Volcanoes

The temperature of the Earth is high enough for rock to melt.

-are vents in Earth's crust through which molten rock flows.
-Do not form everywhere.

Volcanoes

Magma

molten rock stored beneath Earth's surface

Lava

magma that erupts onto Earth's surface

Where Volcanoes Occur

​Most volcanoes form at convergent boundaries

In some convergent boundaries, one plate subducts under another plate. Some rocks contain water within their structure. As the rocks subduct, heat and pressure drive the water out. This water can lower the melting temperature of the mantle

Where Volcanoes Occur

​Most volcanoes form at convergent boundaries

Magma then rises toward the surface and forms volcanoes on the plate that does not subduct. A line of volcanoes forms parallel to the plate boundary directly above the plate that subducted.
The volcanoes in Washington and Oregon, such as Mount Rainier, Mount St. Helens and Mount Hood, formed above the subducting Juan de Fuca Plate.

How Volcanoes Change Earth's Surface

Volcanoes are some of Earth's most distinctive landforms.

Mountains can form over millions of years, but volcanoes can form in hundreds to thousands of years. Sometimes volcano formation happens even more quickly.

How Volcanoes Change Earth's Surface

Volcanoes erupt in two ways. Sometimes, lava can flow over Earth's surface before cooling, hardening, and becoming solid rock. This is called lava flow .

Lava flows can be more then 10 km long and over time can cover large areas surrounding a volcano.

How Volcanoes Change Earth's Surface

Volcanoes can erupt explosively. This kind of eruption can produce tiny pieces of glass made from solidified lava. These pieces are called ash and can be blown high into the atmosphere. When the ash falls back to Earth;s surface, it can cover vast areas.

Ocean Basins

Lava erupts from volcanoes on land.

However, not all lava flows on land. Recall that the land mases that make up North America and Europe began to separate when divergent boundary formed between them. What happened in the sea between these land masses as it became large?

Lava on the Ocean Floor

Lava erupts at both convergent plate boundaries and divergent plate boundaries. This lava hardens and forms new crust. At an oceanic divergent plate boundary, magma rises between two plates and forms new crust. As the plates move apart, more lava rises. The lava fills in the space and forms more ocean crust. The seafloor between North America and Europe is made of ocean crust that formed after the continents began to spread apart.

Mountains on the Ocean Floor

The ocean crust made at divergent plate boundaries is not flat.

Mid-ocean ridges are long, narrow mountains formed by magma at divergent boundaries.

Even though explosive volcanic eruptions usually occur near convergent plate boundaries, more lava erupts at divergent plate boundaries. Three-quarters of all lava erupts at mid-ocean ridges. As lava erupts under water, it hardens into flows and unique shapes, such as pillow lava.