Hawaiian Islands

• 1. Introduction

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  The Hawaiian Islands, called Mokupuni Hawaii in the Hawaiian language, run for some 1500 miles across the North Pacific Ocean. The island of Hawaii anchors the southern end, and Kure Atoll marks the far northern tip. The nearest continent is about 1860 miles from the chain. Europeans once named these islands the Sandwich Islands, but the present name comes from the largest island, Hawaii.

  How did a line of volcanic peaks come to stand so far from any mainland? Why do the islands at one end of the chain look so different from those at the other? And what is it like to live on ground that erupts, shakes, and sometimes sends the sea racing toward the shore?

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• 2. A Hotspot Beneath the Plate

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  About 32 miles every million years is the speed at which the Pacific Plate drifts northwestward over a hotspot in the Earth's mantle. The islands are the exposed peaks of a huge undersea mountain range, the Hawaiian-Emperor seamount chain, built by that hotspot's volcanic activity. As the plate carries each island away from the heat, its volcano goes quiet and erosion starts to wear it down. That is why the northwestern islands are typically older and smaller than the ones in the southeast.

  Scientists estimated the ages of the islands with potassium-argon dating. Kure Atoll, the northwesternmost, comes out at roughly 28 million years old. The southeasternmost island, Hawaii, is only about 400,000 years old.

  Almost all of the hotspot's magma is basalt, so the volcanoes are built almost entirely of that rock. Gabbro and diabase appear in very small amounts. Nephelinite is exposed here too, but it is extremely rare. Basaltic magma is relatively fluid, so most eruptions in Hawaii are of the gentler Hawaiian type. Andesitic magmas, by contrast, feed the more explosive and dangerous eruptions along the margins of the Pacific basin.

  Mauna Loa covers over half of the Big Island and is the largest shield volcano on Earth. From sea level to its summit it rises more than 2.5 miles. The distance from sea level down to the sea floor adds about 3.1 miles more. The Big Island itself is the biggest and youngest island in the chain, and five volcanoes built it. Several of those volcanoes are still alive, and a younger neighbor is growing out of sight beneath the waves.

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• 3. Volcanoes Old and New

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  Haleakalā on Maui is the only active volcano outside the Big Island. On the Big Island, the active ones are Mauna Loa, Mauna Kea, Kilauea, and Hualalai. The volcanoes on the other islands are extinct, because those islands no longer sit over the hotspot.

  Kīlauea erupted nearly continuously from 1983 until it stopped in August 2018. The USGS Hawaiian Volcano Observatory documents recent activity and publishes images and interpretations of it. Over the last 200 years, the only active volcanism has been on the island of Hawaii and at one submerged volcano to the extreme southeast.

  That submerged volcano is Kamaʻehuakanaloa, formerly known as Loʻihi. It is expected to become the newest Hawaiian island once it rises above the ocean surface, somewhere between 10,000 and 100,000 years from now.

  The dangers from these volcanoes are varied. Lava flows can destroy and bury the land around them, and volcanic gas drifts off the vents. Submarine eruptions can disturb the surrounding ocean, and an explosive eruption remains possible. Beyond lava and gas, the volcanoes also shake the ground beneath the communities that sit near them.

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• 4. Counting the Islands

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  The state of Hawaii counts 137 islands in the chain. That figure includes small islands and islets offshore of the major islands, along with the individual islets inside each atoll. Names on the list include Molokini, Lehua, Kaʻula, and Nā Mokulua.

  Eight major islands anchor the chain, and all but Kahoolawe are inhabited. Together the islands cover a total land area of 6423.4 square miles. These eight are also called the Windward Islands.

  West of Niʻihau, the land breaks into partial islands, atolls, and reefs, known as the Northwestern Hawaiian Islands or the Leeward Islands. Midway Atoll is the only one of them with people on it. Midway is also the lone exception to state control. It is an unincorporated unorganized territory of the United States, while everything else is administered as the 50th state.

  Hawaii is the only state that sits entirely on an archipelago. It is also the only state with no geographic connection to North America. Many of the northwestern places carry a Hawaiian name beside the English one, such as Gardner Pinnacles, called Pūhāhonu.

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• 5. Ground That Moves

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  Between 1833 and 1896, roughly four or five earthquakes were reported in Hawaii each year. Much of that early monitoring was done in Hilo by the missionary Titus Coan and by Sarah J. Lyman and her family. Today the work belongs to the observatory run by the USGS.

  Most of the earthquakes are tied to the volcanoes, so activity is highest in the southern part of the chain. The biggest ones cluster along the flanks of active volcanoes such as Mauna Loa and Kilauea. Erosion causes slumping and landslides, while rising magma presses outward. Together they stress the flank until the slope slips and releases an earthquake.

