Volcanism
A terrestrial planet or moon has a crust over a molten, flexible mantle. This can't be stable unless the the crust is really thick. Under the weak or thin places in the crust, liquid mantle tends to push up to the surface. When it reaches the surface of a planet the lava does what it does on Earth; it flows and cools into rock. How it far flows and what shape it takes depends on its temperature and what kind of rock it is. It also depends on the properties of the planet: how strong gravity is and how hot it is. When the lava piles up, it can create the familar volcano. If the lava doesn't pile up, it can just flow over the existing surface, covering whatever was there before it. In a nutshell, that is story of volcanism.
Where do volcanoes form? They occur where it is easiest for mantle to push to the surface; where the crust is weak, thin, or flawed or where the mantle is particulary hot. By mapping the volanoes on a planet, you can start to understand what is going on deep under its surface. For instance, on Earth by finding long regions of volcanic activity you can trace the edges of tectonic plates .
Exactly what kind of volcanic feature is created depends on the amount, temperature, and type of lava. The most familar volcanic feature is the volcano. When lava flows onto the surface and doesn't get far before it cools into rock, more lava builds up on the new rock and forms a volcano. On planets with an atmosphere, the lava transfers its heat to the air and cools quickly. As the lava continues to flow, it builds up on top of the lava that just solidified. Eventually, this forms the familar cone shape. Repeated eruptions at the same site serve to make the volcano bigger and bigger.
Now if the lava doesn't cool fast enough, a cone doesn't form. Instead it spreads out over a very large area, covering whatever was there before. This is the a common way old geological features are buried and a planet's history gets covered.
On some planets, you find lava features that look like huge pancakes. This clearly demonstrates how sometimes lava doesn't form a cute volcano.
There are several reasons the lava might cool slowly. It could just be "extra hot". It could be made of material that becomes a solid at a relatively low temperature. It could be there is a whole lot of lava with so much heat, it can't quickly cool.
Sometimes when lava reaches the surface, it forms a river. The outermost lava cools into rock and insulates the lava below. These lava tubes can flow a long way. They look a lot like water rivers except lava rivers tend to get smaller as they flow downhill while water rivers often get bigger.
Volcanic events do more than just create interesting features for us study. As the lava flows out of the Earth, gases come out along with it. A lot of gas. In fact, all the atmosphere of a planet first flowed with lava before it escaped out of the crust.
Deep underground, there is under a lot of pressure and a lot of heat. Rock will melt under these conditions. But when it melts, some if the rock will become a gas. Under the intense pressure deep below the surface, it will stay trapped in the molten rock. However, if the molten rock moves towards the surface the pressure will drop. If the pressure drops slowly, the gas will gradually release itself. If the rock moves quickly to the surface, it can reach the surface with a lot of gas still trapped. In this case, the trapped gas explodes as it did on Mount St. Helens. Many times this sort of event has killed many people. Where the lava reaches the surface without trapped gas, a lava eruption is far less dangerous. In Hawaii, they often become tourists attractions.
When you look at geologic features, you can gain insights into what kind of lava event occured. Ask yourself how far the lava flowed, which way it flowed, where it came from.