Earth Science Chapter 18

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All of the questions are from the chapter 18 power point in class notes.

Volcanism

All the processes associated with the discharge of magma, hot fluids, and gases.

Volcano

Both the opening in the Earth's crust through which molten rock, gases, and ash erupt and the landform that develops around this opening.

The Ring of Fire

The zone of volcanic activity surrounding the Pacific Ocean.

Convergent Volcanism

Volcanoes form on the overriding plate as subducting plate melts to form magma.

If convergent boundary is ocean/continent, then the chain of volcanoes is where?

On land

What forms when a convergent boundary is ocean/ocean?

A volcanic island arc

Example of a chain of volcanoes on an ocean/continent convergent boundary

Cascade Range of Pacific Northwest

Example of a volcanic island arc on an ocean/ocean convergent boundary

Mariana Islands

Where does most magma reach Earth's surface?

At divergent boundaries

When magma reaches the surface what does it do?

Fills in gap as plates move apart.

What type of magma does divergent volcanism contain?

Basaltic

Almost all divergent volcanism is (1)_________. Except in (2)__________.

(1) underwater. (2) iceland

Hotspots

Areas of volcanic activity that result from plumes of hot solid material that have risen from deep within Earth's mantle. As this rises, it melts to produce magma.

What do hotspots form and how?

Forms islands as plate moves over spot if in water.

Example of a hotspot

Hawaiian Islands

What happens when a hotspot is under land?

It erupts through cracks (called fissures) and forms flood basalts, resulting in lava plateaus.

Fissure

A crack in the ground

Example of a hot spot under land

Columbia River basalts; Deccan Traps, India.

Conduit

Tube-like structure lava travels through to reach the surface.

Vent

Opening on the surface at the end of the conduit.

Crater

Bowl-shaped depression around the vent at the top of a volcano.

Caldera

Forms when the top of a volcano collapses after an eruption empties part of the lava chamber underneath it.

What do types of eruptions and lava that form it determine?

Shape and structure

Shield Volcanoes

Made with basalt. Very fluid and will flow long distances before cooling. Volcanoes have a very broad, circular base with shallow slope. Largest type of volcano. Non-explosive eruptions.

What are shield volcanoes made with?

Basalt

Example of a shield volcano

Mauna Loa, Hawaii.

What type of lava are in cinder cone volcanoes?

Usually basaltic

Cinder Cone Volcano

Molten lava thrown into air from vent, form solid fragments that fall on ground around vent. Cone-shaped mound with oval base and steep sides. Smaller than other types, often found in groups.

Tephra

solid material ejected from a volcano during an eruption

Another name for a composite volcano is a ________.

Stratovolcano

Composite Volcano

Mixed layers of lava flows and more explosive pyroclastic material.

Example of a composite volcano

Mount Rainier, Washington.

3 conditions which allow solid asthenosphere to melt and form magma

1.) Decrease in pressure- divergent zones where lithosphere thinner and exert less pressure.
2.) Increase in temperature- hot spots.
3.) Increase in water in asthenosphere- subduction boundaries.

Viscosity

Resistance to flow. Determined by amount of silica and temperature.

The more silica magma contains then...

The slower the magma moves

Basaltic Magma

<50% Silica, low viscosity, non-explosive. Rifts and ocean hot spots.

Andesitic Magma

50-60% Si, medium viscosity, medium explosivity. Subduction boundaries.

Rhyolitic Magma

>60% Si, high viscosity, high explosivity. Hot spots under continents.

Lava

Magma that reaches the Earth's surface.

Pahoehoe

ropelike in appearance.

What forms pahoehoe?

Hot, quick flowing lava on land

Aa

blocky in appearance.

What forms aa?

Cool, slow moving lava on land

What forms pillow lava?

Lava underwater

Pillow Lava

pillow-like appearance.

Pyroclastic Material

ejected lava when trapped gases escape.

What are ash and lava classified by?

Size

How big is ash?

< 2 mm

How big is lapilli?

2 - 64 mm

How big are blocks and bombs?

> 64 mm

Pyroclastic flow

a dense, superheated cloud of gases and pyroclastic material that travels downhill at high speed.

Plutons

Intrusive, igneous rock bodies.

Batholith

Irregularly shaped mass of coarse-grained igneous rocks that cover at least 100 km2 and take millions of years to form. Common in interior of mountain chains.

Stock

Similar to batholiths but smaller in size.

Laccolith

A lens-shaped pluton with a round top and flat bottom.

Sill

When magma intrudes parallel to layers of rock.

Dike

A pluton that cuts across pre-existing rocks.

Volcanic neck

When the magma in a volcano conduit solidifies.

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