-Sudden earthquake activity & change in temperature, ground tilt &gas compositions
-Use of seismographs, tiltmeters, geodimeter, magnetomers & gravity meters are also helpful because changes in magnetic and gravity measurements around the volcano can indicate whether magma body is rising into it (hence the volcanic eruptions)
Seismographs:
Seismic activity (earthquakes and tremors) always occurs as volcanoes awaken and prepare to erupt and are a very important link to eruptions. Some volcanoes normally have continuing low-level seismic activity, but an increase may signal a greater likelihood of an eruption.
(above is the picture of a seismograph)
Tiltmeters:
A tiltmeter is designed to measure very small changes from the horizontal level, either on the ground or in structures. Tiltmeters are used extensively for monitoring volcanos, the response of dams to filling, the small movements of potential landslides, the orientation and volume of hydraulic fractures, and the response of structures to various influences such as loading and foundation settlement. They may be purely mechanical or incorporate vibrating-wire or electrolytic sensors for electronic measurement. A sensitive instrument can detect changes of as little as one arc second.
Magnetomers:
Magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument. Magnetism varies from place to place and differences in Earth's magnetic field (the magnetosphere) can be caused by the differing nature of rocks.
credits: www.wikipedia.com
www.photobucket.com
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