Hans Cloos, son of Ulrich Cloos and his wife Elisabeth, born Heckel, matriculated at the Gymnasium Kreuzgasse, Cologne, in 1905 and showed an early aptitude for languages and playing the cello. He started studying architecture in Aachen, but soon changed to the study of geology, first at the University of Bonn and from May 1906 at the University of Jena. Following the death of his father he moved to the University of Freiburg, where he was awarded the doctoral degree in 1910.
With help from his uncle he was able to pursue geological research in German South West Africa (now Namibia) in 1911. He investigated the tin occurrences near Omaruru and Karibib and then studied the Erongo Mountains and Brandberg, which he recognised as anorogenic granite intrusions of post-Karoo age. In later years he visited southern Africa again in 1929 and 1936. His investigations led to a number of publications in German journals, which appeared over a long period. They included the following: "Geologische Beobachtungen in Suedafrika. 1. Wind und Wueste im Deutschen Namalande" (1911, 22pp.); "Geologische Beobachtungen in Suedafrika. 2. Geologie des Erongo in Hererolande" (1911, 84 pp.); "Geologische Beobachtungen in Suedafrika. 4. Granite des Tafellandes und ihre Raumbildung" (1919, 37pp.); Der Erongo: Ein vulkanisches Massive im Tafelgebirge des Hererolandes und seine Bedeutung fuer die Raumfrage plutonischer Massen (Berlin, 1919, 238 pp.); "Der Brandberg: Bau, Bildung und Gestalt der jungen Plutone in Suedwestafrika" (with K. Chudoba, 1931, 130 pp.); and "Suedwestafrika. Reiseneindruecke" (1937, 25 pp). The last of these papers included a description of Brukkaros Mountain, which he visited in 1936 in the company of the geologists H. Martin and H. Korn. He published only one paper in a southern African journal, namely "Aufgaben und Methoden heutiger Geologie" in the Journal of the South West African Scientific Society, 1929-1931, Vol. 5, pp. 32-38).
Still in 1911, following his first visit to Namibia, Cloos accepted a contract appointment for two years with a subsidiary of the American Standard Oil Company to search for oil deposits on Java and Borneo, where he obtained extensive field experience. At the conclusion of his contract he was inaugurated as a university lecturer in Marburg in 1914. During World War I (1914-1918) he worked for Krupp-Bergbau in Schlesien (now Silesia, Poland) on the provision of nickel for the steel industry. After the war, in 1919, he was appointed professor of geology at the University of Breslau (now Wroclaw, Poland). During the next few years he made an intensive study of plutons and their internal structure, specifically the structure of the granite mountains in Silesia, which in some ways resembled the Erongo Mountains. The methods that he developed during this period made him one of the pioneers in the study of granite techtonics. On the basis of this work he published, among others, Einfuehrung in die tektonische behandlung magmatischer erscheinungen (granittektonik): Das Riesengebirge in Schlesien (Berlin, 1925).
In 1926 Cloos was appointed professor of geology at the University of Bonn. He investigated the geology of the Scandinavian countries during 1925-1926 and visited the United States in 1927, 1935 and 1948. He made pioneering studies of rock deformation and investigated how continents develop their structure. He used scale models of clay to conduct tectonic experiments which, in combination with field observations, contributed substantially to an understanding of the mechanics of faulting. His book Gesprach mit der Erde (Munchen, 1947), translated into English as Conversation with the earth (New York, 1953, 413 pp), led to his world-wide recognition as one of the most prominent structural geologists. Other books by him included Einfuehrung in die geologie (Berlin, 1936, 503 pp) and Tektonik und magma (Berlin, 1922-1927, 3 vols). Among other honours he was awarded the Penrose medal of the Geological Society of America in 1948. A ridge on the moon was named Dorsum Cloos in his honour.
Cloos became chief editor of the Journal Geologische Rundschau in 1923 and made it internationally renowned. In 1911 he married Elisabeth Grueters, with whom he had four children. They were divorced in 1932, after which he married Frieda Grueters, born Schwab.