In 1902, he was one of the scientists that the United States sent to investigate the volcanic disasters at La Soufrière volcano, St Vincent, and Mont Pelée, Martinique, which he credited with inspiring him to make a life's work out of geology.

Jaggar constructed water flumes bedded by sand and gravel to understand stream erosion and melted rocks in furnaces to study the behavior of magmas.

[2] With the help of the U.S. Navy and the National Geographic Society, Jaggar landed on the steaming shores of Martinique some 13 days after the disaster.

[2] The next 10 years of Jaggar's life brought expeditions to the scenes of great earthquakes and eruptions in Italy, the Aleutians, Central America, and Japan.

With each trip, Jaggar became increasingly concerned that his field studies were but brief, inadequate snapshots of long-term, dynamic, earth processes.

After the 1908 Messina earthquake killed 125,000 people near Mount Etna in Italy, Jaggar declared that "something must be done" to support systematic, ongoing studies of volcanic and seismic activity.

[2][11] After his retirement from the Hawaiian Volcano Observatory (HVO) in 1940, Jaggar remained active at the University of Hawaii, where he was a research associate in geophysics.

Within a year of their conversation, Thurston and other businessmen raised financial backing for the Hawaii Volcano Research Association (HVRA).

[2] In 1912, construction of the HVO began:[4] in February, prisoners sentenced to a term of hard labor by the Territory of Hawaii dug through Kilauea ash and pumice to a thick layer of pahoehoe lava on which to place concrete piers where seismometers would be mounted.

[2] During his early years as director, Jaggar struggled to obtain private endowments with the hope of eventually securing sponsorship by the federal government.

[18]On February 3, 1923, when an 8.4-magnitude earthquake hit the southeastern coast of the Kamchatka Peninsula,[19] Jaggar tried to warn the Hilo harbormaster about the possibility that a tsunami could have been generated.

"[20] In 1935, Jagger called on the United States Army to bomb lava flow that would soon reach the headwaters of the Wailuku River, which supplied water for the town of Hilo.

According to the United States Geological Survey, "Whether or not the bombing stopped the 1935 Mauna Loa lava flow remains a controversial topic today.