Geology 111G

Lecture 6. Plutons and Volcanoes

Plutons

Concordant

Discordant

Volcanoes

Types of Lava

Pyroclastic debris

Morphology

I. Plutons: Igneous rocks masses that form beneath the earth's surface by crystallization from subsurface magma. Plutons are formed of intrusive rocks. Vary from huge, massive bodies of rock to sheetlike features. Concordant plutons have margins parallel to layering of host or country rock. Discordant plutons have margins that cross cut these layers.

A. Tabular plutons: thickness is small relative to other dimensions; sheetlike.

1. Sill: concordant

2. Dike: discordant

B. Massive plutons: subequal dimensions

1. Laccolith: concordant, with layers arched up over top.

2. Stock: discordant, 1-several km in long dimension

3. Batholith: large pluton or composite pluton, consisting of many stocks; hundreds to thousands of square km. Probably intruded at several km (10-15) depth, but exposed by uplift and erosion.

1. Sierra Nevada batholith contains more than a dozen plutons to form a complex 650 km by 60-100 km. Roots of an ancient volcanic mountain chain, termed a volcanic arc.

II. Volcanoes: surface expressions of subsurface magmatism (igneous activity).

A. Types of lavas tend to fall into three categories, determine the type of volcano and the violence of the eruption.

1. Mafic (basaltic magmas ):

a. Less than 50% silica.

b. Form basalt

c. Common at divergent plate boundaries, ocean basins.

d. Low volatiles, low silica.

e. Tendency to form low viscosity lava flows.

2. Intermediate:

a. 50-70% silica

b. Form andesite

c. Common at convergent margins characterized by subduction, where they form volcanic arcs.

d. High volatiles, moderate silica

e. Tendency to form violent eruptions that create mixes of gas and fragmental material that create pyroclastic deposits.

3. Silicic (rhyolitic)

a. Greater than 70% silica

b. Form rhyolite

c. Common at convergent plate margins, both subduction and collisional

d. High volatiles, very high silica.

e. Tend to form local domes that well up like syrup and widespread sheets of pyroclastic material.

B. Pyroclastic debris: extremely common in silicic and intermediate magmas. Consist of ejecta (fragments) thrown from the vent and deposited on the ground by a variety of processes.

1. Tephra: collective term for pyroclastic fragments.

a. Ash (small)

b. Cinders

c. Lapilli

d. Blocks (large)

e. Pumice is a special form of tephra consisting of frothy glass.

2. Processes that deposit pyroclastic debris.

a. Pyroclastic falls: ash falling from an eruption cloud

b. Pyroclastic flows: denser mixes of gas and ejecta ejected laterally, formed by cloud collapse, or even between eruptions by collapse of mountain flank.

1) Ash flow plus glowing cloud.

C. Volcano Morphology is controlled by the dominant material erupted and eruptions style. Volcano is usually some combination of a vent, which commonly forms a crater, and an edifice, the mountain-like feature that makes up the body of the volcano.

1. Shield volcanoes: gently sloping, large base, up to 100 km radius, but may be 10 km high. Vent consists of fissures on flanks and some kind of central vent. Composed entirely of lava.

2. Cinder cones: small, steep sided, prominent central vent. Almost all cinders

3. Composite volcanoes: large, symmetrical volcano composed of both lava flows and pyroclastic deposits. 2-5 km high.

4. Caldera: steep walled, basin shaped depression formed by collapse of edifice or roof rocks into magma chamber.