Terceira Island, Azores

Located in the middle of the Atlantic Ocean about 1,400 kilometers (870 miles) off the coast of Portugal, the Azores archipelago is an autonomous region made up of nine islands and home to almost a quarter of a million people. The islands are grouped into three groups in a chain stretching 600 kilometers (370 miles) across the mid-ocean ridge. The westernmost group of islands lie on the North American tectonic plate, while the central and eastern groups lie on the Eurasian plate.

The Azores islands began to form about 10 million years ago on a mantle hot spot, similar to the formation of the Hawaiian Islands. However, unlike Hawai’i, which sits in the middle of a tectonic plate, the Azores lie on the edge.

The North American, Eurasian, and African Plates meet to form the Azores Triple Junction, a rare type of plate boundary where three divergent ridges meet. (Most triple junctions involve a subduction zone where the plates converge.) The North American and Eurasian plates are moving apart at a rate of 2 to 5 centimeters (0.75 to 2 in) per year, an average rate for a center of propagation. The Eurasian and African plates, however, diverge at a super slow rate of 2 to 4 millimeters (0.08 to 0.16 inches) per year along the 550 kilometers (340 miles) Terceira Rift.

The fault crosses the island of Terceira, shown in the image above, which was acquired on January 3, 2020 by the Operational terrestrial imager (OLI) on Landsat 8. This 400 square kilometer island in the center of the Azores was formed around 400,000 years ago. At the western end, the Serra de Santa Bárbara rises to the highest point on the island at 1,021 meters (3,350 ft) above sea level.

Almost a quarter of the island, dominated by forests and grasslands, has been designated a protected nature reserve. These include Biscoito da Ferraria Natural Forest Reserve in the north and Serra of Santa Barbara in the West.

This detailed view shows the volcanic features of the Serra de Santa Bárbara. The peak includes a double caldera, which formed from summit collapses 25,000 and 15,000 years ago. Also visible are small lava domes that formed on the eastern flank in 1761 during the last volcanic eruption happen on land in Terceira.

The flanks of the volcano are covered with agricultural fields and pastures. The Azores are famous for the production of wine and tea, as well as for cattle, which were brought by settlers in the 15th century. When the armadas of the Iberian empires ruled the seas, the Azores were a port of respite and resupply for ships in the middle of the Atlantic. This was made possible by a deep water anchorage at Angra do Heroísmo (“Bay of Heroism”). The port is sheltered by Monte Brasil, an extinct volcanic cone 200 meters high attached to the southern shore of the island. The second largest city is Praia da Vitória (“Victory Beach”). Both towns were named by Queen Maria II in the early 1830s to commemorate victories in the Portuguese Civil War.

The most recent volcanic activity on the island occurred underwater on the Serreta volcanic ridge, which lies about 10 kilometers (6 miles) west of the island. This eruption began in December 1998 and continued until March 2000. During this eruption, an underwater plume emitted basalt balloons. These hot basaltic lava drops contain volcanic gases that expand, causing lava balls to swell and float on the surface. Once the gas dissipates and the lava is quenched by seawater, the newly formed igneous rocks sink back to the seabed.

NASA Earth Observatory images by Lauren Dauphin, using Landsat data from the United States Geological Survey. Story by Sara E. Pratt.

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