July 13, 2026

Geology of Samoa

Introduction

Figure 1 – Samoa
Credit: Mapsland, Creative Commons
Attribution-Share Alike 3.0 Licence

The Independent State of Samoa (formerly called Western Samoa to differentiate it from nearby American Samoa) is a country of 207,501 people on a group of islands in Oceania, in the South Pacific. The country has a total area of 2,831 square kilometres and consists of two main inhabited islands, Savai'i and Upolu together with two smaller inhabited islands Manono and Apolima. There are also several uninhabited islands including the Aleipata Islands. Nearby countries include: American Samoa, 64 km to the east; Tonga, 889 km to the southwest; and Fiji, 1,152 km to the southwest.

Samoa is a relatively poor country with a per capita GDP (PPP) of $5,962 and a high Human Development Index of 0.708.A developing country, two thirds of the labour force works in agriculture. The rest of the economy consists of services, especially to the tourist sector, and industry. Many Samoans rely on remittances from family members working overseas. In 2024, the top exports of Samoa were insulated wire, refined petroleum, telephones, coconut oil, and integrated circuits. The top destinations were Singapore, New Zealand, Indonesia, Australia, and the United States. In 2024, the top imports of Samoa were refined petroleum, coated flat-rolled iron, poultry meat, and cars. The top origins were New Zealand, China, Singapore, South Korea, and Australia.

For more details on the country, check out the Wikipedia and Grokipedia articles on the country.

Geology

Figure 2 – Tectonic Stetting of Samoa
Credit:National Park Service, US Dept. of the Interior, public domain

Tectonically, Samoa sits on the Pacific Plate near a convergent boundary with the Australian Plate (also called the Indo-Australian Plate), At this boundary, the Pacific Plate subducts under the Australian Plate forming the Tonga and Kermadec trenches (sometimes combined into the Kermadec-Tonga Trench). To the west of Samoa, the Pacific and Australian plates meet at a transform boundary, forming the Paleo-Vitiaz Trench.

Figure 3 – Geology of Samoa
Credit: Figure 2 in Zakharovskyi & Németh, 2022,
Creative Commons Attribution-Share Alike 4.0 International license

The geology of Samoa consists of three main features:

Volcanic rocks;
Alluvium from erosion of the volcanic rocks; and
Coral reef deposits immediately offshore.

Petrologically, the volcanic rocks of Samoa are fairly similar to one another, being mostly olivine-rich basalts; however other volcanic rock types occur. The oldest rocks in Samoa, are the Pliocene to Pleistocene aged Fagaloa Volcanics, 2.15 +/- 0.35 million years ago (Mya), consisting of a a sequence of basaltic lava flows that are found on the northeast potion of Upolu.

Next younger are the Pleistocene aged Salani Volcanics deposited during time coinciding with the Illinoian Glaciation and Sangamonian Interglacial in the North America. The Salani Volcanics are also mostly basalts but also include some andesite and rhyolite.

Also deposited during the time coinciding with the Sangamonian Interglacial were the Vini Tuffs, found on some offshore islands. As suggested by their name, the Vini Tuffs are made up of tuff, a pyroclastic rock.

The Mulifanua Volcanics were deposited during the time coinciding with the Wisconsin Glaciation, the rocks have been aged at 0.022 to 0.030 Mya, although some dispute this dating. The Mulifanua Volcanics are similar to the Salani Volcanics and are mostly basalt.

Following beginning of the Holocene Epoch, the Lefaga Volcanics and the Puapua Volcanics were deposited. These were also predominately basalts. The most recent volcanic deposits are the Aopo Volcanics, which are also basalts deposited during historic times.

Overlying the volcanic deposits, and helping to differentiate the various formations, are alluvial deposits formed by erosion. Finally, offshore, you will find coral reefs made up of carbonate rock.

The small size of Samoa leads to no exploitable mineral resources, the USGS has no Minerals Yearbook on the country. The Mindat.org site on Samoa records no mineral occurrences.

Paleontology

For volcanic islands, there have been a surprising number of fossils uncovered. Almost all of these are Holocene aged bones of marine animals and birds that were hunted (sometimes to extinction) by the earliest inhabitants of the islands. Here are a couple of references if this kind of archaeology interests you:

J. T. Clark, S. Quintus, M. Weisler, E. St Pierre, L. Nothdurft, Y. Feng, 2016, Refining the chronology for west polynesian colonization: New data from the Samoan archipelago, Journal of Archaeological Science: Reports, Volume 6, 2016, Pages 266-274, ISSN 2352-409X, https://doi.org/10.1016/j.jasrep.2016.02.011
Weisler, M. I., Lambrides, A. B., Quintus, S., Clark, J. and Worthy, T.,2016, Colonisation and Late Period Faunal Assemblages from Ofu Island, American Samoa, Journal of Pacific Archaeology, 7(2), pp. 1–19. https://doi.org/10.70460/jpa.v7i2.200

Summary

Figure 4 – Beach on Upolu Island
Credit: Stephen Glauser, Creative Commons
Attribution-Share Alike 2.0 Generic license

For a geoscientist, the main attraction of Samoa, besides its beautiful beaches, would be an opportunity to study the volcanic rocks on the islands. Currently, there are no mineral based industries on the islands to speak of, so further exploration could reveal something worth developing. Although, a lot would depend on whether of not the locals wanted such industry. Perhaps they are satisfied with their island paradise as it is.

One controversial area that the Samoans could benefit from would be offshore exploitation of subsea mining for minerals such as cobalt, nickel, manganese, and rare earth elements, if they can accept the inevitable environmental damage. Already, there are plans for subsea mining exploration in nearby American Samoa, so it is only a matter of time before someone takes a look at Samoa’s territorial seabed. There is a lot of local resistance to this kind of development, so it may not happen. However, the ocean is a big place and pirate operators may just take what they want. This past week, the subject of deep sea mining has come up in American Samoa’s press. So the debate continues.

Overall, Samoa looks like it has a great future with tourism and industries that use the local, currently cheap, labour pool, for industries such as such as electronics assembly. They may want to be careful with endorsing deep sea mining, since the environmental damage could simply be unacceptable. We can only wish them the best, trade offs between development and environmental quality are always difficult.

Standard Caveat

J. Robert Oppenheimer on freedom and scientific inquiry

The purpose of my weblog postings is to spark people's curiosity in geology. Don't entirely believe me until you've done your own research and checked the evidence. If I have sparked your curiosity in the subject of this posting, follow up with some of the links provided here. If you want to, go out into the field and examine some rocks on your own with the help of a good field guide. Follow the evidence and make up your own mind.

In science, the only authority is the evidence.