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How Is a Blockchain Formed and What Does a Block Consist Of?

General Structure of a Blockchain​

A blockchain is a sequential chain of blocks, where each block contains a set of data and is linked to the previous block using cryptographic mechanisms. This structure ensures the integrity and consistency of data across the entire ledger.

New blocks are added strictly sequentially, which makes it possible to unambiguously determine the order of all records and prevents undetectable modification of historical data.

What Is a Block?​

A block is the basic structural unit of a blockchain. It serves as a container for storing data and includes both user-generated information and technical elements required for the network to function.

Each block is created according to predefined rules that depend on the specific blockchain and the consensus mechanism it employs.

Core Components of a Block​

A typical blockchain block includes the following components:

  • Data (payload)
    Usually transactions or other records that must be recorded in the distributed ledger.

  • Merkle Root
    A hash representing an aggregated cryptographic summary of all transactions included in the block.

  • Hash of the Previous Block
    A cryptographic value that uniquely identifies the preceding block in the chain and ensures the linkage between blocks.

  • Service (metadata) information
    May include a timestamp, consensus parameters, and other technical data.

  • NONCE (in blockchains using Proof-of-Work)
    A special number selected by miners to find a valid block hash.

The combination of these elements is used as input data for computing the block’s own hash. The hashing process is described in more detail in Section 1.3.

The Role of NONCE and Block Hash Formation​

In blockchains that use the Proof-of-Work mechanism, the NONCE parameter (Number Used Once) plays a key role in block creation.

Miners iterate over different NONCE values, combining them with the transaction hash and the hash of the previous block, until they find a block hash that satisfies the predefined difficulty condition. Once a valid value is found, the block is considered confirmed and is added to the blockchain.

In public blockchains, this process is typically accompanied by an economic reward in the form of cryptocurrency. In private or specialized blockchains, the NONCE mechanism and reward system may be absent.

Merkle Tree​

To efficiently store and verify a large number of transactions within a block, a data structure known as a Merkle Tree is used.

The process of forming the Merkle root consists of the following steps:

  1. A hash is computed for each transaction based on its input data (addresses, amounts, timestamps, etc.).
  2. Transaction hashes are paired, and the result of each pair is hashed.
  3. The resulting hashes are grouped and hashed at the next level.
  4. This process continues until a single hash remains—the Merkle root.

The Merkle Tree has a binary structure. If the number of transactions is odd, the last hash is duplicated to ensure proper construction of the tree.

The Merkle root is included in the block and participates in the formation of the block’s final hash. As a result, any change to a transaction alters the Merkle root, which in turn changes the block hash and all subsequent block hashes in the chain.

Block Size Limits and Throughput​

In many blockchains, block size is limited. This directly affects the number of transactions that can be included in a single block and, consequently, the network’s throughput. Blockchain throughput is commonly measured in transactions per second (TPS) and depends on several parameters:

  1. Block size — how many transactions can be included in a block.
  2. Block creation speed — how many blocks are produced per unit of time.

The current throughput of Bitcoin is approximately 6–7 transactions per second, which became one of the reasons for the development of second-layer solutions. At the same time, there are more modern experimental solutions with a claimed throughput of up to 10,000 transactions per second, such as Monad.

Summary​

A blockchain is formed as a sequential chain of blocks, where:

  • each block contains data and a cryptographic reference to the previous block,
  • hashing ensures data integrity and protection against tampering,
  • Merkle Trees enable efficient aggregation and verification of transactions,
  • modifying past data is impossible without breaking the entire chain.

Understanding block structure and block formation mechanisms is essential for further study of consensus, security, and economic incentives in blockchain systems.