Editing Geography of Guam (section)

== Geology ==
{{main|Geology of Guam}}
{{further|List of rivers of Guam|List of beaches in Guam}}

The [[Mariana Islands]] lie atop the largely submerged East Mariana Ridge, a part of the [[Izu–Bonin–Mariana Arc|Izu–Bonin–Mariana (IBM) Arc]]. The IBM Arc is a [[plate tectonics|tectonic plate]] [[convergent boundary]] where the west [[Pacific Plate]] subducts the [[Philippine Sea Plate]]. Guam is actually located on the [[Mariana Plate]], a micro plate between the two. The subduction area is marked by the [[Mariana Trench]], the deepest gash in the earth's surface, which includes three deep spots to the south of Guam. From east to west, these are: Nero Deep, which was the deepest known spot in the ocean from 1899 to 1927 at {{Convert|9660|m|ft|abbr=on}}; [[Sirena Deep]], the third deepest measured point at {{Convert|10714|m|ft|abbr=on}}; and [[Challenger Deep]], the deepest point at {{convert|10902|to|10929|m|ft|0|abbr=on}}.

There have been three major eruptions on Guam. the first, the Facpi formation, in the mid [[Eocene]], laid the base of the island and is still the topmost formation along the southwestern coast. The second eruption created the Alutom formation that is still the topmost strata in the middle of the island. The [[Mount Alifan]]-[[Mount Lamlam]] ridge is the remnant of the Alutom formation [[caldera]].<ref name="EIS_Ch 3">{{cite web |title=Guam and CNMI Military Relocation - Final EIS - Chapter 3: Geological and Soil Resources |url=http://www.guambuildupeis.us/documents/final/volume_2/Vol_02_Ch03_Geological_and_Soil_Resources.pdf |website=Guam Buildup Environmental Impact Statement |access-date=14 March 2021 |date=July 2010 |archive-date=25 January 2012 |archive-url=https://web.archive.org/web/20120125063320/http://www.guambuildupeis.us/documents/final/volume_2/Vol_02_Ch03_Geological_and_Soil_Resources.pdf |url-status=live }}</ref>{{rp|7}}

The last volcanic layer, called the Umatac formation, was formed by the third and final eruption, which surrounded this ridge in the south of Guam. Meanwhile, volcanic activity alternately submerged areas of the island, which hosted [[coral reef]]s, and then lifted those reefs, which became [[limestone]]. The island may be divided into four general geophysical regions:{{r|"EIS_Ch 3"|p=4}} the uplifted and relatively flat [[coral]]line [[limestone]] plateau in the north that provides most of the drinking water; the low-rising hills of the Alutom Formation in the center; the mountains of the Umatac formation in the south; and the coastal lowlands ringing most of the island.{{r|"EIS_Ch 3"|p=1}} Much of the coast is protected by a [[fringing reef]].

Soils are mostly silty clay or clay and may be gray, black, brown or reddish brown; acidity and depth vary.<ref>{{cite web |url=https://casoilresource.lawr.ucdavis.edu/gmap/ |title=SoilWeb: An Online Soil Survey Browser &#124; California Soil Resource Lab |publisher=Casoilresource.lawr.ucdavis.edu |date= |access-date=2021-11-24 |archive-date=2021-11-19 |archive-url=https://web.archive.org/web/20211119221418/https://casoilresource.lawr.ucdavis.edu/gmap/ |url-status=live }}</ref> There are four [[List of National Natural Landmarks in Guam|National Natural Landmarks in Guam]] chosen as exemplars of the island's geology: [[Facpi Point]], [[Fouha Point]], [[Mount Lamlam]], and [[Two Lovers Point]].

<gallery>
File:Two lovers point (80103943).jpg|[[Two Lovers Point]], a [[List of National Natural Landmarks in Guam|National Natural Landmark]], is a sea cliff of coralline limestone in northern Guam
</gallery>

=== Earthquakes ===
{{main|List of earthquakes in Guam}}
Guam occasionally experiences earthquakes;<ref>{{cite web|last1=Mueller|first1=Charles S.|display-authors=etal|title=Seismic Hazard Assessment for Guam and the Northern Mariana Islands|url=https://pubs.usgs.gov/of/2012/1015/report/OF12-1015.pdf|publisher=U. S. Geological Survey|access-date=September 23, 2018|archive-url=https://web.archive.org/web/20180923162947/https://pubs.usgs.gov/of/2012/1015/report/OF12-1015.pdf|archive-date=September 23, 2018|url-status=live}}</ref> most with epicenters near Guam have had magnitudes ranging from 5.0 to 8.7. Unlike [[Anatahan]] in the [[Northern Mariana Islands]], Guam is not volcanically active, though [[vog|vog (volcanic smog)]] from Anatahan affects it due to proximity.<ref name="USGS Anahatan Volcano">"Home page of the Anahatan volcano". ''USGS-CNMI'', November 8, 2007. Retrieved November 8, 2007 from {{cite web|url=http://hvo.wr.usgs.gov/cnmi/|title=Home page of Anatahan Volcano, Commonwealth of the Northern Mariana Islands|access-date=November 8, 2007|url-status=dead|archive-url=https://web.archive.org/web/20071013221850/http://hvo.wr.usgs.gov/cnmi/|archive-date=October 13, 2007}}</ref>