Year 8 Science · Unit 3 · Lesson 24
Challenge Worksheet
Learning Goals
Read the graph
The bar chart below shows the approximate time needed for each rock type to form. Study it carefully then answer the questions.
Data: estimated from geological timescales; Ma = millions of years ago (Geoscience Australia, 2023).
(a) Which rock type takes the longest to form, and what geological processes require so much time?
(b) The graph shows that extrusive igneous rocks form in years while intrusive igneous rocks take millions of years. Both come from magma. Use your lesson knowledge to explain this difference.
(c) After a volcanic eruption, which rock type would be the first to form in a landscape? What would it take for sedimentary rock to eventually form from it?
What if...?
Imagine that plate tectonics suddenly stopped, all plate movement ceased permanently. Subduction zones shut down, mid-ocean ridges stop spreading, mountain building stops. Earth's interior is still warm, but no plates are moving. Predict what would happen to the rock cycle over the next 10 million years.
(a) Which rock types would stop forming or become very rare? Explain why, linking each rock type to the tectonic processes that create it.
(b) Which rock type would continue to form, and even accumulate? Explain what surface processes (not tectonic) would still be operating to produce it.
(c) What does this "what if" scenario tell us about the relationship between plate tectonics and the rock cycle? Write a generalisation supported by at least two specific examples.
Wrap Up
In one sentence, what was the main idea of this lesson?