Andesite and rhyolite are lavas with increasing proportions of silica, making them stiffer (more viscous) than basalt. They don't flow so far, producing volcanoes with steeper slopes. When dissolved gases are present in magma, pressure changes cause the gas to escape as the magma reaches the surface. In stiffer lava, the expanding gas can be trapped until it escapes explosively. This can produce airborne material, or tephra, with sizes varying from volcanic "bombs" , several centimetres across, to fine ash. Ash from large explosive eruptions can rise tens of kilometres into the atmosphere and spread thousands of kilometres horizontally and is easily seen from space.

The 1994 eruption of Klyuchevskaya, on the Kamchatka Peninsula

Tephra cones build up around the vents of explosive rhyolitic and andesitic volcanoes as the debris showers down, producing steep slopes of loose material. The highest volcanoes are stratovolcanoes, which emit both viscous lava (usually andesite) and tephra. The lava layers act to protect the tephra layers from erosion, allowing the volcano to build much higher than a simple tephra cone.

Mount St Helens exploded in 1980, devastating the previously-forested landscape.

Volcanoes can be recognised from Earth orbit by their circular form, often with a sizeable eruption crater at the summit. Extinct or dormant volcanoes can be heavily eroded, with a radial drainage pattern. Larger volcanoes may show a sizeable collapse crater, or caldera, where an empty magma chamber has collapsed, such as Ngorongoro in Tanzania.

Ngorongoro Crater, East Africa, is a caldera, left after a huge eruption 3 million years ago.

There is some correlation between the composition of a lava and its position on a tectonic plate. Andesitic stratovolcanoes occur at continental margins where oceanic crust is subducted under continental crust. Rhyolitic magma is erupted mainly from the continental crust. Basaltic lava is extruded all along the mid-ocean ridges and basaltic shield volcanoes are found in the ocean basins. Basaltic lava is also erupted through the continental crust, however, indicating an origin in the mantle, beneath both types of crust.

Igneous rocks do not always make it to the surface before solidifying. Subsequent erosion, however, may expose them to view. Large igneous intrusions into the crust may uplift the overlying rock strata. Batholiths are large intrusions, up to 15 kilometres thick. Sills and dikes are thinner intrusions along weaknesses in the surrounding rock. Sills are sheets, up to 300 meters thick, running between the rock's bedding planes, while dikes run through fractures across the bedding planes. Laccoliths are mushroom-shaped intrusions between bedding planes which have caused the overlying strata to dome upwards.