Volcanoes form for much the same reason that earthquakes occur--Earth's crust is separated into plates that have the ability to move. This movement is an earthquake and, if that quake occurs over plates that go deep enough inside the Earth's mantle, magma, or liquid rock, can seep between the plates and form a volcano. Thus, an earthquake happening in the vicinity of a volcano could lead to magma rising, meaning the potential for a volcanic eruption.
When the top or side of a mountain collapses, it could be an indication that it was actually an inactive volcano all along, or perhaps a dormant volcano that is becoming active again. Usually an earthquake will precipitate and cause mountain collapse. In the case of Washington's Mount St. Helens, what was once a large mountain in a cone shape became something else entirely when it blew out its top and side on May 18, 1980, leaving a massive crater.
When previously dormant volcanoes begin to spew ash, like Washington's Mount St. Helens in 2004, they are becoming active again. This ash falls to the ground, covering it like snow. This ash can suffocate plants, animals and even humans. It has also been known to cause major visibility issues if pilots are flying in the area; a Boeing 747 came precariously close to crashing in Alaska in 1989 when Mount Redoubt erupted due to its massive ash cloud.
A mixture of the ash from the volcano and other hot, volcanic materials can mix with surrounding water (such as rivers, lakes and brooks) to form mudflows. Communities that have been within range of a volcanic eruption have been completely buried by these mudflows.
A mixture of magma pieces that have been blasted into the air, tephra is a clear warning sign that a volcano is on the verge of eruption since magma is the precursor to lava. Tephra is harder, though, and can range in size from ash-like particles to house-sized boulders.
