Понимание структурно-смысловых связей в тексте

Volcanic eruptions are among the Earth’s most powerful and destructive forces. However, they are also creative forces. The Earth’s first oceans and atmosphere formed from the gases given off by volcanoes. In turn, the oceans and the atmosphere created the environment A ______. Volcanoes have also shaped the Earth’s land- scape. Many of our mountains, islands, and plains have been built by volcanic eruptions.

The Earth’s crust is broken into plates B ______. There are 16 major plates. These rigid plates float on a softer layer of rock in the Earth’s mantle. As the plates move about, they push together or pull apart. Most volcanoes are located near the edges of plates.

Deep within the Earth it is so hot that some rocks slowly melt and become a thick flowing substance called magma. C ______, magma rises and collects in magma chambers. Eventually some of the magma pushes through vents and fissures in the Earth’s surface. A volcanic eruption occurs! Magma that has erupted is called lava.

Some volcanic eruptions are explosive and others are not. How explosive an eruption is depending on how runny or sticky the magma is. If magma is thin and runny, gases can escape easily from it. D ______, it flows out of the volcano. Lava flows rarely kill people, because they move slowly enough for people to get out of their way. Lava flows, however, can cause considerable destruction to buildings in their path.

If magma is thick and sticky, gases cannot escape easily. Pressure builds up E ______. In this type of eruption, the magma blasts into the air and breaks apart into pieces called tephra. Tephra can range in size from tiny particles of ash to house-size boulders.

Explosive volcanic eruptions can be dangerous and deadly. They can blast out clouds of hot tephra from the side or top of a volcano. These fiery clouds race down mountain- sides destroying almost everything in their path. Ash erupted into the sky falls back to earth like powdery snow, but snow F ______. If thick enough, blankets of ash can suffocate plants, animals, and humans. When hot volcanic materials mix with water from streams or melted snow and ice, mudflows form. Mudflows have buried entire communities located near erupting volcanoes.

1. that resemble a jigsaw puzzle
2. when plates push together
3. until the gases escape violently and explode
4. that made life possible on our planet
5. because it is lighter than the solid rock around it
6. that doesn’t melt
7. when this type of magma erupts