1

SGES 1302INTRODUCTION TO EARTH SYSTEM

LECTURE 14: Rock Cycle & Magmatism

2

Lecture 14: Rock Cycle & Magmatism

• Rock is defined as any solid mass of mineral, or mineral-like matter that occur naturally as part of the Earth.

• Rocks can be divided into 3 broad groups:

• Igneous Rocks

• Sedimentary Rocks

• Metamorphic Rocks

3

Rock cycle helps us to understand the interrelationships among different parts of the Earth system

It help us to understand the origin of igneous, sedimentary and metamorphic rocks

4

Rock Cycle Magma is a molten material that forms inside the Earth. Eventually magma cools and solidifies : crystallisation

that forms igneous rocks Can occur at the surface (volcanism) or below the

surface (plutonism) Igneous rocks exposed at the surface will undergo

weathering (disintegration & decomposition) Materials formed often move downslope and transported Most sediment ultimately deposited in the ocean, others

along floodplains, swamps, etc.

5

Rock Cycle Sediments undergo lithification (compaction &

cementation) to form sedimentary rock If a sedimentary rock is buried deep enough or intruded

by magma, it will be subjected to high pressure & temperature (metamorphism).

It will turn into a metamorphic rock. If temperature is high enough, melting will occur,

creating magma. The magma will crystallise into igneous rocks and the

cycle continues. Other paths of the rock cycle (see rock cycle diagram).

6

Igneous Rocks: Magma Igneous rocks were the first rocks to form as Earth cooled from a molten

mass to the crystalline rocks of the early crust. Igneous rocks are the rocks that form when molten material (magma)

cools and crystallizes. Magma is often a slushy mix of molten rock, dissolved gases, and

mineral crystals. The common elements present in magma are the same major elements

that are in Earth’s crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), and sodium (Na).

Magma crystallise to form mainly silicate minerals. Of all the compounds present in magma, silica is the most abundant and

has the greatest effect on magma characteristics. Magma is classified as basaltic (42-52% SiO2), andesitic (52-66% SiO2),

or rhyolitic (>66% SiO2), based on the amount of silica it contains. Silica content affects melting temperature and impacts how quickly

magma flows (viscosity).

7

Igneous Rocks: Magma The type of igneous rock that forms depends on the composition

of the magma. Different minerals have different melting points. For example, rocks such as basalt, which are formed of olivine,

calcium feldspar, and pyroxene, melt at higher temperatures than rocks such as granite, which contain quartz and potassium feldspar.

Granite has a melting point that is lower than basalt’s melting point because granite contains more water and minerals that melt at lower temperatures.

In general, rocks that are rich in iron and magnesium melt at higher temperatures than rocks that contain higher levels of silicon.

8

Igneous Rocks: Magma Source of magma: partial melting of rocks, mainly in

the mantle, minor in lower crust Rocks do not entirely melt at one temperature &

pressure because each mineral in the rock has a particular melting point.

Some minerals remains solid, while the melted portion may flow away as magma.

As each group of minerals melts, different elements are added to the magma mixture thereby changing its

composition. If temperatures are not high enough to melt the entire

rock, the resulting magma will have a different composition than that of the original rock.

This is one way in which different types of igneous rocks form.

Main factors controlling the creation of magma: heat, pressure, water & composition of the source rock.

9

Creation of Magma Temperatures in the Earth increases with

depth (geothermal gradient) Average geothermal gradient is 30°C per km

(eg. At the depth of 10 km, the temperature is ~300°C hotter than the Earth’s surface)

With increasing depth, the pressure also increases, and the melting point of rocks generally increases with increasing pressure.

This is because pressure squeezes atoms and ions closer together, therefore requires higher temperature to break their bonds.

A rock that melts at 1100°C at Earth’s surface will melt at 1400°C at a depth of 100 km.

Temperature

Increasing Depth

Lithosphere

Asthenosphere

GeothermalGradient

Melting Z

one (~50 – 250 km)

10

Creation of Magma If pressure of a hot rock is removed or reduced, the rock may reach a

condition where melting occurs : decompression melting. Decompression melting occurs at divergent plate boundaries, where

partial melting of asthenosphere occurs below a thinning lithosphere at oceanic or continental rift zones.

Water, even a small amount lowers the melting point of rocks, especially at high pressures.

Water enhances melting because it dissolves more easily in magmas than in minerals.

The partial melting aided by dehydration of subducted oceanic plate is important mechanism of magmatism at subduction zones.

11

Motion of Magma Once partial melting produces magma, the magma rises towards

the Earth surface due to its bouyancy (less dense than the surrounding rocks).

A magma’s ability to rise is large controlled by its fluidity (viscosity), which is governed by its temperature and composition.

Increasing temperature decreases viscosity of any material: hot magmas flows more easily than relatively cool magmas.

Viscosity generally increases with silica content: silica-rich magma such as felsic magma is more viscous and tends to relatively cool (it crystallise at lower temperature) compared to low-silica magma such as basaltic magma.

Silica-rich magma is less likely to reach the surface compared to low silica magma. Silica-rich magma is more likely to crystallise within the Earth crust.

12

Motion of Magma When magma reaches the surface, it is called lava. Volcanic eruption can be violent or quiet. Crystallisation of lavas forms volcanic or extrusive rocks. Violent volcanic eruption ejects materials high into the atmosphere,

and subsequently these pyroclastic materials fall on the Earth surface to form pyroclastic rocks.

Most magma loses its mobility before reaching the surface. It crystallise at depth to form plutonic or intrusive rocks.

Intrusive rocks will only be exposed at the surface after being uplifted and overlying rocks eroded away.

13

Kilauea Volcano, Hawaii

14

Mt Pinatubo, Philippines Mt St Helen, USA

15