When the ground shakes its usually during an earthquake or volcano eruption. During earthquakes the earth moves up and down rapidly while volcanoes erupt violently. Volcanic eruptions sometimes cause tsunamis because of large amounts of water released into the atmosphere. A red sky appears over the area after an eruption and a yellowish-orange color occurs due to ash particles in the air. After an eruption there may be small black clouds above the site but not enough to cover the sun. Earthquakes occur underground in the crust of the planet causing tremors at surface level. There are various ways to tell if a volcano is active including tiltmeters, seismic sensors and geyser activity. An increase in temperature could indicate an eruption. Some indicators include gas emissions, lava flows and increased pressure within the magma chamber.
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