GEOL 1601 Chapter : Chapter 3 Notes (Yuting Li)
Geology Lab: Chapter 3—Igneous Rocks
Introduction
• Most rocks are aggregates of crystals or grains of one or more minerals
• The individual mineral particles in rocks are generally small with average
dimensions less than 1 centimeter
• Rocks are classified as igneous, sedimentary, and metamorphic based on how they
are formed
o Igneous rocks solidified from melted rock
o Sedimentary rocks formed from material (sediment) deposited on the Earth’s
surface
o Metamorphic rocks formed when preexisting rocks recrystallized, as solids,
under high temperature and pressure beneath the Earth’s surface
• Rock cycle is a global recycling program that links these rocks together
o These basic rock types are linked by a set of processes that routinely
converts igneous rocks to sedimentary and metamorphic rocks, sedimentary
rocks to igneous and metamorphic rocks, and metamorphic rocks to igneous
and sedimentary rock
Igneous Rocks
• The most spectacular way in which igneous rocks form is by volcanic eruption
o Process:
▪ Happens when molten rock, called magma, rises to the surface
▪ Magma then either simply flows onto the surface as lava or violently
exploded at the surface due to rapid bubble formation within
▪ The forming bubbles increase the volume of the magma so
dramatically that it shoots up and out of the volcano
• Igneous rocks that cool on the surface are called extrusive rocks
o Cool quickly because air, rain, and snow rapidly remove a lot of heat
• Igneous rocks that cool beneath the surface are called intrusive rocks
o The trapped magma cools slowly because the surrounding rocks form an
insulating jacket that helps the magma to retain its heat
• Why igneous rocks are studied:
o Many people live close to volcanoes so they study the ancient deposits of
individual volcanoes to understand their likely future eruptive style
o Much of the Earth’s crust, both continental and oceanic, is made of igneous
rocks so they study them to understand how our crust formed and why our
planet is so different from the other rocky planets
o Igneous rocks commonly form at tectonic plate boundaries so the study of
ancient igneous rocks tell us a great deal about the history of the Earth
o Hot magmas drive the circulation of a lot of hot water that pick up a number
of important metals and can deposit them to create metal ore deposits. These
deposits form a basis of our modern technological economy.
• Igneous rocks differ widely in texture, mineral composition, and appearance
Texture