  This kind of seismicity is unusual because the driving forces change as volcanic activity rises and falls. Flank earthquakes typically strike at depths of 5 to 20 km. Being so shallow, and so hard to predict, they raise the hazard for local infrastructure and communities.

  From 1974 to 2003, Hawaii accounted for 7.3% of reported United States earthquakes of magnitude 3.5 or greater. That was a total of 1533 quakes. Only Alaska and California had more.

  On the 15th of October 2006, a magnitude 6.7 earthquake struck off the northwest coast of the island of Hawaii, near Kona. A magnitude 5.7 aftershock followed about five minutes later. Damage on most of the Big Island was minor to moderate. Rock slides closed several major roads, and several water mains ruptured. Power outages lasted from several hours to days. Honolulu, nearly 150 miles from the epicenter, felt the shaking, and no deaths or life-threatening injuries were reported.

  On the 4th of May 2018, a magnitude 6.9 earthquake hit the zone of volcanic activity around Kīlauea. Earthquakes and landslides here have also been known to push the ocean itself into motion.

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• 6. Waves from Near and Far

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  Waves from distant Pacific earthquakes cross the ocean at 400 to 500 miles per hour. Tsunamis of this kind can reach coasts thousands of miles from where the quake struck. Most of the tsunamis that hit Hawaii begin this way, with an earthquake somewhere else in the Pacific.

  The islands can also produce their own. Explosive volcanic activity can trigger a tsunami. More than a million years ago, a catastrophic collapse, a debris avalanche, struck the island of Molokaʻi. That underwater landslide likely sent tsunamis across the sea. The Hilina Slump on the island of Hawaii is another place where a large landslide and tsunami could one day begin.

  Hilo has suffered more from tsunamis than any other city on the Big Island. The shape of Hilo Bay squeezes and builds up the rushing water. Coastal cities have warning sirens to alert residents.

  On the 27th of February 2010, a tsunami from an earthquake in Chile reached the islands. It was relatively minor. Even so, emergency officials used the latest technology and ordered evacuations in case a major event developed. The Governor called it a "good drill" for the next major event.

  On the 11th of March 2011, an earthquake in Japan sent another tsunami toward Hawaii. It was also relatively minor, and officials again ordered evacuations as a precaution. It still caused about $30.1 million in damages.

  Water arrives in gentler forms too, most often as rain carried in on the wind.

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• 7. Rain and Trade Winds

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  Average summer temperatures in the islands are about 84 °F (29 °C). In winter they sit near 79 °F (26 °C). The islands are tropical, yet altitude and surroundings give them many different climates.

  Trade winds bring most of the rainfall, which falls on the north and east flanks. Those windward sides get their rain through orographic precipitation. Coastal areas generally, and the south and west leeward flanks especially, tend to be drier.

  In the lowlands, most precipitation arrives during the winter months, from October to April. Drier conditions generally prevail from May to September. Tropical storms and occasional hurricanes tend to occur from July through November.

  Because temperatures stay so constant through the year, the chance of dangerous thunderstorms is low. Whoever first stepped ashore met this weather, but exactly when that happened is still argued.

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• 8. Settlers and Species

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  Explorers from Tahiti or the Marquesas Islands were the first to find and settle Hawaii. The date of that first settlement is still debated. Kirch's textbooks on Hawaiian archeology place the first Polynesian settlements at about 300 C.E., though his more recent estimates run as late as 600. More recent surveys of carbon-dating evidence put the first arrivals at around 940-1130 C.E.

  The islands hold a multitude of endemic species. After people arrived, first the Polynesians, non-native trees, plants, and animals were introduced. Rats and pigs were among them, and they preyed on native birds and invertebrates that had evolved without such predators.

  European and American colonization brought a larger shift. Large-scale single-species export agriculture and livestock grazing replaced multi-species farming systems. Forests were cleared, towns grew, and treeless grasslands spread. Many species that relied on forest habitat and food went extinct. Many of the endemic species that remain are considered endangered.

  On the 15th of June 2006, President George W. Bush issued a proclamation creating Papahānaumokuākea Marine National Monument under the Antiquities Act of 1906. It covers the northwestern Hawaiian Islands and their surrounding waters, and it forms the largest marine wildlife reserve in the world. In August 2010, UNESCO's World Heritage Committee added Papahānaumokuākea to its list of World Heritage Sites. On the 26th of August 2016, Barack Obama greatly expanded the monument, quadrupling it from its original size.

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