Sunset Crater is a beautiful cinder cone in the San Francisco Volcanic Field that erupted betwee 1050 and 1100 AD. It is the most recent eruption from this volcanic field. This specific cinder cone likely won't erupt again, although the volcnic field underneath is expected to erupt again in a location further east. The location is a national monument.
Arizona's volcanoes each last erupted about 900 years ago
The state of Arizona has two volcanic fields that meet the most commonly-used definition of an active volcano. Geologists most typically define "active" as any volcano that has erupted in the past 10,000 years and is thought likely to erupt again.
Although Arizona’s active volcanoes haven’t erupted in approximately 900 years, this is a short amount of time geologically, and they are each expected by geologists to erupt again someday. Nobody knows when it will happen.
For basic volcanology terms - such as stratovolcano, cinder cone, caldera, etc - see my article here on InfoBarrel - Types of Volcanoes and Volcanology Terms.
Arizona volcano #1 of 2: Uinkaret
This is Vulcan's Throne at the northern edge of the Grand Canyon, a cinder cone that is part of the Uinkaret Volcanic Field. Black lava flows that came from this volcano can clearly be seen flowing over the edge of the canyon. This occurred about 73,000 years ago.
Located north of the Grand Canyon in the northwestern corner of the state, this is one of two active volcanoes in Arizona, the other as mentioned being the San Francisco Volcanic Field near the city of Flagstaff.
This volcanic field has more than 200 cinder cones, and associated lava flows. The highest of the volcanoes is Mount Trumbull, which is also the oldest, having come into existence about 3.6 million years ago.
The most intensive activity occurred between 500,000 and 600,000 years ago during the Pleistocence Epoch, and the most recent eruption occurred about 1,000 years ago. Some of the lava flows went over the edge of the Grand Canyon, and at times blocked the Colorado River. The dams have since eroded, although some features remain from these lava flows within the canyon and draping over the walls of the canyon.
The most recent eruption originated from two vents just 2 miles (3 km) south of Mount Trumbull, and a large pool of lava spread both to the north and south. Only the southwestern portion of the cinder cone remains, as most of it was destroyed in the eruption that occurred in about 1100 AD.
Another famous cinder cone associated with this volcanic field, located near the edge of the Grand Canyon, is called Vulcan’s Throne (in the photo above).
A chain of volcanic fields begins here and extends northward into Utah.
Arizona volcano #2 of 2: San Francisco
Arizona's San Francisco Peaks in winter. The tallest peak is Mount Humphreys, and it is the highest elevation point in the state. This is an ancient extinct stratovolcano associated with the still-active San Francisco Volcanic Field nearby.
This volcanic field is associated with a nearby stratovolcano, which is believed to be extinct and no longer a possibility for an eruption, which is often referred to as the San Francisco Peaks and includes the highest point in Arizona, Humphreys Peak, at an elevation of 12,633 feet (3,851 meters).
Sunset Crater Volcanic National Monument and its main attraction, Sunset Crater, is the site of the most recent eruption from the volcanic field in about 1050 to 1100 AD. Sunset Crater itself peaks at 8,042 feet (2,451 meters), and stands about 1,100 feet (335 meters) above the surrounding terrain.
Of the several hundred volcanoes in the area, the next-most-recent eruption by a volcano other than Sunset Crater was 71,000 years ago.
At Sunset Crater Volcano National Monument, visitors can hike around the base, although hiking to the summit is not permitted. The park entrance is about 15 miles (24 km) from the city of Flagstaff, along US Highway 89. I’ve been there and found it super fascinating.
The volcanic field has typically erupted every few thousand years, and is expected to erupt again someday, although no one knows precisely when. USGS geologists have said that when an eruption does occur, it may be further east as the North American plate slowly moves over what is believed to be a hot spot underneath, and that Sunset Crater itself won’t likely erupt ever again.