How do you know the rock type of your volcano? 

 Volcanic rocks have different names, just like we give names to different plants. Different volcanoes can have different rock types, and geologists collected hand specimens of volcanic rocks in different areas of the volcano using a hammer or a shovel, depending on the deposits, lava flows or tephra layers. To give a name to the different rocks, the samples need to be analyzed in a geochemical laboratory to determine its elemental composition. Commonly used techniques for such analyses are X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). These instruments provide information about the concentration of the most abundant element oxides (e.g., SiO2, K2O, Na2O), which is the basis for classifying the rock hand specimen into its name. It is important to note that volcanic regions often have a variety of rock types due to the complex geological processes involved in the formation and evolution of volcanoes. In our contribution we put together for the first time the information about Holocene volcanic eruptions (younger than 10,000 years) and the rock names. This will help geologists and volcanologists to better understand why different volcanoes produce different rock types and different eruptive styles. 

How to classify and give a name to a volcanic rock? 

The so-called “TAS diagram” (Total Alkali-Silica diagram), introduced by Le Bas et al. (1986), is commonly used for defining and classifying volcanic rock types based on their chemical composition. The TAS diagram plots the weight percent of total alkalis (Na2O + K2O) on the Y-axis and the weight percent of silica (SiO2) on the X-axis of each rock sample. The diagram is divided into different fields or regions that represent specific rock types based on their composition. The diagram is typically divided into different fields or regions representing specific rock types, and there are in total about 15 rock names, ranging from basalt, which has low silica and alkalis to rhyolite which has the highest silica as well as very high alkalis. 

Why volcanic rock names matter? what can different volcanic rock types teach us about volcano behavior? 

 Volcanic rock type is important because it provides valuable information about the behavior and characteristics of volcanoes, eruption styles, magma composition, volcanic history, and potential hazards. Some rock types produce much more commonly explosive eruptions than others, and some volcanoes produce mainly one rock type, whereas others produce a range of compositions through magma evolution. Understanding these aspects is essential for assessing volcanic hazards, predicting future eruptions, and managing the risks associated with volcanic activity.  

Global distribution of volcanic rocks  

 The different types of volcanic rocks found around volcanoes can be plotted via a density map. Geoscientists can look at this colorful rock type map and immediately spot important pieces of evidence concerning the eruptive history of the volcanic region. For instance, in the map below, the red area found in New Zealand represents basaltic rock type dominated in the Auckland volcanic fields and Rhyolitic type or rocks along the Taupo caldera. Through such a map, we can explore the whole world volcanoes with their characteristics in chemical compositions (rock types), such as where is the most Rhyolitic region in the Earth? Do they produce the most explosive type of eruptions in history?