Earth & Life Science – Lecture 5: Magma, Structural Geology, and Stratigraphic Principles

These flashcards review key concepts from Lecture 5, covering magma formation, structural features (folds and faults), and the fundamental stratigraphic laws used to interpret Earth’s rock record.

What is magma?

Hot, semi-liquid molten rock composed of partially melted mantle material and subducted oceanic plates, located beneath Earth’s surface.

What is magmatism?

The set of processes beneath Earth’s crust involving the formation, movement, and evolution of magma.

Through what main process is magma generated in the lower crust and upper mantle?

Partial melting of solid mantle and crustal rocks.

Name the three primary factors that trigger partial melting to form magma.

(1) Increase in temperature, (2) Decrease in pressure, and (3) Addition of volatiles such as water or CO₂.

How does an increase in temperature produce magma?

Heat conducted from hotter surrounding rocks raises a rock’s temperature until it begins to melt.

How does a decrease in pressure lead to magma formation?

During mantle convection, rising rocks experience lower pressure, allowing them to melt (decompression melting).

How does the addition of volatiles cause melting?

Introducing water or CO₂ lowers a rock’s melting point, producing magma through flux melting.

In structural geology, what is a fold?

A bend in rock layers created by compressional forces.

What is an anticline?

An upward-arching fold with the oldest rocks at its core.

What is a syncline?

A downward-curving fold (trough) with the youngest rocks at its core.

What is a monocline?

A simple step-like bend where otherwise horizontal layers tilt in one direction before flattening again.

Define a fault.

A fracture or zone of fractures between two blocks of rock along which movement has occurred.

Describe a normal fault.

A fault produced by tensional forces where the hanging wall block moves downward relative to the footwall.

Describe a reverse fault.

A fault produced by compressional forces where the hanging wall block moves upward relative to the footwall.

What is a strike-slip fault?

A fault with primarily horizontal motion where blocks slide past each other along a nearly vertical fault plane.

What does stratigraphy study?

The description, correlation, and interpretation of layered (stratified) rocks.

What is stratification?

The bedding or layering that forms in sedimentary (and some igneous) rocks.

State the Law of Superposition.

In an undeformed sequence of sedimentary layers, the oldest layer lies at the bottom and the youngest at the top.

State the Law of Inclusion.

Rock fragments enclosed within another rock are older than the host rock that contains them.

State the Law of Cross-Cutting Relationships.

Any rock body or fault that cuts across another rock unit is younger than the unit it disrupts.

State the Law of Original Horizontality.

Sedimentary layers are initially deposited as flat, horizontal sheets.

What do unconformities indicate in the geologic record?

Gaps in time caused by erosion or non-deposition of sediments.

Explain the Principle of Faunal Succession.

Fossils appear in rock layers in a definite, recognizable order, allowing correlation of strata across regions.

Who was Nicolas Steno and why is he important to stratigraphy?

A 17th-century scientist whose work on rock layers established foundational stratigraphic principles such as superposition and original horizontality.