Warm Up 11/18 The broad, slightly dome-shaped volcanoes of Hawaii are ____. a. cinder cone volcanoes c. pyroclastic volcanoes b. composite cone volcanoes d. shield volcanoes The most violent volcanic eruptions are associated with what type of volcano? a. composite cones c. fissure eruptions b. shield volcanoes d. cinder cones A magma’s viscosity is directly related to its ____. a. age c. silica content b. depth d. color Answers: 1) d. 2) a. 3) c.

Intrusive Igneous Activity Chapter 10, Section 2

Plutons Plutons – structures that result from the cooling and hardening of magma at depth They can only be studied after uplift and erosion have exposed them to the surface Intrusive igneous bodies, or plutons, are generally classified according to their shape, size, and relationship to the surrounding rock layers

Sills and Laccoliths Sills and laccoliths are plutons that form when magma is intruded close to the surface Sill – forms when magma is injected along sedimentary bedding surfaces, parallel to the bedding planes For a sill to form, the sedimentary rock must be lifted to a height equal to the thickness of the sill Laccoliths – forms when magma intrudes into sedimentary bedding surfaces, collects as a lens-shaped mass that pushes the overlying strata upwards

Sills

Laccoliths

Concept Check How are sills and laccoliths alike? They are both plutons formed by magma intrusions close to the surface How are sills and laccoliths different? They differ in shape and usually composition

Dikes and Batholiths Dikes – plutons that form when magma is injected into fractures, cutting across preexisting rock layers Many dikes form when magma from a large magma chamber invades fractures in the surrounding rocks Batholiths – the largest intrusive bodies, must have a surface exposure greater than 100 square kilometers Batholiths may form the core of mountain ranges

Dikes

Batholiths

Basic Igneous Structures

Origin of Magma Earth’s crust and mantle are composed primarily of solid, not molten, rock, and the liquid iron in the outer core is too dense to travel all the way to the surface Geologists conclude that magma originates when essentially solid rock, located in the crust and upper mantle, partially melts The most obvious way to generate magma from solid rock is to raise the temperature above the level at which the rock begins to melt

The Role of Heat The rate of change of temperature as you move down into the upper crust is ~20-30º per kilometer Geothermal Gradient – the change in temperature with depth At a depth of ~100 km, temperature ranges from 1400ºC and 1600ºC, almost the melting point of rock Subduction zones add friction (another way to produce heat) and water (which lowers the melting temperature of rock) to give the added heat needed to melt the crustal rocks Hotter mantle rocks can rise and intrude into crustal rocks, melting them

Geothermal Gradient

Concept Check What is a geothermal gradient? The change in temperature with depth

Magma Formation at Subduction Zones

The Role of Pressure Pressure also increases with depth Melting occurs at higher temperatures as you go deeper into Earth, because an increase in the confining pressure causes an increase in the rock’s melting temperature Reducing that confining pressure lowers a rock’s melting temperature Decompression Melting – triggered when the confining pressure drops enough to melt the rock

Decompression Melting

Assignment Read Chapter 10 Do Chapter 10 Assessment