VolcanoesBy: Rachel Clarke

How Volcanoes form on Divergent Boundaries

In the mantle, convection currents can break apart, and cause a crack in the Earth’s surface, allowing the magma to flow up from the mantle, which hardens on the surface, and builds up.

How Volcanoes form on Convergent Boundaries

The older plate sinks beneath the other plates and sinks into the mantle and makes a deep-ocean trench. Water in the sinking plate eventually leaves the crust and rises into the wedge of the mantle above it. The melting point of the mantle in the wedge is lowered. The mantle partially melts and the magma that forms as a result, and can break through the ocean's crust, creating volcanoes.

How the Lava/ Magma Moves Through the Volcano during an Eruption

All volcanoes have a system of passageways, through which the magma moves.

During an eruption, as the magma rises, the pressure of the rock on the magma decreases.

When the volcano erupts, the force of expanding gases pushes magma from the chamber through the pipe until it flows or explodes out of the vent.

How Lava is Identified

Hot, low-silica lava which is thin, runny, and flows easily.

Pahoehoe forms fast-moving hot lava that is thin and runny.

Aa forms cooler, thicker, and slow-moving lava.

High-silica lava is thick and sticky.

How Lava flows after it Leaves the Volcano

An explosive eruption will powerfully throw the lava into the air, causing it to break into fragments, which cool rapidly. The smallest pieces are called volcanic ash. The pebble-sized pieces are called cinders. The larger pieces are called bombs, and they can range from golf ball sized to the size of a car.

Eruptions

They both cause damage from their crater’s rims.

A quiet eruption can cover a large area of land with a thick layer of lava.

The eruption of a volcano can last as long as a decade to more than a million years.

Geologist classify a volcano either dormant, active, or extinct to describe it’s stage of activity.

If the volcano drifts away from it’s hot spot, it may become extinct

Landforms Volcanoes Form

When a volcano erupts it can make landforms out of lava, ash, and other materials.

These landforms include shield volcanoes, cinder cone volcanoes, composite volcanoes, and lava plateaus. Other landforms include calderas, which are the huge holes left by the collapse of volcanoes.

Large eruptions can empty the main vent and magma chamber beneath a volcano. At some spots on Earth's surface, thin layers of lava pour out of a vent and harden on top of previous layers. Lava can flow out of several cracks in an area. Low-silica lava can travel for miles before cooling. After millions of years, lava floods can build up high level layers. This is called a lava plateau.

Over time, forces such as flowing water, wind, or ice can strip away layers of the crust and expose the hardened magma. Features formed by magma include volcanic necks, dikes, and sills as well as dome mountains and batholiths.

Landforms Volcanoes Form *continued*

A dome mountain forms when uplift pushes a large body of hardened magma toward the surface. The hardened forces the layers of rock to bend upward into a dome shape. Eventually, the rock above the dome mountain wears away, leaving it exposed.

Batholiths are the core of many mountain ranges. Over millions of years, the rock is worn away, allowing the batholith to curve upward. Flowing water and grinding ice curve the batholith into the mountains.

Questions:

How does a volcano form on a divergent boundary? When a crack in the mantle forms, magma is forced

out of it, and after build up it becomes a volcano. What kind of lava is sticky and thick? High-silica lava What kind of landforms do volcanoes form? Batholiths, other volcanoes, lava plateaus, dome

mountains, and calderas. What is one of the fragments lava gets split up into? Volcanic ash, cinders, or bombs.