Patrick Marshall was born on 22 December 1869 in Sapiston, Suffolk, England. He was the son of Emily Louisa Merielina Rogers and her husband, John Hannath Marshall, vicar of Sapiston. In 1876 the family emigrated to New Zealand and settled at Kaiteriteri, Nelson. John Marshall died in 1879 and his widow and children went back to England, where Patrick attended school in Bury St Edmunds. His mother remarried and in 1882 the family returned to New Zealand, to live in Wanganui. From 1883 to 1888 Patrick attended Wanganui Collegiate School, where he had an outstanding record. At Canterbury College he studied geology and biology under Professor F. W. Hutton, graduating BA and BSc in 1891 and gaining the Senior Scholarship in geology. In 1892 he studied at the University of Otago for an MA under Professor G. H. F. Ulrich.
Marshall's first appointment, from 1893 to 1895, was as lecturer in natural science at the Lincoln School of Agriculture, where his first independent research was in entomology. For the next five years he was senior science master at Auckland College and Grammar School and during this time completed a thesis on the Auckland volcanic field, for which he was awarded a DSc in 1900. On 19 January 1900 he married Ruth Mary Dudley at Auckland.
In 1901 Marshall was appointed lecturer in geology at Otago University, and in 1908 promoted to professor of geology and mineralogy. His years at Otago were very productive in research. He accompanied the Philosophical Institute of Canterbury's 1907 expedition to the subantarctic islands and worked on two regional bulletins for the New Zealand Geological Survey. In Dunedin he became widely known as a brilliant teacher, and, outside the university, for his involvement in patriotic causes. On the outbreak of war in 1914 he volunteered for active service but was not accepted.
Bitter disputes on scientific matters with Professor James Park, director of the Otago School of Mines, may have been a factor in Marshall's decision in 1916 to resign from the university to accept the headmastership of his old school, Wanganui Collegiate. His time there, during a difficult period, lasted a little over five years, and he left early in 1922. He spent some time at the British Museum in London doing research for a major palaeontological paper, and then from 1924 until retirement in 1940 was employed as geologist and petrologist by the New Zealand Public Works Department in Wellington. His work there, although centred on geological problems associated with engineering projects, enabled him to develop several important studies of general application.
With minor interruptions Marshall's research career spanned 50 years and covered most fields of geology. At Otago probably his most important research was on the volcanic rocks of the Dunedin district. He also wrote widely used textbooks on the geography and geology of New Zealand, and the first of many papers on Pacific islands and the structure of the Pacific basin. In 1912 he proposed that the true border of the south-west part of the ocean basin is a major structural line extending along the sea floor from New Zealand to Fiji and thence westwards to the New Hebrides (Vanuatu) and the Solomon Islands. He named this the andesitic line, to mark an important change in the composition of the rocks across it, and in later years it was called the Marshall line by some writers. It is now recognised as the boundary between two of the major plates of the Earth's crust.
While at Otago Patrick Marshall became involved in two major controversies, one concerning the age subdivision of the older sedimentary rocks of New Zealand, the other the extent of glaciation over the South Island and parts of the North during the glacial period. In the former his views proved to be mistaken, but in the latter he held correctly that glaciation had not been as extensive as James Park and others had asserted. The first controversy was little known outside geological circles, but the arguments about glaciation became known to a wide audience, as lengthy reports of local meetings on the subject were published in the Dunedin press.
In the Public Works Department Marshall carried out important research on beach gravels and the formation of fine-grained sediment. In the 1930s he showed that many of the siliceous volcanic rocks widespread in the central North Island resulted from the deposition and welding of hot fragments from clouds of debris and incandescent gases emitted during great eruptions. He called them ignimbrite, a term now widely used both in New Zealand and overseas.
During his career Marshall gave outstanding service to the New Zealand Institute (later the Royal Society of New Zealand). He was one of the original fellows of the institute, and its president in 1924 and 1925. He served on many committees, including one which advised the government on the setting-up of the Department of Scientific and Industrial Research in 1926. From 1912 to 1918 he was a member of the Senate of the University of New Zealand. He was also active in the Australasian (later Australian and New Zealand) Association for the Advancement of Science and was its president in 1946. He was awarded the Hector and Hutton memorial medals by the Royal Society of New Zealand, and in 1938 was elected a correspondent of the French Académie des sciences coloniales. In 1949 he was awarded an honorary DSc by the University of New Zealand.
Patrick Marshall was of distinguished appearance, tall and, especially in his younger days, athletic. He was a gifted and persuasive speaker, in his teaching, on committees, and in patriotic addresses during the First World War, in which he espoused imperialistic ideals. When at school and university he excelled in sport, particularly rugby (in which he represented Canterbury province, Canterbury College and Otago University), cricket and tennis (he was New Zealand doubles champion with his brother, J. M. Marshall, in 1893). He also became a keen mountaineer and was a founding member of the New Zealand Alpine Club. His interest in his research lasted almost to the end of his life. He died at his home in Lower Hutt on 10 November 1950 and was survived by his wife and a daughter and son. Ruth Marshall died on 11 December 1960.
Marshall was one of the most versatile and productive geologists of his period and was among the best-known New Zealand scientists abroad. Inevitably much of his purely descriptive work has been superseded by later research. Apart from his teaching at Otago, his most lasting contribution lies in his work on aspects of coastal erosion and in volcanology, fields in which he was among the leading investigators of his day.