Island Mountain is a resistant formation of volcanic

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

 and metavolcanic rock

Metavolcanic rock

In geology, metavolcanic rock is a type of metamorphic rock. Such a rock was first produced by a volcano, either as lava or tephra. Then, the rock was buried underneath subsequent rock and was subjected to high pressures and temperatures, causing the rock to recrystallize...

 in the Franciscan Assemblage

Franciscan Assemblage

The Franciscan Assemblage is a geological term for an accreted terrane of heterogeneous rocks found on and near the San Francisco Peninsula. It was named by geologist Andrew Lawson who also named the San Andreas Fault which bounds the Franciscan Assemblage....

 of southwestern Trinity County. The Eel River

Eel River (California)

The Eel River is a major river system of the northern Pacific coast of the U.S. state of California. Approximately 200 miles long, it drains a rugged area in the California Coast Ranges between the Sacramento Valley and the ocean. For most of its course, the river flows northwest, parallel to the...

 makes an abrupt "S" curve around the mountain in its otherwise north-northwesterly flow between the California Coast Ranges.

History

Island Mountain was the final obstacle to completion of the Northwestern Pacific Railroad

Northwestern Pacific Railroad

The Northwestern Pacific Railroad is a regional railroad serving California's North Coast. The railroad currently runs on 62 miles of the 462 mile main line, stretching from Schellville, California to Eureka, California...

 between San Francisco and Eureka, California

Eureka, California

Eureka is the principal city and the county seat of Humboldt County, California, United States. Its population was 27,191 at the 2010 census, up from 26,128 at the 2000 census....

. Railroad construction proceeded northerly down the west side of the Eel River canyon

Canyon

A canyon or gorge is a deep ravine between cliffs often carved from the landscape by a river. Rivers have a natural tendency to reach a baseline elevation, which is the same elevation as the body of water it will eventually drain into. This forms a canyon. Most canyons were formed by a process of...

 and bridged the river at the "S" curve to enter 4313 ft (0.816858090894404 mi; 1.31 km) tunnel

Tunnel

A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...

 number 27 through the mountain. There was a golden spike ceremony downstream of the mountain at Cain Rock

Cain Rock, California

Cain Rock is an locality in Humboldt County, California. It is located on the Northwestern Pacific Railroad south-southeast of Alderpoint, at an elevation of ....

 in 1914 to complete the railroad. The redwood framing of tunnel 27 burned on 6 September 1978, and a significant length of the tunnel collapsed. Tunnel rebuilding costing 3.8 million dollars was completed on 7 December 1979.

Mining

A volcanogenic massive sulfide ore deposit

Volcanogenic massive sulfide ore deposit

Volcanogenic massive sulfide ore deposits are a type of metal sulfide ore deposit, mainly Cu-Zn-Pb which are associated with and created by volcanic-associated hydrothermal events in submarine environments....

 within the mountain was mined after rail service became available. Mining

Mining

Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...

 recovered 4100 tons of copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, 140,000 ounces of silver

Silver

Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

, and 8,600 of gold

Gold

Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

 between 1915 and 1930. Minerals identified within the ore body include chalcanthite

Chalcanthite

Chalcanthite, whose name derives from the Greek, chalkos and anthos, meaning copper flower, is a richly-colored blue/green water-soluble sulfate mineral CuSO4·5H2O. It is commonly found in the late-stage oxidation zones of copper deposits...

, chalcopyrite

Chalcopyrite

Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical composition CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.On exposure to air, chalcopyrite...

, copiapite, goslarite

Goslarite

Goslarite is a hydrous zinc sulfate mineral with formula: ZnSO4·7H2O. It is a secondary mineral which develops after primary zinc minerals within the zone of oxidation and in old sphalerite mine workings. It is unstable at the surface and dehydrates readily. It is transparent to translucent and...

, pyrite

Pyrite

The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...

, and pyrrhotite

Pyrrhotite

Pyrrhotite is an unusual iron sulfide mineral with a variable iron content: FeS . The FeS endmember is known as troilite. Pyrrhotite is also called magnetic pyrite because the color is similar to pyrite and it is weakly magnetic...

. Volcanic rock was quarried from Island Mountain by Northwestern Pacific Railroad for heavy riprap

Riprap

Riprap — also known as rip rap, rubble, shot rock or rock armour or "Rip-rap" — is rock or other material used to armor shorelines, streambeds, bridge abutments, pilings and other shoreline structures against scour, water or ice erosion.It is made from a variety of rock types, commonly granite or...

 used as far away as Humboldt Bay

Humboldt Bay

Humboldt Bay is a natural bay and a multi-basin, bar-built coastal lagoon located on the rugged North Coast of California, United States entirely within Humboldt County. The regional center and county seat of Eureka and the college town of Arcata adjoin the bay, which is the second largest enclosed...

.