Tall , conical volcano that is made of a layer of hardened lava, ash and tephra is referred as stratovolcanoes.
Step-by-step explanation:
A stratovolcano requires more than thousands year time to form. Various type of magma type are erupted from this volcano they are rhyolite, basalt,dacite and andesite. When eruption occurs huge amount of gases are evolved. In case of stratovolcanoes periodic explosion takes place.
The lava that comes out is highly viscous and the lava cools very fast and hardens before going too far. Mt. St. Helens is one of the example of stratovolcano.