi Domain (MD) grains are frequency independent because the values of high frequency susceptibility and low frequency susceptibility are close to each other.In Figure 7 the values of the susceptibility with low and high frequencies are shown.This makes superparamagnetic grains not found in the sample [34].From the data that has been obtained in tables 3 and 4 and see [35].Based on [35] that in nature there are two categories of minerals, namely magnetic minerals and non-magnetic minerals.The sample of Mount Lawu volcanic material belongs to the Ilmenite (FeTiO 3 ) group with very weak magnetic properties.Antiferromagnetic materials are strongly influenced by external magnetic fields.If the external magnetic field is not large or practically nonexistent, then the material is not affected by the others.

X-Ray Fluorescence (XRF) Result
The results XRF on the Mount Lawu rock sample aim to see the elements contained therein.These elements serve as the initial basis for viewing the compounds contained.Then, from these compounds we can predict that the sample contains minerals according to [35].Here is presented XRF data according to table 6.Table 6 shows that the samples taken contain the constituent elements.These elements are Fe, Si, Al, Ti, Ca, K, and others.In the name of the sample LM 8, the Fe content is the largest with 60.31%.However, it has a low susceptibility value which is second only to the sample code LM 7. The MS value will be hindered in the research process because there are still Si impurities which can be quite high at 21.8% and other impurities as much as 2.43%.The element Fe is the most important element sought to see its magnetic mineral content.Theoretically, the higher the Fe content has a positive correlation with MS.The sample with the name LM 4, which has the highest susceptibility value 5263,733 x 10 -8 m 3 /Kg contains 50.37%Fe.However, it contains the least amount of impurities, which is 19.4%.In accordance with the theory which states that the susceptibility value is high if the Fe content is high, note that the impurities must be small.

Conclusion
From the data that has been obtained and the initial testing has been done using Bartington MS2 with MS2B sensor, susceptibility value in the sample ranges from 1467,2 x 10 -8 m 3 /Kg -5262,733 x 10 -8 m 3 /Kg.We know that the mineral contained is Ilmenite (FeTiO 3 ) based on MS data.In the sample not indicated of superparamagnetic content because it has (  %) which ranges from 0.032 -1.262% or less than 2%.From this value, it can also be seen that the Mount Lawu sample is included in the Multi-Domain (MD) type, which means that the Mount Lawu sample is very easily influenced by an external magnetic field.The grain size is greater than 10μm.

Figure 1 .
Figure 1.Geological Map of Mount Lawu.It can be seen that volcanic material is scattered in all directions [22].
eight samples were taken around mid to late 2021.

Figure 4 .
Figure 4. Sample preparation process to obtain mineralogy results.

Figure 5 .
Figure 5. Plot results between low field susceptibility   and   % Table4shows that the percentage of frequency dependent susceptibility (fds%) ranges from 0.032 -1.262%.According to[30] the values that are not too different between low-frequency and high-frequency MS identify the grains contained in the sample of Mount Lawu volcanic material including multi-domain (MD) and according to[32] the multi-domain (MD) material has a grain size of 10μm.Based on[33] Multi Domain (MD) grains are frequency independent because the values of high frequency susceptibility and low frequency susceptibility are close to each other.

Figure 6 .
Figure 6.Comparison of low field (  ) susceptibility values with high field ( ℎ ) samples of Mount Lawu volcanic material.

Table 1 .
Sample categories scattered in all directions.

Table 2 .
The Value of Frequency dependent susceptibility (  %) 2-10 %Mixing occurs between SP granules and granules that do not contain SP, or size in <0.005  High   10-14 % Almost all SP grains (>75%)

Table 3 .
Magnetic susceptibility data that have been measured and   and  ℎ

Table 4 .
Data   and %

Table 5 .
Magnetic minerals are contained in the volcanic material of Mount Lawu.

Table 6 .
Element content from lawu mount