{"id":775,"date":"2012-09-27T00:14:47","date_gmt":"2012-09-26T23:14:47","guid":{"rendered":"http:\/\/all-geo.org\/volcan01010\/?p=775"},"modified":"2012-09-27T14:05:45","modified_gmt":"2012-09-27T13:05:45","slug":"pumicelithicsproportions","status":"publish","type":"post","link":"https:\/\/all-geo.org\/volcan01010\/2012\/09\/pumicelithicsproportions\/","title":{"rendered":"A visual estimate of the proportions of mixtures: pumice vs. lithics"},"content":{"rendered":"

When a volcano erupts explosively, the tephra<\/a> that comes out is a mixture of material that was molten at the time and bits of other old, cold rock that happened to get caught up in the blast.\u00a0 These are referred to as the juvenile<\/em> and the lithic<\/em> components and consist of bubbly, pumice-like fragments and of chunks of lava and hydrothermally-altered material, respectively.<\/p>\n

\"\"<\/a>

Lithic-rich deposits from the explosive Hekla 4 eruption. The yellow-grey-white grains are juvenile pumice, the dark grains are lava lithics that were caught up in the eruption. I reckon that there are up to 40% lithics here. How about you? Click to enlarge.<\/p><\/div>\n

The relative proportions of juvenile and lithic grains can tell us something about the eruption.\u00a0 For example, if the deposits are full of lithics, it might indicate that the vent where the material was erupting was getting wider or that underground water was coming into contact with the magma and turning explosively into steam, breaking up the surrounding rocks in the process.<\/p>\n

The best way to measure the relative proportions of juvenile and lithic grains in a sample is to get out the tweezers and separate them by hand, but this is often impractical, and a lot of time a visual estimate is sufficient.\u00a0 Visual estimates are a lot better if you have some known values to compare them with.\u00a0 That’s where the chart comes in.<\/p>\n

Visual estimates of the proportions of mixtures<\/h3>\n

The following chart gives examples of what mixtures of different proportions look like.\u00a0 Download it, print copies, use it whenever you need to make visual estimates of proportions of mixed materials.<\/p>\n

\"\"<\/a>

A chart demonstrating the visual appearance of mixtures of different proportions. Click to see the full-size version, or use 'Right-click, Save As...' to download a copy.<\/p><\/div>\n

Make your own charts<\/h3>\n

The charts were made using the Python<\/a> programming language.\u00a0 The source code is given below so that you can make and customise your own plots.\u00a0 If you are learning Python, try messing around with the code.\u00a0 First try changing the numbers and sizes of grains; next change the number of plots and give the grey grains different shapes to the black ones; finally, try plotting mixtures of 3 different grain types, each with different size distributions.\u00a0 Happy hacking!<\/p>\n

\r\nimport numpy as np\r\nimport matplotlib.pyplot as plt\r\n\r\nnumber=1000\r\nx = np.random.rand( number )\r\ny = np.random.rand( number )\r\npercentages=[1, 2, 5, 10, 20, 30, 50, 75]\r\nfig = plt.figure(figsize=(8.27,11.69))\r\n\r\nfor i in range(8):\r\n    percent=percentages[i]\r\n    black = np.round( percent*(number\/100.0) )\r\n    color = np.array( black*['black'] + (number-black)*['lightgrey'] )\r\n    np.random.shuffle(color) #randomise so black aren't all at the bottom\r\n    # define a marker shape here to make something that looks more like rocks\r\n    marker=[(0.17,0.12),(0.15,0.42),(0.14,0.75),(0.35,0.87),(0.68,0.90),\r\n            (0.98,0.86),(0.90,0.29),(0.72,0.14),(0.45,0.05),(0.17,0.12)]\r\n    ax=plt.subplot(4,2,i+1)\r\n    plt.scatter(x, y, color=color, s=60, linewidth=0.25, edgecolors='black',\r\n                marker=(marker,0))\r\n    plt.xlim(0,1)\r\n    plt.ylim(0,1)\r\n    plt.title('%i%% black' % (percent), fontsize=12)\r\n    ax=plt.gca()\r\n    ax.xaxis.set_ticklabels([])\r\n    ax.yaxis.set_ticklabels([])\r\nplt.subplots_adjust(left=0.05, right=0.95, bottom=0.05, hspace=0.15, wspace=0.1)\r\nplt.suptitle('Visual estimates of proportions of mixtures', fontsize=18, y=0.97)\r\nplt.suptitle('http:\/\/all-geo.org\/volcan01010\/2012\/09\/pumicelithicsproportions',\r\n             x=0.95, y=0.045, horizontalalignment='right',\r\n             verticalalignment='top', fontsize='x-small')\r\nplt.savefig('PumiceLithicsProportions.png', dpi=150)\r\nplt.ion()\r\nplt.show()\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"
When a volcano erupts explosively, the tephra that comes out is a mixture of material that was molten at the time and bits of other old, cold rock that happened to get caught up in the blast.\u00a0 These are referred … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-775","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/all-geo.org\/volcan01010\/wp-json\/wp\/v2\/posts\/775","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/all-geo.org\/volcan01010\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/all-geo.org\/volcan01010\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/all-geo.org\/volcan01010\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/all-geo.org\/volcan01010\/wp-json\/wp\/v2\/comments?post=775"}],"version-history":[{"count":16,"href":"https:\/\/all-geo.org\/volcan01010\/wp-json\/wp\/v2\/posts\/775\/revisions"}],"predecessor-version":[{"id":792,"href":"https:\/\/all-geo.org\/volcan01010\/wp-json\/wp\/v2\/posts\/775\/revisions\/792"}],"wp:attachment":[{"href":"https:\/\/all-geo.org\/volcan01010\/wp-json\/wp\/v2\/media?parent=775"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/all-geo.org\/volcan01010\/wp-json\/wp\/v2\/categories?post=775"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/all-geo.org\/volcan01010\/wp-json\/wp\/v2\/tags?post=775"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}