Earth Science Unit 1 Lesson 8 Metamorphic Rocks

Tadpoles and caterpillars undergo a change in physical form called metamorphosis. Tadpoles change into frogs, and caterpillars change into butterflies.

Metamorphic rocks also undergo physical changes. They form when heat and pressure change preexisting rock. The preexisting rock is the parent rock. It can be igneous, sedimentary, or even another type of metamorphic rock. In this lesson, you will learn how metamorphic rocks form and how people classify and use them.

OBJECTIVES

Describe how metamorphic rocks form. Compare and contrast different types of metamorphism. Identify common characteristics of metamorphic rock.VOCABULARY

contact metamorphism changes in rocks caused by magma seeping into crustal rock; occurs at high temperature and low pressure

dynamic metamorphism

changes in rocks caused by tectonic plates rubbing sideways against one another; occurs at low to high temperatures and high pressure

foliation layers or bands found in metamorphic rock

hydrothermal metamorphism

changes in rocks caused by chemicals in hot water; occurs at low temperature and low pressure

metamorphosis physical, chemical, or structural change

regional metamorphism

changes in rocks caused by tectonic plates pushing together; occurs at low to high temperatures and medium pressure

Heat

The Formation of Metamorphic Rock

Have you ever baked a cake, made chocolate chip cookies, or baked brownies? The raw batter or dough you start with is very different from the end product. Heat from the oven changes the raw material into fluffy cake, chewy cookies, or moist brownies.

Similarly, heat produced inside of Earth can change rocks into new types of rocks.

Heat produced from the geothermal gradient and intrusive magma breaks the crystalline structure of the minerals in rocks

The newly deconstructed minerals rearrange themselves and combine with other chemicals to create new minerals. New rocks, called metamorphic rocks, are created when the minerals hardens.

Temperature increases as you move from the crust down toward Earth's core. On average, the temperature increases one degree Celsius for every 30 meters,

or 100 feet, of depth. This is called the geothermal gradient, which is the gradual increase of temperature as you go deeper into the Earth's crust.

As new rock forms on the surface, it pushes older rock deeper into the crust. Geologic events associated with plate tectonics can also plunge rocks deep into the crust.

Heat is applied to the rocks that are forced downward into the crust and they undergo metamorphosis.

Intrusive magma can also create metamorphic rocks. Magma from the mantle seeps or gets pushed into the crust. When the magma cools, it forms intrusive igneous rock. The heat produced from

the magma causes the surrounding rocks that are not melted to change into metamorphic rock.

If temperatures become high enough, rock melts back into magma. When the magma cools, it is new igneous rock. Metamorphic rocks form before complete melting occurs.

Pressure

Have you ever tried to move a bucket or wheelbarrow full of rocks? Rocks are heavy. The weight of rocks can exert an incredible amount of pressure.

If pressure becomes high enough, mineral structures break down and reorganize.

Sometimes, pressure is exerted unequally. Stress from tectonic plates can compress rock in only one direction. This type of

pressure is directed pressure. Metamorphic rocks formed from directed pressure have a layering condition,

called foliation. Foliated rocks break along these layers, creating thin, flat sheets.

Pressure from the weight of overlying rock is equal in all directions. It is similar to when you are underwater in a swimming pool. You can feel equal pressure all around you.

This type of pressure is confining, or hydrostatic, pressure. Metamorphic rocks do not foliate when they form under confining pressure.

Types of Metamorphism

Types of Metamorphism

Metamorphism Temperature Pressure Rocks Formed

hydrothermal low low rich ore deposits

contact high low hornfels

regional low - high medium marble, slate, schist, gneiss

dynamic low - high high mylonite

Imagine you wanted to paint your room blue. Could you go to the paint store and simply ask for a gallon of blue paint? No- because there are many different shades of blue, from navy to powder blue.

Similarly, there are also many different kinds of metamorphic rocks. The type of metamorphism determines the kind of rock.

There are four main types of metamorphism; hydrothermal, contact, regional, and dynamic.

In hydrothermal metamorphism, hot water interacts with preexisting rock. The water contains dissolved minerals that chemically change the surrounding

rock. Hydrothermal metamorphism is the most common and widespread type. It

happens at low temperature and low pressure.

Contact metamorphism also happens at low pressure, but at high temperature. Magma from an intrusion contacts preexisting rock. It seeps into tiny spaces and

heats the rock. Contact metamorphism affects a small area and forms fine-grained, non-foliated rock.

Regional metamorphism affects a much larger area than contact metamorphism. Pressure from a large-scale geologic event, such as mountain building, causes

regional metamorphism. It can happen at any temperature and medium pressure. Pressure from regional

metamorphism is directed, which forms foliated rock. The movement of tectonic plates also causes dynamic metamorphism. When two bodies of rock slide past one another along a fault line, the friction

generates heat. The heat and the mechanical forces of crushing and grinding change the rock.

Dynamic metamorphism is not very common because it only happens along fault lines; like regional metamorphism, it can happen at any temperature.

These four types of metamorphism determine the type of metamorphic rock and help to draw distinctions between the various types.

Common Metamorphic Rocks

Have you ever been to the Lincoln Memorial in Washington, D.C.? The 99-foot building is limestone covered with white marble.

Marble is a common metamorphic rock formed from limestone. People often use marble for ornate buildings and statues. The ancient Greeks and Romans made most of their statues from marble.

Slate is another common metamorphic rock. It forms from shale or mudstone. Unlike marble, slate is foliated. When slate breaks, it forms thin, flat sheets. This makes slate a popular building material for pathways, roof shingles, and floors.

Quartzite is a strong metamorphic rock formed from sandstone. Under high temperatures, the quartz grains in sandstone fuse together. This makes quartzite extremely strong and resistant to weathering. That is why people often use quartzite for landscaping.