Geology · Year 12 · Mineralogy and Petrology · 2.º Período

Metamorphism and Metamorphic Rocks

Investigate the changes in rocks subjected to high heat and pressure without melting. Differentiate between contact and regional metamorphism and their resulting rock types.

TL;DR:Metamorphism is the study of transformation. This topic examines how existing rocks change in response to heat and pressure without melting. Students differentiate between contact metamorphism, driven by the heat of igneous intrusions, and regional metamorphism, driven by the immense pressure and heat of mountain-building events. This is a critical component of the OCR Geology specification, requiring a deep understanding of solid-state recrystallisation.

• National Curriculum Attainment Targets
• OCR Geology AS/A-level: 2.4.1 Metamorphic processes
• OCR Geology AS/A-level: 2.4.2 Metamorphic rock classification

About This Topic

Metamorphism is the study of transformation. This topic examines how existing rocks change in response to heat and pressure without melting. Students differentiate between contact metamorphism, driven by the heat of igneous intrusions, and regional metamorphism, driven by the immense pressure and heat of mountain-building events. This is a critical component of the OCR Geology specification, requiring a deep understanding of solid-state recrystallisation.

Students will learn to identify metamorphic textures, specifically the difference between foliated rocks (like schist and gneiss) and non-foliated rocks (like marble and quartzite). They also explore the concept of metamorphic grade and index minerals, which allow geologists to map the intensity of tectonic activity across a region. This unit connects mineralogy, tectonics, and structural geology.

Students grasp this concept faster through structured discussion and peer explanation, where they must justify the 'metamorphic grade' of a sample based on its mineral assemblage and texture.

Key Questions

1. What are the primary agents of metamorphism?
2. How does regional metamorphism differ from contact metamorphism?
3. What causes foliation in metamorphic rocks?

Watch Out for These Misconceptions

Common Misconception

Metamorphism involves melting the rock.

What to Teach Instead

If a rock melts, it becomes igneous. Metamorphism happens entirely in the solid state. Using the analogy of 'toasting bread' (changing texture/chemistry without turning it into liquid) helps clarify this boundary.

Common Misconception

Pressure always creates foliation.

What to Teach Instead

Foliation only occurs if there is directed pressure and the rock contains platy minerals like mica. Marble and quartzite are non-foliated because their minerals (calcite/quartz) are equidimensional. A collaborative investigation into mineral shapes helps surface this error.

Active Learning Ideas

Simulation Game

The Metamorphic Bake-Off

Using layers of different coloured plasticine and glitter, students apply heat (warmth from hands) and directed pressure to model foliation. They observe how 'minerals' (glitter) align perpendicular to the pressure, mimicking the formation of slate and schist.

40 min·Small Groups

Stations Rotation

Metamorphic Aureoles

Students examine a map of an igneous intrusion and rock samples from different distances. They must arrange the samples in order of increasing metamorphic grade and explain the changes in crystal size and mineralogy.

50 min·Small Groups

Think-Pair-Share

Parent Rock Match-Up

Students are given a list of metamorphic rocks and a list of protoliths (parent rocks). They must pair them up (e.g., Limestone to Marble) and explain what physical changes occurred during the transformation.

20 min·Pairs

Frequently Asked Questions

What is the main difference between contact and regional metamorphism?

Contact metamorphism is caused primarily by high temperature from a nearby magma body and affects a small area (an aureole). Regional metamorphism is caused by both high pressure and temperature over vast areas, usually during continental collisions or mountain building.

How does foliation form in metamorphic rocks?

Foliation forms when directed pressure (stress) is applied to a rock. Platy minerals, such as micas, rotate or grow in a preferred orientation perpendicular to the direction of maximum pressure, creating a layered or banded appearance.

What are the best hands-on strategies for teaching metamorphism?

Using physical models to demonstrate directed stress is vital. Beyond plasticine, using 'index mineral' maps where students must contour zones of different minerals (like chlorite, garnet, sillimanite) helps them understand how geologists reconstruct ancient pressure and temperature conditions in the field.

What is a protolith?

A protolith is the original, unmetamorphosed 'parent' rock from which a metamorphic rock is formed. For example, the protolith of slate is shale, and the protolith of marble is limestone. Identifying the protolith is a key step in understanding the geological history of a metamorphic rock.