Chapter 4 Flashcards
Magma and Igneous Rocks
How is the process of freezing magma similar to that of freezing water? How is it different?
Both processes involve the liquid cooling to a solid, with atoms arranging in a structured form. The difference is that magma solidifies into a variety of minerals at different temperatures, forming an interlocking texture, while water freezes uniformly into ice crystals.
What is the source of heat on Earth? How did the first igneous rocks on the planet form?
Earth’s heat comes from radioactive decay and residual heat from planetary formation. The first igneous rocks formed as molten material from the early Earth’s mantle and crust cooled and solidified.
Describe the three processes that are responsible for the formation of magmas.
Magmas form by (1) decompression melting, as pressure decreases; (2) addition of volatiles, which lowers melting temperature; and (3) heat transfer from rising hot magma, which can melt surrounding rock.
Why are there so many different types of magmas? Does partial melting produce magma with the same composition as the parent rock from which it was derived?
Variations in source rock composition, melting conditions, and crystal fractionation cause diverse magmas. Partial melting creates magma richer in silica and other low-melting-point elements, different from the parent rock.
Why do magmas rise from depth to the surface of the Earth?
Magma is less dense than surrounding solid rock, creating buoyant forces that drive it upwards. Pressure differences also push magma toward the surface.
What factors control the viscosity of a melt?
Viscosity is controlled by silica content, temperature, and volatile content. High silica content and lower temperatures increase viscosity, while higher volatile content decreases it.
What factors control the cooling time of a magma within the crust?
Cooling time is affected by the depth of intrusion, size and shape of the magma body, and the presence of surrounding rock that insulates or dissipates heat.
What is the difference between a sill and a dike, and how do both differ from a pluton?
A dike cuts across rock layers, while a sill is parallel to them. A pluton is a large, deep-seated intrusive body formed by the cooling of magma underground, often larger than dikes or sills.
How does grain size reflect the cooling time of a magma?
Larger grains indicate slower cooling, allowing crystals to grow. Finer grains reflect faster cooling, where crystals don’t have time to grow as large.
What does the mixture of grain sizes in a porphyritic igneous rock indicate about its cooling history?
It indicates a two-stage cooling process: slow cooling initially forms large crystals, followed by rapid cooling that creates a fine-grained matrix.
Describe the way magmas are produced in subduction zones.
Magmas form as water and volatiles from the subducting plate lower the melting temperature of the overlying mantle wedge, causing partial melting and magma formation.
What process in the mantle may be responsible for causing hot-spot volcanoes to form?
Mantle plumes, which are columns of hot, buoyant rock rising from deep within the mantle, can cause melting and lead to hot-spot volcanism as they reach the lithosphere.
Describe how magmas are produced at continental rifts. Why can you find both basalt and rhyolite in such settings?
Decompression melting at rifts creates mafic magma (basalt), while partial melting of continental crust forms felsic magma (rhyolite), resulting in both rock types.
What is a large igneous province (LIP)? How might the formation of LIPs have affected the Earth System?
An LIP is an extensive region of igneous rock formed from massive volcanic eruptions. These events may have influenced climate and biotic extinctions by releasing large amounts of gases into the atmosphere.
Why does melting take place beneath the axis of a midocean ridge?
At mid-ocean ridges, decompression melting occurs as the mantle rises and pressure decreases, enabling partial melting and magma formation.
Bowen’s Reaction Series
A sequence describing the order of mineral crystallization in cooling magma, with minerals forming at high temperatures crystallizing first, followed by those that crystallize at progressively lower temperatures.
Crystalline Igneous Rock
Rock composed of interlocking crystals formed through the solidification of magma or lava, where the rate of cooling affects crystal size.
Dike
A sheet-like intrusion of igneous rock that cuts across preexisting rock layers, often formed when magma forces its way through cracks in the surrounding rock.
Extrusive Igneous Rock
Rock formed from magma that erupted onto Earth’s surface and solidified, typically fine-grained due to rapid cooling.
Flood Basalt
A vast outpouring of basaltic lava that covers large areas of land, often associated with the formation of large igneous provinces.
