Lava is magma that reaches the Earth's surface and is erupted.

Volcanic Rocks

There are two major forms of volcanic rocks

1. Rocks formed from the direct cooling of a lava flows. These are the volcanic igneous rocks such as basalt, andesite and rhyolite.
2. Rocks formed from solid fragmental volcanic materials. These are called pyroclastic materials and rocks. Pyroclastic means fiery fragments.

Lava Flows

Basaltic lava flows come in two forms:

1. Pahoehoe - very fluid flows that cool into a ropey texture

2. Aa - cooler, slower flows that solidify into a jagged blocky basalt.

Andesitic lavas are thicker and slower and cool into layered rock.

Felsic lavas are extremely thick, and flow only with great difficulty. These often cool into a volcanic glass material called obsidian.

Pyroclastic Materials

Any composition of lava can produce pyroclastic materials. Pyroclastic fragments are ejected from the volcano as solid fragments, often solidifying as they leave the volcanic vent, or fly through the air.

Pyroclastic fragments are referred to as tephra.

Tephra comes in all sizes:

• Volcanic bombs - large, often streamline fragments ranging in size from 2 inches to the size of a school bus.
• Lapilli - Latin for "little stones", these fragments are gravel sized.
• Cinders - sand sized fragments.
• Ash - finest volcanic material.

Pyroclastic rocks are formed from the accumulation of tephra and include:

• Volcanic breccias - contain large angular fragments embedded in ash and cinders. These are often formed by collapse and landslides on the volcano.
• Tuff - consolidated cinders and ash
• Welded Tuff - fused volcanic glass, ash and cinders. Formed when ash and cinders are deposited at very high temperatures, often in a pyroclastic flow or nuee ardente.

Volcanic Land Forms

Shield Volcanoes

• formed from basaltic (mafic) lavas
• very gentle slopes, convex in shape
• volcano is broad, like a shield layed on the ground, hence the name.
• eruptions can come from a summit vent, or from fissures on the flanks of the volcano
• summit can have calderas
• shield volcanoes are the largest by volume, both on the Earth and other planets
• Mauna Loa on the big island of Hawaii is a classic example

Composite Volcano (Stratovolcano)

• andesitic (intermediate) composition lavas, alteernating eruptions of pyroclastic material and lavas
• form large symmetric cones, concave flanks get steeper toward summit
• cones are composed of alternating layers of pyroclastics and lava flows... pyroclastic layers are steep, lava flows cover the pyroclastic layers preventing their erosion
• most beautiful volcanoes, often capped with snow and glaciers
• most violent volcanoes, usually destroyed by explosive eruptions
• Mount St. Helens, Mt. Rainier, Vesuvius and Mt. Fuji are examples

Cinder Cone

• usually basaltic to intermediate magma
• made entirely of pyroclastic material, mostly cinder and lapilli
• small, symmetic cones with uniform steep slopes, usually less that 1000 feet high
• short lived volcanoes with most material coming from one central vent, vent is relatively large compared to the size of the cone
• erode quickly since pyroclastic fragments are not cemented together

Calderas

• large (> 1km) depressions caused by collapse after a volcanic eruption
• shield volcanoes often have calderas near their summits
• Crater Lake is an example of a stratovolcano summit that collapsed into a caldera about 6000 years ago
• Some very large calderas are not associated with a single volcanic peak. Yellowstone NP is the site of several caldera eruptions and collapses. These calderas are 10's of miles across and erupted nearly 1000 times as much material as Mt St Helens between 1 and 2 million years ago.
• Long Valley, CA, near the Mammoth Mtn. Ski resort is the site of another large caldera, which like Yellowstone, has erupted enormous amounts of material in the last 1 million years.
• Ash deposits from large caldera eruptions can often be found thousands of miles away. Crater Lake and Yellowstone ash are found east of the Mississippi river. These ash deposits provide excellent means of dating the rocks that surround them. 
• Ash deposits amid other lake sediments are critical to dating deposits containing early hominid fossils in east Africa.

Eruptive Styles

• Plinean - vertical blast often to very high altitudes in the atmosphere, ash fall, tuffs

• Pelean - pyroclastic flows (nuée ardente), ash flow, welded tuffs

Predicting Eruptions

Short term predictions based on monitoring of precursory events (things that reliably happen before an eruption):

• Seismicity: Magma movement is marked by earthquake activity that produces a "ringing" type seismogram. These earthquakes are known as harmonic tremors. Monitoring the depth and movement of seismicity is also key to prediciting eruptions.
• Tiltmeters: These monitor the tilt of the flanks of the volcano as it swells when magma rises.
• SO₂: Rate of release of sulfur dioxide gas (SO₂) is another important monitoring tool.
• Heat flow: measurements of increased heat flow indicate rising magma