Answer:
Gabbro and basalt
Explanation:
The oceanic crust is mainly made of gabbro rocks and different types of basalt rocks.
They are called fringing reefs. They grow directly from shore and form borders along the shoreline.
When a fringing reef continues to grow upward from a volcanic island that has sunk entirely below the sea level, an atoll is formed. Atolls are usually circular or oval in shape, with an open lagoon in the center.
The fringing reef is the most common type of coral reef. They however tend to be very sensitive to human induced pollution in the water and are easily destroyed by such.
Answer:
potassium-argon dating
Explanation:
Potassium-argon (K-Ar) dating technique is the most appropriate dating method for establishing the age of a volcanic ash. It is most applicable for dating minerals and rocks more than 100,000 years old. It is based on measuring the product of the radioactive decay of an isotope of potassium (k) into argon (Ar). Since potassium is a common element found in many volcanic ash layer, the time since re-crystallization is calculated by measuring the ratio of the amount of argon 40 (Ar-40) accumulated to the amount of potassium 40 (K-40) remaining in these volcanic ashes.
