Scott Rowland: It depends on what part of the Earth you are looking at. In areas that are not volcanic, volcanic activity isn't doing any shaping at all. North America east of the Rockies, for example, isn't being affected currently by volcanism and hasn't been affected by volcanism for at least a couple hundred million years. Many other parts of the Earth fall into the same category, for example almost all of Africa, most of South America, and essentially all of Australia and India. Table -->

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	Galileo imaged the Prometheus volcano on Io, with a caldera and lava flowing through fields of sulfur dioxide snow. Scientists said it is remarkably similar to the Kilauea volcano in Hawaii, though Prometheus is much larger.
	Olympus Mons is taller than any terrestrial volcano. Mars Global Surveyor captured this view in 1998.
	Lava from Hawaii's Kilauea volcano flows down a scarp that was previously cut by a wave. The lava heads toward the water in this Sept. 3 photograph.
	The lava reaches the water. Steam forms constant acidic plume hazardous to your health. It also adds land to the island over time.
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Where there are active volcanoes, of course, they have a large effect on the building and shaping of the landscape. The most obvious effect is that erupting volcanoes usually produce growing edifices.

This growth is almost always too small for people to observe in a lifetime except on a minor scale (the creation of new cinder cones or individual lava flows, for example). But over thousands, tens of thousands, and hundreds of thousands of years large topography-altering structures are built.

Some eruptions produce negative topography and this often occurs very rapidly. The main example of this is caldera collapse associated with large explosive eruptions. The most recent example of this was at Pinatubo during the June 1991 eruption.

Erupting and growing volcanoes not only alter topographic features but they can dam streams and rivers. They can produce huge amounts of sediment that choke river drainages or get mobilized all at once to produce deadly lahar (mudflows).

Volcanic gases may kill vegetation even when eruptions are not occurring, leaving slopes prone to erosion. So although the destruction of a town by a lava flow, for example, is an extreme event in the life of a human being, in the overall scheme of changing the Earth's surface it is relatively small.

The internal heat is what drives plate tectonics and in turn, plate tectonics causes magma to be produced. Certainly that is the case with magma produced at spreading centers (both oceanic and continental) and subduction zones. It is harder to blame plate tectonics for producing hotspot magmas. Plate tectonics is also what produces most of the big-scale landforms we see today -- mountain belts, lakes, etc.

Rowland: Most volcanoes grow endogenously as well as exogenously.

In Hawaii for example, we know that there are many intrusions down the rift zones that don't result in eruptions on the surface. The same is probably true for most volcanoes. It is only when these volcanoes become old and eroded that we are able to see these intrusive bodies (although their emplacement can be monitored seismically and by tracking surface deformation).

Again though, even this internal volcano growth is confined only to the bodies of volcanoes themselves and therefore is pretty surficial with respect to growing a planet. The exception to this is the idea of magmas underplating the tectonic plates on which the volcano is growing.

This has been proposed and seismic data have hinted at now-solidified igneous bodies underlying a couple of the older Hawaiian volcanoes, but it is not a process that is understood very well. The huge batholiths of the Sierra Nevada, for example, are definitely large-scale surface features and they are usually thought to be the now-solidified subsurface portions of now-eroded-away volcanic structures.

SPACE.com: When we see pictures of fantastic eruptions and lava flows on Hawaii nowadays, how significant is the process compared to Earth's early geologic history?

Rowland: Probably not very significant. Earth's early geologic history, depending on how you define "early," lasted for a really long time. In that period of time, way more happened than we can really appreciate. By comparison, the current eruption of Kilauea is pretty miniscule. Even the entire Hawaiian/Emperor chain is pretty tiny both spatially and temporally.

Rowland: This kind of depends on who you ask and also on at what scale you are looking. Some in the planetary community might howl at this but ... it is probably a stretch to say that you can understand the history of a planet by looking at a single volcano. It isn't a total waste of time though.

You can say things about the environment into which an eruption occurred by looking at aspects of the volcanic products. The differences between eruptions that occur in dry environments vs. those that occur under water or where there is abundant surface water are obvious, and many people have used this environment-from-volcanic-products approach to studying Mars.

There are also differences between the eruptions and volcanic forms that occur from magmas rich in dissolved volatiles vs. those poor in dissolved volatiles. These can be assessed and from them people make inferences about the richness in volatiles of the mantle of a planet. Once you know this you can start saying things about whether or not that particular mantle has undergone multiple episodes of partial melting.