The shape test
If the mountain is a steep cone of stacked lava and pyroclastic layers, USGS language points to a composite volcano, also called a stratovolcano. If it is a wide, low dome of overlapping fluid flows, USGS language points to a shield volcano [USGS Volcano Hazards Program: About Volcanoes, read 2026-10-25].
USGS notes that some composite volcanoes rise more than 8,000 feet above their surroundings. Ojos del Salado in Chile is cited there as the tallest composite volcano on Earth (summit elevation 22,615 feet); Mount Rainier is the tallest in the U.S. at 14,410 feet. Famous composites include Mount Fuji, Mount St. Helens, Mount Shasta, and Mount Hood.
Shield examples in the same USGS overview include the Hawaiian chain: Kīlauea and Mauna Loa, which USGS calls the world's largest active volcano. Smaller shields in northern California and Oregon are described as up to 3 or 4 miles wide and 1,500 to 2,000 feet tall.
The names describe construction style, not which eruption is worse. Shields can bury roads under long basalt flows for weeks. Stratovolcanoes can send ash and pyroclastic density currents that erase valleys in minutes. Hazard depends on magma, water, and where people live.
USGS keeps both shapes in the same primer because travelers mix them up on sight. Steep plus layered ash and lava usually means composite/strato. Broad plus gentle slope usually means shield.
Why the slopes differ
USGS explains shield growth as lava pouring from the summit or along rift zones that radiate like bicycle spokes. Overlapping flows construct the shield shape over time.
Composite cones stack both lava and fragmental deposits. Because ash and sticky lava do not travel as far as fluid basalt, the edifice steepens. USGS also notes that repeated cinder-and-lava eruptions from one vent can eventually build a composite volcano from overlapping layers.
British Geological Survey teaching pages make the same viscosity point in slope numbers: sticky products favor roughly 30-35° stratovolcano slopes, while fluid basalt shields often sit under about 10° [BGS: How volcanoes form, checked 2026-10-25]. Use USGS for U.S. examples; BGS for the slope shorthand.
Viscosity and eruption style drive the silhouette. Fluid basalt spreads; stickier andesite or dacite piles up and often puts more mass into the air as tephra. Over many eruptions those habits build different edifices.
USGS teaching examples include Hawaiian shields such as Mauna Loa and Kīlauea, and many Cascade stratovolcanoes. The primer is about process, not a complete world catalog.
Slope angle is a field clue, not a lab assay. Confirm with geologic maps and monitoring pages when travel or insurance depends on it.
Related landforms USGS keeps separate
Cinder cones are the simplest USGS type: piles of bubbly cinder around a vent, often only hundreds of feet high. They are not shields and not full stratovolcanoes, though repeated activity at one vent can grade into a composite.
Lava domes are thick lava that piles over a vent instead of flowing away. USGS treats them as an eruption phenomenon that often grows on composite volcanoes, not as a third primary "mountain type" in the same sense as shield versus composite.
For another FactCrumbs earth-science comparison, see Glacier vs iceberg.
USGS also covers cinder cones, lava domes, and other landforms in the same educational set. Those are separate construction habits, not small stratovolcanoes or baby shields.
When news copy calls every smoking mountain a volcano without a type, the missing adjective is often the useful part. Steep layered cone versus broad basalt pile changes which historical eruptions are fair analogs.
Field mnemonic: steep and layered → stratovolcano/composite; wide and gently sloping basalt → shield. Read the local geologic summary before you treat the mnemonic as a hazard forecast.