Earthquakes

• Piles of rocks line the crater's bottom.Tom Brakefield/Stockbyte/Getty Images
  
  Magma movement produces vibrations and earthquakes. Because an eruption requires magma to move, an increase in earthquake activity is a good indicator of a possible eruption. If you are nearby, you may be able to feel the ground shake or see rocks tumble down the slopes of a volcano. Earthquakes can also turn previously clear water muddy. Scientists use seismometers to detect these earthquakes. Volcanic earthquakes are usually less than a magnitude 3 and occur within 10 kilometers of the surface.
  
  

Ground Deformation

• The bulge at Mount St. Helens grew an average of 5 feet a day before the eruption.Getty Images/Getty Images News/Getty Images
  
  When more magma gathers in the magma chamber beneath a volcano, it causes the volcanic cone to swell or bulge. Sinking can also occur when magma erupts in the caldera. An example of sinking, or subsidence, occurred near Mount Nyiragongo. Locals noticed that the rocks previously above Lake Kivuwater's water level had submerged. Mount St. Helens grew, or bulged, in the weeks prior to its eruption. Scientists use satellite data and tiltmeters to measure ground deformation.
  
  

Steam and Gas Emission

• Steam and gas rise from an active volcano in Hawaii.Thinkstock/Comstock/Getty Images
  
  The solubility of gases and water in magma decrease as the pressure decreases. This causes sulfur dioxide, hydrochloric acid and carbon dioxide gases to escape when the magma is close to the surface. Scientists use a correlation spectrometer, or COSPEC, to monitor these gas emissions. The amount and proportion of gases often change before an eruption. The fumarole, a vent that steam and gases escape through, may increase in size as a volcano becomes more active. When more gases escape, nearby plants may die and lakes at the summit of a volcano may change in color or pH.
  
  

Temperature Increases

• A volcanic crater can form when a volcano subsides.Hemera Technologies/Photos.com/Getty Images
  
  Volcanoes can erupt many times over thousands of years. After a volcanic crater forms, it fills with water. As magma moves closer to the surface, the temperature of lakes and hot springs in the crater increases. Crater Lake's average surface temperature increased by 8 degrees Celsius prior to the eruption of Mount Mazama. Scientists use thermal cameras and satellite data to track temperature changes.